Biomedical subjects

J C McGrath

Publications and source records attributed to J C McGrath.

At least 19 recordsLinked to original sources

PubMed1992

The effect of ethanol on responses of the isolated rabbit ileocolic artery.

Antagonist drugs were used to separate the purinergic and adrenergic contributions involved in the sympathetic vasopressor responses produced by electrical field stimulation in the ileocolic artery of the rabbit. Blocking drugs were applied either alone or in various combinations and sequences. The effect of ethanol was studied in conditions of alpha-adrenoceptor blockade, P2x purinoceptor desensitisation, or in the absence of antagonists. From these studies it is concluded that ethanol has a selective potentiating effect on alpha-adrenoceptor-mediated responses.

Adenosine Triphosphate↗

PubMed1992

Endogenous nitric oxide modulates sympathetic neuroeffector transmission in the isolated rabbit lateral saphenous vein.

The rabbit isolated lateral saphenous vein (RLSV) has a heterogeneous population of alpha-adrenoceptors. Responses to electrical field stimulation, in the presence of cocaine, exhibit both alpha 1- and alpha 2-adrenoceptor-mediated components. The present study examined sympathetic neuroeffector transmission and the response to exogenous catecholamines after inhibition of nitric oxide (NO) synthesis with N omega-nitro-L-arginine methyl ester (L-NAME). A comparison of the response in the presence and absence of a functional endothelium was also carried out. L-NAME potentiated the first and second components of the response to nerve stimulation on the order of 300 and 500%, respectively. L-NAME also significantly potentiated responses to norepinephrine (NE), phenylephrine (PE), and UK 14304. Selective antagonism of the first phase was seen with prazosin (alpha 1-antagonist, 0.1 microM) and the second phase with rauwolscine (alpha 2-antagonist, 1 microM). In the presence of L-NAME, the remaining (uninhibited) components were potentiated. Removal of endothelial function induced by gentle rubbing of the intimal surface abolished potentiation to exogenous NE, PE, and UK14304 by L-NAME. However, a significant degree of potentiation of the neurogenic response was observed in the rubbed tissues in response to L-NAME. This suggests that there may be a nonendothelial source of NO that can modulate the neurogenic response to electrical field stimulation.

Adrenergic alpha-Agonists↗

PubMed1992

Contributions of alpha 1-adrenoceptors, alpha 2-adrenoceptors and P2x-purinoceptors to neurotransmission in several rabbit isolated blood vessels: role of neuronal uptake and autofeedback.

1. The roles of autofeedback and neuronal uptake in neurotransmission produced by electrical field stimulation in several rabbit isolated blood vessels were examined. 2. Blocking drugs were used to separate the possible purinergic and noradrenergic contributions to the end organ response: prazosin, antagonist at postjunctional alpha 1-adrenoceptors; rauwolscine and yohimbine, antagonists at pre- and postjunctional alpha 2-adrenoceptors; alpha,beta-methylene ATP, desensitizing agent at postjunctional P2x-purinoceptors. In addition to desensitizing postjunctional P2x-purinoceptors, alpha,beta-methylene ATP potentiated the noradrenergic component of the nerve-induced responses. 3. In the presence of an intact neuronal uptake mechanism, the vessels showed different contributions of purinergic (via P2x-purinoceptors) and noradrenergic (via alpha 1-adrenoceptors and alpha 2-adrenoceptors) components to the end organ response to nerve stimulation: saphenous artery (approximately equal contributions from P2x-purinoceptors and alpha 1-adrenoceptors), ileocolic artery (mainly P2x-purinoceptors with a smaller contribution from alpha 1-adrenoceptors), plantaris vein (mainly alpha 1-adrenoceptors with a small contribution from alpha 2-adrenoceptors and P2x-purinoceptors) and saphenous vein (alpha 1-adrenoceptors). 4. The presence of alpha 2-adrenoceptor-mediated autofeedback could be demonstrated for both purinergic and noradrenergic components of the nerve-induced responses in the artery preparations. In the veins, potentiation of nerve-induced responses by alpha 2-adrenoceptor antagonists could not be studied due to blockade of postjunctional alpha 2-adrenoceptor-mediated vasoconstriction. 5. Blockade of neuronal uptake with cocaine potentiated the noradrenergic component of the nerve-induced responses. Both alpha 1-adrenoceptor- and alpha 2-adrenoceptor-mediated components were potentiated, with a relatively greater potentiation of the alpha 2-adrenoceptor-mediated component. In the case of saphenous vein an alpha 2-adrenoceptor-mediated component which was previously absent was uncovered.6. Blockade of neuronal uptake with cocaine had no effect or reduced the purinergic component of responses, the latter effect presumably due to enhanced alpha 2-adrenoceptor-mediated autofeedback.7. In the presence of cocaine, nerve-induced responses in the saphenous vein were biphasic. Rauwolscine potentiated the first phase and inhibited the second phase thus demonstrating effects of pre- and postjunctional alpha 2-adrenoceptor-mediated activation in the same preparation.8. In conclusion, neuronal uptake and autofeedback processes play important and complex interacting parts in determining the relative contributions of alpha 1,- and alpha 2-adrenoceptors and P2.-purinoceptors in the end organ response to neurotransmission in blood vessels.

Animals↗

PubMed1992

The influence of endothelin-1 on human foeto-placental blood vessels: a comparison with 5-hydroxytryptamine.

1. The vasoconstrictor effect of endothelin-1 (3 x 10(-11) M-10(-7) M) was studied in successive generations of blood vessels of the foeto-placental vascular tree. These were the human umbilical arteries and veins, primary surface chorionic plate arteries, secondary chorionic plate arteries, tertiary surface chorionic plate arteries and veins and the secondary stem villus arterioles. The responses to endothelin-1 were compared with those to 5-hydroxytryptamine (10(-9) M-10(-5) M). Arterial preparations were gassed with 2.5% O2, 8% CO2 balance N2 and venous preparations were gassed with 5% O2, 6% CO2 balance N2 to simulate the conditions prevalent in utero. The influence of increasing the oxygen tension to 16% (that prevalent at birth) on the response to endothelin-1 on the umbilical arteries was also investigated. 2. All the arterial vessels tested were some ten times more sensitive to endothelin-1 than to 5-hydroxytryptamine and the venous preparations were ten times more sensitive to endothelin-1 than were their equivalent arteries. Increasing oxygen tension did not affect the responses to endothelin-1 in the umbilical artery. 3. Whilst the amplitude of the endothelin-1-induced response was uniform throughout the foetoplacental vascular tree, including the stem villus arterioles, the maximum response to 5-hydroxytryptamine decrease with successive generations and it had no significant effect on the stem villus arterioles.The ratios of the responses to 10- M endothelin-1: 10-7M 5-hydroxytryptamine in human umbilical arteries, primary surface chorionic plate arteries, secondary chorionic plate arteries,tertiary surface chorionic plate arteries and the secondary stem villus arterioles in the vessels (listed in order of decreasing vessel size) were 1:1.2, 1:0.36, 1:0.33, 1:0.35 and 1:0.04 respectively.4. In conclusion, endothelin-1 is a powerful vasoconstrictor at all levels of the foeto-placental vascular system including the stem villus resistance vessels. It may play an important role in maintaining foeto-plancental vascular resistance at the low oxygen tension which exists in this vascular system in utero.

Blood Vessels↗

PubMed1991

Prostaglandin E2 and fetal oxygen tension synergistically inhibit response of isolated fetal rabbit ductus arteriosus to norepinephrine.

We wished to determine the effect of prostaglandin E2 (PGE2) on the response of the ductus arteriosus to norepinephrine (NE) and whether any effect of PGE2 was influenced by O2 tension. The vessel was isolated from fetal New Zealand White rabbits and studied in vitro. The response to NE was assessed in terms of both sensitivity (pEC50) and maximum contractile response (MCR) as determined from cumulative concentration-response curves. PGE2 caused a concentration-dependent inhibition of ductal sensitivity to NE. This was maximal in nanomolar concentrations of PGE2, in which ductal sensitivity to NE was decreased by approximately 50 times. This action was only minimally potentiated by 3% O2 (simulating fetal O2 tension, PO2). PGE2 could also inhibit the MCR to NE, but this was entirely dependent on PO2. In 95% O2, PGE2 had no effect on the MCR to NE, whereas in fetal PO2, PGE2 decreased the MCR. This decrease was maximal in 1 nM PGE2, in which the MCR in 3% O2 was about one fifth the size of the MCR in 95% O2. In 1 nM nM PGE2, 10% O2 also largely abolished the inhibitory effect of PGE2 on ductal MCR to NE, indicating that this effect of O2 is also observed in the range of the physiologic increase of arterial PO2, tension that occurs at birth. PGE2 also inhibited ductal sensitivity to 5-hydroxytryptamine, histamine, and potassium. We conclude that physiologic and therapeutic concentrations of PGE2 inhibit sensitivity of the ductus arteriosus to certain vasoconstrictors and can inhibit the maximum response to NE in fetal but not increased PO2.

Animals↗

PubMed1991

A comparison of the effects of angiotensin II and Bay K 8644 on responses to noradrenaline mediated via postjunctional alpha 1-and alpha 2-adrenoceptors in rabbit isolated blood vessels.

1. The effects of angiotensin II (AII) and Bay K 8644 on responses to noradrenaline (NA) mediated via postjunctional alpha 1- and/or alpha 2-adrenoceptors have been compared in three isolated venous preparations from the rabbit, the lateral saphenous vein, the left renal vein and the ear vein. 2. A similar action of AII and Bay K 8644 was observed only in the lateral saphenous vein; each potentiated responses to NA after isolation of a homogeneous population of postjunctional alpha 2- adrenoceptors. However, even in this preparation the mechanism of action for these agents was not identical. The sensitivity of KCl-induced contraction to changes in extracellular calcium ions (reflecting activation of voltage-dependent Ca2+ channels) was enhanced by Bay K 8644 but reduced by AII. 3. All produced a selective facilitation of responses mediated via postjunctional alpha 2-adrenoceptors. In the lateral saphenous vein it reduced the effectiveness of prazosin and facilitated responses after isolation of alpha 2-adrenoceptors with phenoxybenzamine and rauwolscine. It directly enhanced responses to NA in the ear vein, where only alpha 2-adrenoceptors are involved. In contrast, AII did not influence responses mediated via postjunctional alpha 1-adrenoceptors in the left renal vein (even after the receptor reserve had been removed with phenoxybenzamine) nor the 'rauwolscine-resistant' component of responses to NA in the saphenous vein. 4. Bay K 8644 enhanced contractile responses to NA mediated both via alpha 2-adrenoceptors, in the lateral saphenous vein, and via alpha 1-adrenoceptors in the left renal vein. Thus, unlike angiotensin II, no preferential effect was apparent. 5. Bay K 8644 was inactive against responses to NA in the rabbit isolated ear vein. Since the sustained component of responses to NA in this preparation is dependent upon the influx of extracellular Ca2 , these observations suggest that the influx of Ca2+ stimulated by NA is mediated via receptor-operated (1,4-dihydropyridine-resistant) Ca2 + channels.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗

PubMed1991

Postjunctional alpha-adrenoceptors in the rabbit isolated distal saphenous artery: indirect sensitivity to prazosin of responses to noradrenaline mediated via postjunctional alpha 2-adrenoceptors.

1. Under normal experimental conditions, the rabbit isolated distal saphenous artery appears to contain a homogeneous population of postjunctional alpha 1-adrenoceptors. Prazosin competitively antagonized responses to noradrenaline (NA) with a pA2 value of 8.6, while a relatively high concentration of rauwolscine (1 microM), produced only a 2 fold rightward displacement of the NA cumulative concentration-response curve (CCRC). 2. Despite the fact that angiotensin II (AII) was without effect on responses to NA or phenylephrine in this preparation, this peptide made responses to NA less susceptible to the antagonistic action of prazosin. This was particularly evident on the lower portion of the CCRC for NA. These results suggest that in the presence of AII, NA produces contractile responses by an action mediated through a prazosin-resistant adrenoceptor. 3. An attempt was made to isolate a homogeneous population of postjunctional alpha 2-adrenoceptors by use of a receptor protection procedure involving the combination of rauwolscine and phenoxybenzamine. After the protection protocol no responses were observed to the alpha-adrenoceptor agonists NA, phenylephrine or UK-14304. In the presence of angiotensin II however, concentration-dependent contractions were observed to each of these agonists. Under these conditions the rank order of potency, UK-14304 greater than NA greater than phenylephrine, is consistent with that of an effect mediated through postjunctional alpha 2-adrenoceptors. 4. The responses to NA, after the protection protocol, in the presence of AII, were susceptible to the selective alpha 2-adrenoceptor antagonist, rauwolscine (1 microM), but resistant to the selective alpha 2-adrenoceptor antagonist prazosin (0.1 microM). Furthermore, the combination of rauwolscine (1 microM) and prazosin (0.1 I microM) was no more effective in blocking responses to NA than was rauwolscine (1 microM) alone. These results are consistent with the presence of a homogeneous population of postjunctional alpha 2-adrenoceptors. 5. Inducing a small degree of tone with a low concentration of the selective alpha 1-adrenoceptor agonist, phenylephrine, markedly increased the threshold sensitivity to the selective alpha 2-adrenoceptor agonist UK- 14304, in a manner analogous to that seen with All. 6. The results in the present study indicate that responses mediated via postjunctional alpha 2-adrenoceptors in the rabbit isolated distal saphenous artery are dependent upon a degree of vascular smooth muscle stimulation by some other receptor system. It is hypothesized that under normal experimental conditions, this function is fulfilled by stimulation of alpha l-adrenoceptors, while after alpha 1-adrenoceptor blockade the necessary positive influence can be provided by stimulation of All receptors. The implications for such an interaction between postjunctional alpha-adrenoceptor subtypes in demonstrating prazosin-resistant, rauwolscine- or yohimbine-sensitive responses in isolated blood vessels is discussed.

Adrenergic alpha-Antagonists↗

PubMed1991

The effects of nifedipine on alpha 2-adrenoceptor-mediated contractions in several isolated blood vessels from the rabbit.

1. The effects of the dihydropyridine calcium channel blocker, nifedipine, on noradrenaline-induced contractile responses have been examined in several isolated blood vessels from the rabbit, with particular emphasis on responses mediated via postjunctional alpha 2-adrenoceptors. 2. In the isolated renal vein, ear vein, distal saphenous artery, saphenous vein and plantaris vein, 0.1 microM and 1 microM nifedipine reduced responses elicited by 54 mM KCl by more than 70%. The remaining responses were abolished by alpha-adrenoceptor blockade, suggesting the involvement of noradrenaline released from neurones activating a dihydropyridine-resistant mechanism. 3. In the renal vein (alpha 1-), ear vein (predominantly alpha 2-), distal saphenous artery (alpha 1- greater than alpha 2-), saphenous vein and plantaris vein (alpha 2- greater than alpha 1-), 0.01 microM and 0.1 microM nifedipine produced concentration-related reductions in the maximum response to noradrenaline. However, 1 microM nifedipine was no more effective than 0.1 microM nifedipine and the reduction in the maximum varied from 10-25% of the control response. Thus, a sizeable component of the alpha-adrenoceptor-mediated response in all blood vessels is resistant to dihydropyridine calcium channel blockers and this appears to be unrelated to the alpha-adrenoceptor subtype involved. 4. Following irreversible inactivation of alpha 1-adrenoceptors and isolation of functional alpha 2-adrenoceptors in the saphenous vein, plantaris vein and distal saphenous artery (the latter requiring the presence of angiotensin II), the effect of nifedipine on responses to noradrenaline was increased. However, a component of the alpha 2-adrenoceptor response in each preparation was present even after the concentration of nifedipine was increased to 1 microM. 5. In the saphenous vein, a preparation in which it has been demonstrated previously that alpha 2-adrenoceptor-mediated responses are highly dependent upon the presence of extracellular calcium ions, partial depolarization with 20mM KCl failed to increase the inhibitory effect of 0.1 microM nifedipine. This suggests the involvement of dihydropyridine-resistant Ca2+ channels. The possible relationship between these dihydropyridine-resistant Ca2+ channels, alpha-adrenoceptor subtypes and 'receptor-operated' Ca2 + channels is discussed.

Animals↗

PubMed1991

Different sensitivities of rabbit isolated blood vessels exhibiting co-transmission to the slow calcium channel blocker, nifedipine.

1. Antagonist drugs were used to separate the purinergic and adrenergic contributions as well as the adrenoceptor sub-types involved in the sympathetic vasoconstrictor responses produced by electrical field stimulation in rabbit isolated ileocolic and proximal saphenous arteries. Blocking drugs were applied either alone or in various combinations and sequences. 2. Nifedipine attenuated vasoconstrictor responses to sympathetic nerve stimulation both in the presence and in the absence of alpha-adrenoceptor blocking agents. However in the presence of alpha,beta-methylene ATP, nifedipine produces at best only a small attenuation of the vasoconstrictor response. 3. These results suggest that the purinergic component of the response to sympathetic nerve stimulation, at least in these tissues, can be antagonized by nifedipine, whereas the alpha 1-adrenoceptor-mediated response is relatively resistant.

Adenosine Triphosphate↗

PubMed1991

Influence of angiotensin II on the alpha-adrenoceptors involved in mediating the response to sympathetic nerve stimulation in the rabbit isolated distal saphenous artery.

Under normal experimental conditions, sympathetic nerve-mediated responses to electrical field stimulation in the isolated distal saphenous artery of the rabbit are sensitive to prazosin (0.1 microM) and so, by definition, are mediated by alpha 1-adrenoceptors. In the presence of angiotensin II (A II, 0.05 microM) however, a component of the response to nerve stimulation became resistant to prazosin. This 'uncovered' response was virtually abolished by the selective alpha 2-adrenoceptor antagonist rauwolscine (1 microM), a concentration that in the absence of A II had enhanced nerve-mediated responses. Exposure to A II therefore, allows the clear demonstration of a role for postjunctional alpha 2-adrenoceptors in mediating the contractile response to sympathetic nerve stimulation in this arterial preparation.

Angiotensin II↗

PubMed1991

The role of endogenous thromboxane in contractions to U46619, oxygen, 5-HT and 5-CT in the human isolated umbilical artery.

1. The effects of selective thromboxane antagonists and a thromboxane synthase inhibitor on the contraction to 9,11-dideoxy-11 alpha,9 alpha-epoxymethano-prostaglandin F2 alpha (U46619) and oxygen in the human umbilical artery (HUA) were examined. The effect of the antagonists on contractions to both 5-hydroxytryptamine (5-HT) and 5-carboxamidotryptamine (5-CT) were also examined. 2. U46619 (0.3 nM-10 microM) contracted the HUA. This contraction was antagonized by two selective thromboxane receptor antagonists EP092 (10 nM-1 microM) and GR32191B (10 nM-1 microM). The contraction was not affected by the selective thromboxane synthase inhibitor, dazoxiben (10 nM-1 microM). 3. When the oxygen tension was increased from 16 mmHg to 120 mmHg, the HUA transiently contracted. Both thromboxane antagonists inhibited this contraction in a concentration-dependent manner with 1 microM almost completely abolishing the response (the oxygen-induced contraction of the control preparation normally increases with a second exposure to 120 mmHg oxygen). 4. In low (16 mmHg) oxygen, responses to both 5-HT and 5-CT were unaffected by both thromboxane receptor antagonists at concentrations up to 1 microM. In high oxygen (120 mmHg) responses to both 5-HT and 5-CT were biphasic in nature, with an additional initial high sensitivity phase, which was abolished by a cyclo-oxygenase inhibitor. In high oxygen, EP092 and GR32191B blocked this initial phase in a concentration-dependent manner, returning sensitivity to 5-HT and 5-CT to that seen in low oxygen. 5. The thromboxane synthase inhibitor, dazoxiben, at concentrations greater than 10 nm inhibited the contraction to 120 mmHg oxygen and at 1 microM, dazoxiben almost abolished the response. In low oxygen, the response to 5-HT was unaffected by dazoxiben at concentrations up to 10 microM. In high oxygen, the initial phase of the contraction to 5-HT was inhibited by concentrations greater than 10 nm, with no effect on the maximum response. 6. The results show that thromboxane receptor antagonism or blockade of thromboxane synthesis selectively attenuates oxygen-induced contractions and those responses to 5-HT and 5-CT which are dependent on high oxygen for their expression. This suggests that the contractions caused by high oxygen tension, and the enhancement of the contractile effects of low concentrations of 5-HT and 5-CT in the presence of high oxygen tension are mediated by endogenously released thromboxane A2.

15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5↗

PubMed1991

Are there more than two types of alpha-adrenoceptors involved in physiological responses?

The application of the four main techniques available for examining the properties of alpha-adrenoceptors, gene coding, radioligand, biochemical and functional methods, has reinforced the earlier subclassification (alpha 1- and alpha 2-subtypes). These different techniques have not yet yielded subclassifications which entirely align when the subtypes have been examined in detail, although a prime reason for this is that the different techniques have not yet all been applied to the same receptors. There is evidence available on each level to indicate that there are further subtypes within the alpha 1- and alpha 2-adrenoceptors as originally defined pharmacologically, with the possible exception of functional evidence at cellular and tissue level for subtypes of alpha 2. The present extension of the subclasses to three each (A,B,C) for alpha 1 and alpha 2 does not withstand cross-examination on every level and seems unlikely to withstand further probing. It remains true that the set of subtypes of alpha-adrenoceptor which can be activated or blocked by drugs in functioning intact tissue preparations and which are shown in Fig. 4 has not been added to by knowledge derived from ligand binding or molecular biology. These techniques have bolstered confidence in the existing functioning categories and have enabled acceleration of drug screening programmes to find new, potent, highly selective antagonists for the known receptors. Rather than taking the categorization of receptors away from the recognition site to the larger molecule, they have reinforced its supremacy. The advantages being gained from the molecular biology of receptors lie in understanding cell signalling and, on the wider genetic scale, on the place of the receptors, among other elements of the cell, in regulation and function within the organism. From the physiologist's point of view, classical pharmacology can still be relied on as the basis for understanding the specificity of drugs which activate or block plasmalemmal receptors and to keep the numbers of these receptors within reason, while molecular biology may increasingly provide the tools to study the subsequent physiological events. A few years ago, one of us commented (in jest!) that the availability of so many different techniques for examining the properties of alpha-adrenoceptors may eventually provide the ultimate answer that there are forty-two subtypes, but that along the way, the original reason for requiring classification might become lost (McGrath, 1983 b).(ABSTRACT TRUNCATED AT 400 WORDS)

Adrenergic alpha-Agonists↗

PubMed1991

Angiotensin II causes vascular hypertrophy in part by a non-pressor mechanism.

Angiotensin II, when given in low doses, raises blood pressure slowly. When tested in vitro on vascular smooth muscle cells, it has mitogenic and trophic effects; it is not known if it has these effects in vivo. Our purpose was to determine whether vascular hypertrophy develops during slow pressor infusion of angiotensin II and, if so, whether it is pressure induced. Three experiments were done in rats infused subcutaneously with angiotensin II (200 ng/kg/min) by minipump for 10-12 days. Experiment 1: Angiotensin II gradually raised systolic blood pressure (measured in the tail) from 143 +/- 2 to 208 +/- 8 mm Hg (mean +/- SEM), significantly suppressing plasma renin and increasing threefold (NS) plasma angiotensin II. There was no loss of peptide in the pump infusate when tested at the end of the experiment. Experiment 2: In the perfused mesenteric circulation, vasoconstrictor responses to norepinephrine, vasopressin, and KCl were enhanced in rats given a slow pressor infusion of angiotensin II, but sensitivity of responses was not altered. This combination of changes suggests that vascular hypertrophy develops during slow pressor infusion of angiotensin II. Experiment 3: Vessel myography was done after angiotensin II infusion with and without a pressor response. Angiotensin II raised systolic blood pressure, increased heart weight, and produced myographic changes of vascular hypertrophy in the mesenteric circulation, increasing media width, media cross-sectional area, and media/lumen ratio. Hydralazine given with angiotensin II prevented the rise of pressure and the cardiac effect but not the vascular changes. Two-way analysis of variance showed that angiotensin II significantly increased media width, media cross-sectional area, and media/lumen ratio, all independent of hydralazine. Thus, although hydralazine inhibits the pressor and cardiac effects of angiotensin II, suggesting a pressor mechanism for the cardiac change, it does not inhibit structural vascular change, which suggests that at least part of the effect has a non-pressor mechanism.

Angiotensin II↗

PubMed1990

Effects of endothelin-1 on isolated vascular beds from normotensive and spontaneously hypertensive rats.

Endothelin-1 and noradrenaline induced dose-dependent pressor responses in isolated in situ blood perfused mesenteric arterial beds and isolated tail arterial beds of anaesthetised spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). In the tail the sensitivity and maximum to either agonist were the same in SHR and WKY whereas in the mesenteric bed the maximum pressor responses to both agonists were increased in SHR. This effect of endothelin-1 may contribute to the greater increase in blood pressure it induces in anaesthetised SHR compared with WKY.

Animals↗

PubMed1990

Effects of pre-contraction with endothelin-1 on alpha 2-adrenoceptor- and (endothelium-dependent) neuropeptide Y-mediated contractions in the isolated vascular bed of the rat tail.

1. The pressor effects to bolus doses of the alpha 2-adrenoceptor agonist UK-14,304 were studied in the isolated vascular bed of the perfused rat tail before and after increasing the perfusion pressure with infusions of endothelin-1. Those of neuropeptide Y were studied before and after pre-constriction with endothelin-1 or 5-hydroxytryptamine. The pressor effects of neuropeptide Y were studied before and after functional disruption of the endothelium with the detergent CHAPS. 2. Endothelin-1 and the alpha 1-adrenoceptor agonist phenylephrine induced dose-dependent vasoconstriction, endothelin-1 being some 10(4) times more potent than phenylephrine [log dose (mol) of the ED50 for endothelin-1 and phenylephrine: -11.8 +/- 0.2 (n = 7), -8.2 +/- 0.2 (n = 5) respectively]. 3. Under control conditions, at basal perfusion pressures, UK-14,304 and neuropeptide Y were virtually inactive as vasoconstrictors. Following a sustained increase in perfusion pressure by infusions of endothelin-1 (2.5-10 nM at 0.8 ml min-1), however, both UK-14,304 and neuropeptide Y induced dose-dependent pressor responses and both were some 10(2) times more potent than phenylephrine [log dose (mol) of the ED50 for UK-14304 and neuropeptide Y: -10 +/- 0.5 (n = 6), -10.3 +/- 0.4 (n = 6) respectively]. Responses to neuropeptide Y also were uncovered when vascular tone was increased with 5-hydroxytryptamine (5-20 nM) [log dose (mol) of the ED50 for neuropeptide Y: -10.2 +/- 0.2 (n = 6)]. 4. Pre-constriction-induced pressor responses to UK-14,304 were inhibited by 1 microM rauwolscine whilst those to neuropeptide Y were inhibited by disruption of the endothelium. Removal of the endothelium had no significant effect on the pressor responses to 4pmol or 8pmol endothelin-1 and had no effect on the increase in perfusion pressure induced by the endothelin-1 infusions but did decrease the time-course of pressor responses to bolus injections of endothelin-1. Endothelial disruption had no significant effect on the vasoconstriction induced by all but one of the doses of phenylephrine administered [log dose (mol) of the ED5o for phenylephrine after CHAPS: -8.6 + 0.2 (n = 5)], indicating that the responsiveness of the vascular smooth muscle was not destroyed by CHAPS. This treatment did, however, slow the onset and prolong the time course of the phenylephrine-induced responses. 5. These results indicate that, in the isolated vascular bed of the rat tail, pressor responses to both alpha 2-adrenoceptor- and neuropeptide Y receptor-activation are uncovered by agonist-induced preconstriction including that to endothelin-1. Neuropeptide Y-induced vasoconstriction was endotheliumdependent.

Animals↗

PubMed1990

Heterogeneity of alpha 2-adrenoceptors in rat cortex but not human platelets can be defined by 8-OH-DPAT, RU 24969 and methysergide.

1. Saturation experiments indicated that [3H]-yohimbine binding was specific, saturable and labelled a single population of sites in rat cerebral cortex (Kd 5.3 +/- 0.9 nM, Bmax 121 +/- 10 fmol mg-1 protein) and human platelets (Kd 0.7 +/- 0.1 nM, Bmax 152 +/- 10 fmol mg-1 protein). 2. The alpha 2-adrenoceptor antagonists, yohimbine, rauwolscine, WY 26703, idazoxan and BDF 6143 displaced [3H]-yohimbine binding to each tissue in a simple manner, with high affinity and Hill slopes close to unity. 3. The alpha 1-adrenoceptor agonist, oxymetazoline and the antagonist prazosin inhibited the binding of [3H]-yohimbine to rat in a complex manner consistent with an interaction at more than one site. However, indoramin and WB 4101 only appeared to interact with one site. In contrast, in human platelets, all antagonists gave rise to monophasic displacement curves with Hill slopes close to unity suggesting a single site of interaction. 4. The 5-hydroxytryptamine (5-HT) receptor ligands, 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), RU 24969, and methysergide inhibited the binding of [3H]-yohimbine to rat cortex with high and low affinity, consistent with an interaction with two populations of binding sites. However, inhibition of [3H]-yohimbine binding to human platelets suggested a single site of interaction. The low affinity of 5-HT, 5-carboxyamidotryptamine (5-CT) and dipropyl-5-CT indicated that [3H]-yohimbine was not labelling a 5-HT1-like site in rat cortex. 5. The ability of 8-OH-DPAT, RU 24969 and methysergide in addition to prazosin and oxymetazoline to differentiate [3H]-yohimbine binding provides additional pharmacological evidence for heterogeneity within rat cortical alpha 2-adrenoceptors. However, if the two sites in rat cortex that are differentiated by the 5-HT ligands represent (alpha 2A- and alpha 2B-adrenoceptor subtypes as defined by prazosin and oxymetazoline, then they do not correspond to the population of sites in human platelets. As receptor classification should be linked to affinity of drugs rather than tissue distribution, the current classification of alpha 2-adrenoceptor subtypes does not appear to be satisfactory.

8-Hydroxy-2-(di-n-propylamino)tetralin↗

PubMed1990

Alpha 2-adrenoceptor subtypes and imidazoline-like binding sites in the rat brain.

1. The binding of [3H]-yohimbine and [3H]-idazoxan to rat cortex and hippocampus is rapid, reversible and of high affinity. Saturation data indicate that a single population of binding sites exist for [3H]-yohimbine in the cortex (Bmax 121 +/- 10 fmol mg-1, protein; Kd 5.2 +/- 0.9 nM) and hippocampus (Bmax 72 +/- 6 fmol mg-1 protein; Kd 5.8 +/- 0.7 nM). [3H]-idazoxan labels one site in the cortex (Bmax 87 +/- 8 fmol mg-1 protein; Kd 4.1 +/- 0.9 nM) and hippocampus (Bmax 30 +/- 6 fmol mg-1 protein; Kd 3.5 +/- 0.5 nM), when 3 microM phentolamine is used to define non-specific binding. A second distinct [3H]-idazoxan binding site (Bmax 110 +/- 21 fmol mg-1 protein; Kd 3.6 +/- 0.07 nM) is identified in rat cortex if 0.3 microM cirazoline is used to define non-specific binding and 3 microM yohimbine is included to prevent binding to alpha 2-adrenoceptors. 2. Displacement studies indicate that the alpha 1-adrenoceptor antagonist prazosin and the 5-HT1 ligands 8-OH-DPAT, RU 24969 and methysergide differentiate [3H]-yohimbine binding into two components; a high and low affinity site. In contrast the displacement of [3H]-idazoxan by each ligand was monophasic. 3. The affinities of 8-OH-DPAT, RU 24969 and methysergide determined against [3H]-idazoxan binding to the cortex and hippocampus correlate significantly with the binding site displaying low affinity for prazosin and previously designated alpha 2A. In contrast, a poor correlation exists for the high affinity site for prazosin designated alpha 2B. 4. [3H]-idazoxan, in the presence of 3 microM yohimbine, labels a site that displays high affinity towards cirazoline, naphazoline and guanabenz, but low affinity towards clonidine, p-aminoclonidine, adrenaline, noradrenaline and the alpha 2-adrenoceptor antagonists yohimbine, rauwolscine, WY 26703 and BDF 6143. 5. The results of this study indicate that [3H]-yohimbine labels two sites; the alpha 2A- and alpha 2B-adrenoceptors whereas [3H]-idazoxan labels an alpha 2-adrenoceptor with a profile consistent with the alpha 2A-adrenoceptor subtype. In addition, [3H]-idazoxan labels an imidazoline binding site in the rat cortex that is pharmacologically distinct from alpha 2-adrenoceptors. The low affinity of clonidine and p-aminoclonidine indicates that the imidazoline-like binding site in rat cortex is different from the site labelled by [3H]-clonidine and [3H]-p-aminoclonidine in human, rat and bovine brain stem, providing evidence of potential heterogeneity within this class of binding sites.

8-Hydroxy-2-(di-n-propylamino)tetralin↗

PubMed1990

The influence of the initial stretch and the agonist-induced tone on the effect of basal and stimulated release of EDRF.

1. The effects of initial stretch and degree of agonist-induced tone on acetylcholine-induced relaxations were examined in rings of rat isolated aorta. The relaxation to acetylcholine was antagonized by atropine and almost completely abolished by haemoglobin. Relaxation to sodium nitroprusside was similar in rings with an intact or disrupted endothelium but that to isoprenaline was greater in intact preparations. 2. In preparations with either an intact or disrupted endothelium there was a similar length-dependent increase in the resting tension of the aortic rings. The size of the contractile response to phenylephrine (1 microM) was dependent on the initial length (and hence degree of stretch) of the preparation in both rubbed and unrubbed tissues. The absolute difference in contractile response between rubbed and unrubbed was greatest at 1.8 mm and less at the other lengths tested, including the optimum degree of stretch for contraction i.e. 2.4 mm. 3. The absolute acetylcholine-induced relaxation (only seen in rings with an intact endothelium) was dependent on the initial length (and hence degree of stretch) of the preparation and was maximum at 2.4 mm. The proportionate relaxation (i.e. expressed as a percentage of induced tone) was also length-dependent being optimal at 1.5 mm. 4. The sensitivity of the vessels to acetylcholine varied depending on the level of agonist-induced tone. When tone was low, acetylcholine sensitivity was high (at [NA] 0.03 microM: pIC50 = 7.36 +/- 0.07), when the concentration of noradrenaline was increased the tone increased and the acetylcholine sensitivity was low (at [NA] 0.3 microM: pIC50 = 6.57 +/- 0.07). 5. The absolute sensitivities and maximum relaxations induced by acetylcholine are discussed in relation to the initial degree of stretch (and hence length of the preparation) or the degree of agonist-induced tone.

Acetylcholine↗