Biomedical subjects

B Wilffert

Publications and source records attributed to B Wilffert.

At least 19 recordsLinked to original sources

PubMed1992

Veratridine activates a silent sodium channel in rat isolated aorta.

To investigate the existence of silent Na+ channels, isolated rat aorta was treated with veratridine (0.1 mM) and the resulting Ca2+ uptake was determined. After 30-min incubation the total tissue uptake of Ca2+ and Ca2+ uptake increased from 2.325 +/- 0.017 to 2.614 +/- 0.080 nmol/mg wet weight (ww) and from 162.6 +/- 9.7 to 218.1 +/- 13.0 pmol/mg ww, respectively. The veratridine-induced Ca2+ uptake was blocked by tetrodotoxin (1 microM; to 17 +/- 5%) but not altered by amiloride (10 microM-1 mM). Activation of Na+/Ca2+ exchange by Na+ removal increased Ca2+ uptake from 74.2 +/- 4.5 to 97.3 +/- 5.3 pmol/mg ww, which was suppressed by amiloride (10 microM-1 mM). Nifedipine (10 nM) and verapamil (0.1 microM) at concentrations at which depolarization-induced Ca2+ uptake was diminished did not attenuate veratridine-induced Ca2+ uptake. Phenytoin at 0.1 mM reduced the Ca2+ uptake induced by veratridine or by depolarization. R 56865 (0.1 microM) and R 59494 (1 microM), novel anti-ischemic compounds inhibiting slowly inactivating Na+ channels, suppressed the veratridine-induced but not the depolarization-induced Ca2+ uptake. Guanidinium uptake was increased by veratridine (0.1 mM) from 371.2 +/- 7.2 to 574.8 +/- 45.9 pmol/mg ww. These results suggest that the rat aorta possesses a Na+ channel which is electrically silent under normal conditions but could be activated by veratridine.

Amiloride↗

PubMed1992

The effect of some alpha-adrenoceptor antagonists on spontaneous myogenic activity in the rat portal vein and the putative involvement of ATP-sensitive K+ channels.

In the present study we showed that the alpha-adrenoceptor antagonists phentolamine, yohimbine, prazosin, corynanthine and idazoxan, when cumulatively applied in high concentrations (1-100 mumol/l), can increase spontaneous myogenic activity in the rat portal vein. 5-Methyl-urapidil and rauwolscine were ineffective in this respect. Pretreatment with phenoxybenzamine in a concentration of 1 mumol/l (20 min), which results in alkylation of all functional alpha-adrenoceptors in the rat portal vein, was unable to antagonize the increase in spontaneous myogenic activity elicited by phentolamine. Antazoline (1-100 mumol/l), a H1 antagonist and 2-substituted imidazoline which is devoid of alpha-adrenoceptor blocking properties, exhibited similar effects on spontaneous myogenic activity as its structurally closely related analogue phentolamine. Since phentolamine is reported to interact with ATP-sensitive K+ channels we investigated the role of K+ channels in more detail. The K+ channel openers cromakalim and diazoxide elicited a decrease in spontaneous myogenic activity. Glibenclamide (0.3-3 mumol/l), a selective blocker of ATP-sensitive K+ channels in cardiac and pancreatic tissues, and phentolamine (1-10 mumol/l) shifted the concentration-response curves of cromakalim and diazoxide concentration dependently to the right. Yohimbine showed only a modest effect in the highest concentration (100 mumol/l) applied. E-4031 (0.01-0.3 mumol/l), a sotalol derivative and one of the most selective blockers of the delayed rectifier current (Ik) in cardiac tissue, was a potent contractile agent when added to the rat portal vein in the same way as the alpha-adrenoceptor antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Triphosphate↗

PubMed1992

Cardiac effects of R 79595 and its isomers (R 80122 and R 80123) in an acute heart failure model. A new class of cardiotonic agents with highly selective phosphodiesterase III inhibitory properties.

R 79595 (N-cyclohexyl-N-methyl-2-[[[phenyl (1,2,3,5-tetrahydro-2 oxoimidazo [2,1-b]-quinazolin-7-yl) methylene] amin] oxy] acetamide) and its isomers represent a novel class of compounds with phosphodiesterase (PDE) inhibitory and cardiotonic (positive inotropic) actions. The cardiac effects of this class of compounds were investigated in the hexobarbital-depressed heart-lung preparation of the guinea-pig. After induction of heart failure (reduction of cardiac output to 25% of the initial value) cumulative addition of R 79595 or its isomers R 80122 (E-isomer) and R 80123 (Z-isomer) concentration-dependently reversed the cardiac depressant effects of hexobarbitone-Na. With regard to reconstitution of contractility and cardiac function R 80122 (E-isomer) was 10 fold more potent than R 79595 (1:1 mixture of the isomers) and nearly 100 fold more potent than R 80123 (Z-isomer). Furthermore, the cardiotonic action of the most potent isomer (R 80122) was compared to the effects of several positive inotropic reference compounds. The order of cardiotonic potency was as follows: (-)-adrenaline > R 80122 = adibendan > digitoxin > milrinone = enoximone > theophylline. Adibendan (EC50 value: 6.7 +/- 1.8 x 10.-8 mol/l), which showed cardiotonic effects in the same concentration range as R 80122 (EC50 value: 6.1 +/- 1.3 x 10(-8) mol/l), was significantly (p < 0.01) less effective than R 80122 with respect to the maximally induceable increase in cardiac output (CO). The cardiotonic effects of R 80122 could be observed in the low concentration range of 3 x 10(-8) to 1 x 10(-6) mol/l, whereas enoximone (EC50 value: 1.2 +/- 0.1 x 10(-5) mol/l) and milrinone (EC50 value: 8.9 +/- 3.5 x 10(-6) mol/l) elicited positive inotropic effects at 100 fold higher concentrations. Digitoxin was 10 fold less and theophylline was 300 fold less potent than R 80122 with regard to reconstitution of heart function. The cardiotonic effects of R 80122 were not accompanied by an increase in heart rate as found with milrinone, theophylline or (-)-adrenaline in this model. Furthermore, the PDE inhibitory effect of R 79595 and its E-isomer R 80122 were investigated in partially purified isoenzymes from guinea-pig ventricles. The IC50 values of R 79595 and R 80122 on PDE I-IV were compared to the IC50 values of adibendan, milrinone, enoximone and theophylline. The selectivity of an inhibitor for PDE III was evaluated by division of its IC50 values on PDE I, II and IV by the IC50 value on PDE III. R 80122 was the most potent and selective PDE III inhibitor.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1992

Ouabain-induced changes of calcium and potassium in slices of hippocampus of the rat: comparison to hypoxia and effect of R 56865.

Simple and reliable in vitro models of cerebral ischaemia are important for the identification of antiischaemic/antihypoxic compounds. Alterations of the concentrations of potassium and calcium were recorded in slices of hippocampus of the rat. The slices were subjected to hypoxia in the presence and absence of intoxication with glucose or ouabain (1 mmol/l). Normoxic slices of hippocampus showed an extracellular space of 57% and a tissue concentration of potassium of 45 mmol/kg wet wt. A cellular concentration of potassium of 92 mmol/kg was calculated. Hypoxia, in the presence of glucose, only slightly reduced tissue concentrations of potassium and did not influence concentrations of calcium. Omission of glucose during hypoxia led to tissue concentrations of potassium below 10 mmol/kg, within 10-30 min of hypoxia. Concentrations of calcium only increased from 3.3 to 3.5 mmol/kg after 30 min of hypoxia, without glucose. Intoxication with ouabain is proposed as alternative experimental model of ionic movements, associated with cerebral ischaemia/hypoxia. Tissue concentrations of potassium fell rapidly to values below 10 mmol/kg, within 5 min and concentrations of calcium rose to 5.2 mmol/kg, within 30 min of intoxication with ouabain. In quantitative terms, the model for cerebral ischaemia with intoxication with ouabain is suggested to be superior to the model based on hypoxia without glucose. To verify intoxication with ouabain as an experimental model for ischaemic/hypoxic insults, the effect of an investigational drug with antiischaemic/hypoxic properties (R 56865) was evaluated in the model. The drug R 56865 produced dose-dependent attenuation of the fall in tissue concentrations of potassium, between 3 x 10(-7) and 5 x 10(-6) mol/l.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1992

Flunarizine and R 56865 suppress veratridine-induced increase in oxygen consumption and uptake of 45Ca2+ in rat cortical synaptosomes.

The effect of the anti-ischemic compounds flunarizine and R 56865 on the veratridine-induced uptake of Ca2+ and Na+ was observed in cortical synaptosomes in the rat. The veratridine-induced uptake of Na+ and Ca2+ was determined by means of a measurement of synaptosomal oxygen consumption and a method for the uptake of 45Ca2+, respectively. Veratridine (10(-5) M) was found to induce a 3-fold increase in synaptosomal oxygen consumption (uptake of Na+) and uptake of 45Ca2+, both of which were inhibited by tetrodotoxin (10(-5) M). Nitrendipine (10(-5) M) and omega-conotoxin (5 x 10(-7) M) were ineffective on the veratridine-induced response. Nimodipine (10(-5) M) suppressed the veratridine-induced uptake of 45Ca2+ but also diminished the unstimulated uptake of 45Ca2+. The veratridine-induced uptake of Na+ was not influenced by nimodipine. Flunarizine (3 x 10(-6)-10(-5) M), as well as R 56865 (10(-6)-10(-5) M), attenuated the veratridine-induced uptake of both Na+ and 45Ca2+. In conclusion, the veratridine-induced uptake of Na+ and 45Ca2+ was shown to be closely correlated to the activity of Na+ channels but not to voltage-operated Ca2+ channels. Secondly, flunarizine and R 56865 seemed to evoke their effects by interfering with the permeability of Na+ channels. Since veratridine-induced uptake of Na+ and Ca2+ shares some similarities with ischaemia-induced uptake of Na+ and Ca2+, it is proposed, that flunarizine and R 56865 exert their anti-ischaemic effects by reducing ischaemia-induced Na+ and Ca2+ load, probably by inhibiting a TTX-sensitive Na+ channel.

Animals↗

PubMed1992

Alpha 1-adrenoceptor-mediated Ca(2+)-entry from the extracellular fluid and Ca(2+)-release from intracellular stores: no role for alpha 1A,B-adrenoceptor subtypes in the pithed rat.

1. In the present study, we tested the hypothesis that in the pithed rat preparation two subtypes of the alpha 1-adrenoceptor are linked to two different signal transduction mechanisms, both of which contribute to vasoconstriction, one facilitating Ca(2+)-entry from the extracellular fluid (alpha 1A) and one promoting the release of Ca2+ from intracellular sources (alpha 1B). 2. The selective alpha 1A-adrenoceptor antagonist, 5-methyl-urapidil, and the selective alpha 1B-adrenoceptor antagonist, chloroethylclonidine, were unable to discriminate between alpha 1-adrenoceptor-mediated pressor responses, which relied on an entry of extracellular Ca2+ sensitive to nifedipine and an intracellular release of Ca2+ insensitive to nifedipine, respectively. 3. Chloroethylclonidine, 12.5 and 25 mg kg-1 i.v., were equieffective, and had only minor effects on alpha 1-adrenoceptor-mediated increases in diastolic blood pressure. This could be associated with a small decrease in the receptor-reserve of the pithed rat preparation due to irreversible receptor blockade by this antagonist. These data indicate that chloroethylclonidine-sensitive alpha 1-adrenoceptors constitute only a minor fraction of the total alpha 1-adrenoceptor population on rat arterial resistance vessels. 4. Chloroethylclonidine behaved as a partial agonist eliciting a small increase in baseline diastolic blood pressure which could be inhibited by Ca(2+)-entry blockade with nifedipine. 5. Chloroethylclonidine potentiated the pressor responses elicited by the alpha 2-adrenoceptor agonists UK-14,304 and azepexole (B-HT 933). 6. No evidence was found in the pithed rat that alpha 1-adrenoceptor-mediated Ca(2+)-entry from the extracellular fluid and Ca(2+)-release from intracellular stores are mediated by alpha 1A and alpha 1B-adrenoceptors, respectively.

Adrenergic alpha-Antagonists↗

PubMed1992

In vitro pharmacology of R 80122, a novel phosphodiesterase inhibitor.

The cardiac in vitro effects of R 80122, a novel phosphodiesterase (PDE) inhibitor, were investigated and compared with those of the reference compound milrinone and of the calcium-sensitizer adibendan. In guinea pig left atria, both milrinone and R 80122 increased contractile force; 10 microM milrinone was equieffective to 1 microM R 80122. The rate of spontaneously beating atria was not altered by R 80122 in the concentration range of 0.01-0.3 microM. Higher concentrations (1-10 microM) led to a statistically insignificant increase of 20%. Milrinone's effect on frequency was more pronounced and amounted to 21% at 10 microM and to 40% at 100 microM. Adibendan increased heart rate (HR) by 10% at a concentration of only 0.03 microM. This effect was not enhanced any further by increasing the concentration. In papillary muscle, the positive inotropic effects of both milrinone and R 80122 were inhibited by carbachol, indicating involvement of cyclic AMP. Further indications for a cyclic AMP-dependent action were obtained by induction of slow action potentials and synergism with isoprenaline. In electrophysiologic measurements, milrinone reduced action potential duration (APD) in a high concentration whereas R 80122 had no effect. Action potential changes elicited by a toxic concentration of ouabain were reduced by R 80122. Relaxation of rat aortic rings contracted by KCl and relaxation of guinea pig aortic rings contracted by norepinephrine (NE) was comparable for both milrinone and R 80122. R 80122 also caused relaxation of canine coronary arteries constricted with prostaglandin F2 alpha (PGF2 alpha) both with and without endothelium. NE-induced contractions in canine gastrosplenic arteries were not affected by R 80122. Cardiac contractility that had been impaired to various degrees by pentobarbital or by aging was restored to control values by both milrinone and R 80122. R 80122 enhanced cardiac contractility at lower concentrations than milrinone with no concomitant increase in frequency or shortening of the action potential, which may be advantageous for treatment of heart failure.

Action Potentials↗

PubMed1991

45Ca2+ movements induced by Ca2+ chloride in isolated rat aorta under K(+)-free conditions.

Increasing the extracellular Ca2+ concentration induced a dihydropyridine-insensitive contraction in the isolated rat aorta bathed in K(+)-free solution. To obtain further insight into the mechanism of this contraction 45Ca2+ uptake measurements were carried out with isolated rat aorta. Increasing the extracellular Ca2+ concentration from 0.63 to 5.0 mM enhanced the 45Ca2+ uptake from 186 +/- 3 to 359 +/- 12 dpm/mg ww (n = 8). Under K(+)-free conditions increasing the extracellular Ca2+ concentration from 0.63 to 5.0 mM enhanced the 45Ca2+ uptake from 184 +/- 4 to 541 +/- 6 dpm/mg ww (n = 12) and elicited an increase in tension of 10.0 +/- 0.4 mN (n = 9). The Ca2(+)-induced 45Ca2+ uptake was not affected by the Ca2% channel antagonists, nifedipine (L-type), flunarizine (L- and T-type) and omega-conotoxin (N-type). Amiloride, a blocker of the Na+/Ca2+ exchange, indomethacin, a blocker of prostaglandin synthesis, the local anaesthetic, lidocaine, and the Ca2+ overload blocker. R 56865, had no effect on Ca2(+)-induced 45Ca2+ uptake. Additionally, R 56865 did not affect the Ca2(+)-induced contractile response. The inorganic Ca2+ entry blocker, lanthanum, reduced both the non-stimulated and stimulated 45Ca2+ uptake and inhibited the Ca2(+)-induced contractile response. We would like to suggest that the 45Ca2+ uptake induced by Ca2+ under K(+)-free conditions enters the cell via the leakage Ca2+ channel, because lanthanum was an effective antagonist and a significant contribution of any other Ca2+ pathway could not be demonstrated.

Amiloride↗

PubMed1991

Differences between full and partial alpha-adrenoceptor agonists in eliciting phasic and tonic types of responses in the longitudinal smooth muscle of the rat portal vein.

The aim of the present investigation was to study, taking into account both quantitative and qualitative differences, the influence of full and partial alpha-adrenoceptor agonists on spontaneous myogenic activity in the rat portal vein. We found that the alpha-adrenoceptor agonists cirazoline, adrenaline, noradrenaline, phenylephrine, St 587, Sgd 101/75, B-HT 920 and UK-14,304 could increase the amplitude of the phasic myogenic contractions in the rat portal vein with apparent differences in EC50 and Emax values. In addition to an increase in phasic myogenic activity, the alpha-adrenoceptor agonists cirazoline, adrenaline, noradrenaline, and phenylephrine were also able (in higher concentrations) to increase the basal tone of the rat portal vein preparation, again with apparent differences in EC50 and Emax values. Changing the extracellular Ca2+ concentration from 0.9 mmol/l to 2.5 mmol/l had no influence on the phasic character and the concentration range in which St 587 and UK-14,304 increased spontaneous myogenic activity, although changes in amplitude and frequency of the spontaneous myogenic contractions were less pronounced at a higher extracellular Ca2+ concentration (2.5 mmol/l). By the use of Schild analysis with the competitive alpha-adrenoceptor antagonists prazosin (pA2 = 8.74) and 5-methyl-urapidil (pA2 = 8.37), it was established that the contractile responses to St 587 were mediated by the same alpha 1-adrenoceptor subtype as the phasic and tonic type of contraction elicited by phenylephrine as described in a previous study. The concentration-response curve of UK-14,304 was significantly shifted to the right by low concentrations of prazosin (3 nmol/l-30 nmol/l), indicating stimulation of alpha 1-adrenoceptors by UK-14,304 in the rat portal vein.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenergic alpha-Agonists↗

PubMed1991

Veratridine-induced intoxication in the isolated left atrium of the rat: effects of some anti-ischemic compounds.

Veratridine-induced Na+ and Ca2+ uptake was used as a simulation of ischemia-induced Na+ and Ca2+ uptake. Therefore, electrically driven (1 Hz) isolated left atria of the rat were intoxicated with veratridine and the 45Ca2+ uptake was determined. Veratridine (10(-4) mol/l) increased the 45Ca2+ uptake from 575 +/- 13 to 2320 +/- 86 dpm/mg ww (n = 20). The total tissue content of 45Ca was elevated from 4328 +/- 132 to 5136 +/- 303 dpm/mg ww (n = 13). The veratridine-induced 45Ca2+ uptake was completely suppressed by tetrodotoxin (10(-7) and 10(-6) mol/l), whereas amiloride (6.10(-6) mol/l) and phentolamine (10(-6) and 10(-5) mol/l) exhibited no effect on the veratridine-induced 45Ca2+ uptake. Nifedipine (10(-7) and 10(-6) mol/l) was ineffective on veratridine-induced 45Ca2+ uptake. Verapamil (10(-5) mol/l) suppressed the veratridine-induced 45Ca2+ uptake, but the 45Ca2+ uptake in the absence of veratridine was also suppressed by verapamil (10(-6) and 10(-5) mol/l). The novel anti-ischemic compounds R 56865 (10(-8)-10(-5) mol/l) and R 59494 (10(-8)-10(-5) mol/l) totally abolished veratridine-induced 45Ca2+ uptake. It is speculated that Ca2+ enters the cell via a Na+ channel which changes its selectivity upon veratridine treatment. Consequently, R 56865 and R 59494 could display their protective effect by either inhibiting the modified Na+ channel or preventing the transition of the normal Na+ channel to its altered state.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1991

The role of alpha 1-adrenoceptor subtypes in the phasic and tonic responses to phenylephrine in the longitudinal smooth muscle of the rat portal vein.

The purpose of this investigation was to determine whether alpha 1-adrenoceptor subtypes (co)exist in the rat portal vein and, if so, whether they could be functionally associated with the phasic and tonic types of contraction as a response to alpha 1-adrenoceptor stimulation by phenylephrine. A low Ca2+ concentration (0.9 mmol/l) in the Tyrode solution enabled us to quantify changes both in the phasic myogenic activity and in the basal tone of the rat portal vein preparation very precisely. We used both competitive and non-competitive alpha-adrenoceptor antagonists which have been employed successfully by other investigators to discriminate between alpha 1-adrenoceptor subtypes in vascular and other tissues. Schild analysis showed that the competitive alpha-adrenoceptor antagonists prazosin, phentolamine, yohimbine, corynanthine, idazoxan, rauwolscine and 5-methyl-urapidil could not distinguish between the phasic and tonic responses to phenylephrine and/or different alpha 1-adrenoceptor subtypes in the rat portal vein. However, when we compared our pA2 values with those found to be representative indicators according to subclassifications based on the use of selective antagonists in different tissues, the alpha 1-adrenoceptors in the rat portal vein appeared to belong to the alpha 1L- or alpha 1a-subtype. This subclassification was not in accordance with the data obtained with the irreversible alpha-adrenoceptor antagonist chloroethylclonidine. However, the validity of this alkylating agent as a tool for receptor classification was restricted, at least in the rat portal vein, by its effects on receptor reserve.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenergic alpha-Antagonists↗

PubMed1991

Differential effect of R 56865 on ouabain binding to isolated sarcolemma and intact atrial tissue of guinea-pig.

1. R 56865 (N-[1-[4-(4-fluorophenoxy)-butyl]-4-piperidinyl]-N-methyl- 2-benzothiazolamine) is a compound known to antagonize cardiac glycoside intoxication. Therefore, the effect of the compound on ouabain binding to intact cardiac tissue as well as cardiac membrane preparations was investigated. 2. The binding of ouabain to highly purified sarcolemmal membranes was not influenced by R 56865 1 x 10(-6) mol l-1 (ouabain: KD = 1.3 x 10(-7) mol l-1, Bmax = 160 pmol mg-1; ouabain + R 56865: KD = 1.4 x 10(-7) mol l-1, Bmax = 168 pmol mg-1). 3. In contrast to the results in purified membranes, the binding of ouabain (10(-8) mol l-1 to 5 x 10(-7) mol l-1) to intact atria was significantly reduced. 4. Ouabain, 5 x 10(-7) mol l-1, led to a transient positive inotropic effect of about 220% followed by a developing negative inotropic effect after 3 h. R 56865, 10(-7) mol l-1, led to a maximal positive inotropic effect of about 290% also followed by a delayed decline of contractile force. A tenfold higher concentration of R 56865 led to sustained positive inotropic effect of about 250% in the same time interval. 5. The different effects of R 56865 on ouabain binding in subcellular preparations and intact tissue do not support the view that R 56865 interferes directly with the action of ouabain on Na/K-ATPase. An indirect effect, which may be mediated by a lowered intracellular sodium load is discussed.

Animals↗

PubMed1991

Calcium channels in the brain as targets for the calcium-channel modulators used in the treatment of neurological disorders.

Recent investigations of calcium channels in brain cells by voltage-clamp techniques have revealed that, in spite of electrophysiological similarities, the pharmacological properties of these channels differ considerably from channels in peripheral tissues, e.g., heart and smooth muscle. Therefore, instead of extrapolation from results obtained on cardiac or smooth muscle cells, a reclassification of calcium-channel modulators applied in disorders of the brain appears necessary. The present article reviews some pertinent observations from groups involved in neuronal calcium-channel characterization and draws the attention to major pharmacological differences on neurons in comparison to those in the heart and in smooth muscle. For certain therapeutic applications in neurology particularly, the low-voltage activated types of calcium channels deserve considerable attention.

Animals↗

PubMed1990

Effects of inorganic cations on K(+)-, 5-hydroxytryptamine- and noradrenaline-induced contractions of the isolated rat jugular vein and aorta.

We investigated the inhibitory effects of 1 mM of the inorganic cations, La3+, Cd2+, Mn2+, Ni2+ and Co2+ on contractions induced by K+ (100 mM) and 5-hydroxytryptamine (5-HT, 10(-5) M) in the isolated rat jugular vein and on contractions induced by K+ (100 mM), 5-HT (10(-5] and noradrenaline (NA, 10(-5) M) in the rat aorta. In the venous preparation, both K(+)- and 5-HT-induced Ca2+ influx could be suppressed completely by all cations studied. In the aorta, the K(+)-induced Ca2+ influx was antagonized completely only by Cd2+. The other cations were less effective and Mn2+ was completely ineffective. Similarly to that in the venous preparation, the 5-HT-induced Ca2+ influx in the aorta was abolished by all the cations. A similar picture was found for the NA-induced Ca2+ influx with the exception of Mn2+, which antagonized the NA-induced contractions only slightly. Although organic calcium entry blockers (CEBs) (nifedipine, diltiazem, flunarizine and gallopamil) inhibited NA-induced contractions to the same extent as did Mn2+, a combination of organic CEBs and Mn2+ abolished the NA-induced Ca2+ influx completely. Apparently, organic CEBs and Mn2+ block different components of the NA-induced Ca2+ influx pathway.

Animals↗

PubMed1990

An approach to differentiate between noradrenaline-elicited contractile processes in the rat isolated aorta.

The aim of the present study was to assess the different processes contributing to the contraction induced by noradrenaline (NA, 1 mumol/l) in the rat isolated aorta. Pretreatment with maximally effective concentrations of nifedipine or cromakalim reduced the NA-induced contraction to 80 +/- 3.5% or 63 +/- 2.0%, respectively, without alteration of the shape of the response. After pretreatment with Mn2+, NA caused a transient phasic contraction followed by a sustained tonic component, comparable to the response obtained in "Ca2(+)-free" medium. Ryanodine--in the presence of extracellular Ca2(+)-caused a slight increase in resting tension, but did not modify the NA-induced contraction. In "Ca2(+)-free" medium the contraction elicited by NA consisted of a transient phasic and a sustained tonic component. The amplitude of the phasic contraction decreased exponentially with the time of exposure to "Ca2(+)-free" medium. The phasic component was identified as elicited by Ca2+ released from the sarcoplasmic reticulum (SR) by means of ryanodine. If Ca2+ depleted tissues (80 min in "Ca2(+)-free" solution) were exposed to Ca2+ in the presence of Mn2+ or cromakalim, the NA-induced phasic response was inhibited, suggesting that Mn2+ and cromakalim blocked the refilling of the store. It can be concluded that activation of alpha 1-adrenoceptors in the rat aorta by NA elicits Ca2(+)-entry processes which have a different sensitivity to nifedipine, cromakalim and Mn2+. The Ca2+ released from SR contributes about 20% to the overall contractile response. Our data suggest that the depleted SR can be refilled from the extracellular space via a direct cromakalim- and Mn2(+)-sensitive pathway.

Animals↗

PubMed1990

Effects of the selective beta 1-adrenoceptor antagonist, nebivolol, on cardiovascular parameters in the pithed normotensive rat.

In the pithed rat we investigated the cardiovascular properties of d,l-nebivolol and its enantiomers. We used the increase in heart rate elicited by (-)-adrenaline and (-)-noradrenaline as a model for studying beta 1-adrenoceptors. A leftward shift of the logarithmic dose-pressor response curve of (-)-adrenaline reflects beta 2-adrenoceptor-blocking properties. The blood pressure responses of methoxamine, B-HT 920 and serotonin (5-HT) were studied in order to test whether d,l-nebivolol has alpha 1-, alpha 2- and 5-HT2-receptor-blocking properties. Furthermore, the interaction of d,l-nebivolol with the peripheral sympathetic neurotransmission was investigated in pithed rats by electrical stimulation of the spinal cord. d,l-Nebivolol and d-nebivolol (threshold concentration 10(-8) mol/kg) were demonstrated to be selective beta 1-adrenoceptor antagonists. l-Nebivolol was a factor of 1,000 less potent as beta 1-adrenoceptor blocker. Up to a dose of 10(-5) mol/kg, d,l-nebivolol appeared to have neither alpha 1-, alpha 2-, beta 2-, 5-HT2-, angiotensin II-receptor antagonistic, calcium entry blocking, converting enzyme inhibiting nor direct vasodilating properties and did not interact with the sympathetic neurotransmission in the vascular wall. An explanation for an antihypertensive effect independent of beta-adrenoceptor blockade as found in spontaneously hypertensive rats and man could not be found in this model, therefore we suggest that this blood-pressure-lowering effect does not originate from conventional peripheral mechanisms.

Adrenergic alpha-Agonists↗

PubMed1990

Ca2+ influx insensitive to organic Ca2+ entry blockers contributes to noradrenaline-induced contractions of the isolated guinea pig aorta.

We determined the contribution of intracellular Ca2+ to the noradrenaline (NA, 3 X 10(-5) mmol/l)-induced contraction of the isolated guinea pig aorta. Since only about 55% of the NA-induced contraction could be attributed to intracellular Ca2+ release, we assumed that a Ca2+ influx component contributes to the NA-induced contraction. This influx component proved resistant to the organic calcium entry blockers (CEBs) nifedipine, diltiazem, flunarizine and gallopamil which, in contrast, antagonized K(+)-induced Ca2+ influx completely. Conversely, the NA-induced Ca2+ influx component could be antagonized by the inorganic cations La3+, Cd2+, Mn2+, Ni2+ and Co2+. 45Ca2+ uptake experiments also revealed that both KCl and NA induce Ca2+ influx of which only the latter one is resistant to nifedipine. It was concluded that in the guinea pig aorta NA activates a receptor-operated channel through which Ca2+ can be translocated from the extracellular space to the cytosol to contribute directly to contraction.

Animals↗

PubMed1990

Uptake of catamphiphilic drugs into erythrocytes and muscular tissue correlates to membrane enrichment and to 45Ca displacement from phosphatidylserine monolayers.

The uptake of the catamphiphilic drugs flunarizine, R 56865 [N-1-4-(4-fluorophenoxy)butyl-4-piperidinyl-N-methyl-2- benzothiazolamine], verapamil, diltiazem, lidocaine and the noncatamphiphilic drug nitrendipine into erythrocytes and erythrocyte membranes was measured at pH 7.2 in a Ringer's solution at 22 degrees C. The uptake was concentration proportional between 10(-8) and 10(-6) mol/l for all drugs investigated; the erythrocyte/medium ratio (E/M) was constant after 20 min in all experiments. In rat aortas and left atria drug uptake was measured in a 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid buffered medium (pH 7.2, 32 degrees C). Equilibrium was reached after 360 min and the tissue/medium ratio (T/M) was concentration proportional between 10(-8) and 10(-6) mol/l for all drugs. E/M or T/M ratios, respectively, increased in the order: erythrocytes less than aortas less than left atria. However, the particular drug/phospholipid ratios were relatively constant for the drugs investigated, indicating that the membranes were a major distribution phase for these drugs. For the catamphiphilic drugs, the E/M or T/M ratios correlated linearly with the EC50 for 45Ca displacement from phosphatidylserine monolayers, but were poorly correlated with the octanol-water coefficient of the unchanged molecules. The EC50 for 45Ca displacement from phosphatidylserine monolayers is a measure for the uptake of the protonated species of catamphiphilic drugs into erythrocytes or tissue and may be a good estimate of tissue uptake of these drugs in general.

Binding Sites↗