PubMed Health⌕ Search

Biomedical subjects

C Y Kwan

Publications and source records attributed to C Y Kwan.

At least 145 records · Page 8Linked to original sources
PubMed1990

Evidence against the role of alpha 1-adrenoceptor reserve in buffering the inhibitory effect of nifedipine on the contractility of canine vascular smooth muscle.

The relationship between the postsynaptic alpha 1-adrenoceptor reserve and the sensitivity of vasoconstriction induced by alpha-adrenoceptor agonists to the dihydropyridine Ca2+ entry blocker nifedipine was investigated in isolated muscle strips of dog mesenteric artery (DMA) and saphenous vein (DSV). The amplitudes of the contractile responses of DMA induced by phenylephrine were the same as those in DSV in the presence and in the absence of extracellular Ca2+. The use of 3 x 10(-9) M phenoxybenzamine to irreversibly block the alpha 1-adrenoceptors revealed a marked difference in the size of the alpha 1-adrenoceptor reserve between DMA (40%) and DSV (7%). In spite of a larger receptor reserve, the contractile responses induced by phenylephrine in DMA were more sensitive to nifedipine compared with those in DSV. These results suggest that the postsynaptic alpha 1-adrenoceptor reserve in vascular smooth muscle, at least in DMA and DSV, does not play an important role in buffering the inhibitory effect of nifedipine on the contractile response to a full agonist of alpha 1-adrenoceptors. Other factors, such as the difference in the membrane depolarizing effect, the ability to utilize intracellular Ca2+ for contraction, and the possible existence of alpha 1-adrenoceptor subtypes, may contribute to the different inhibitory effects of nifedipine on these blood vessels.

Animals↗
PubMed1990

Changes of beta-adrenoceptors in the aortic muscle cells of spontaneously hypertensive rats.

The characteristics of [125I]monoiodocyanopindolol (ICYP) binding to beta-adrenoceptors of cultured aortic smooth muscle cells derived from 4-week-old spontaneously hypertensive rats (SHR) and the Wistar-Kyoto normotensive rats (WKY) were examined. During optimization of the binding assays, we found that the specific binding of ICYP by intact cells was masked by a high level of nonspecific ICYP accumulation in intact cells presumably owing to the lipophilic nature of ICYP. Optimal specific ICYP binding requires that the cells be gently lysed with hypotonic dilution followed by a freeze-and-thaw cycle. Under most experimental conditions tested, the total number of ICYP binding sites in WKY aortic muscle cells was considerably and consistently smaller than that in SHR cells. There was no difference in the Kd values for ICYP binding to SHR and WKY cells. However, when ICYP binding was carried out using crude membrane fractions with well-defined plasma membrane content isolated from aortic muscle strips of adult rats, we found no difference in the number of beta-adrenoceptor sites between SHR and WKY. Morphological evidence indicated that cultured SHR aortic muscle cells contained a greater proportion of larger cells with multinuclear features. These results suggest that an increase in the number of beta-adrenoceptor density per cell in SHR may be associated with cellular hypertrophy of aortic smooth muscle cells. We conclude that under cultured conditions, a higher incidence of polyploid smooth muscle cells in the SHR as compared with WKY was expressed earlier than under in vivo conditions. Therefore, the interpretation of results obtained from cultured cell studies in relation to under in vivo conditions should be exercised with caution.

Animals↗
PubMed1990

Effects of MeCh, thapsigargin, and La3+ on plasmalemmal and intracellular Ca2+ transport in lacrimal acinar cells.

The Ca2(+)-mobilizing actions of the muscarinic receptor agonist, methacholine (MeCh), and the microsomal Ca2+ pump inhibitor, thapsigargin, were investigated in lacrimal acinar cells. As previously shown for parotid cells (J. Biol. Chem. 264: 12266-12271, 1989), thapsigargin activates both internal Ca2+ release and Ca2+ entry from the extracellular space without increasing cellular inositol phosphates. The inorganic Ca2+ antagonist La3+ inhibited MeCh- or thapsigargin-activated Ca2+ entry. However, when added before MeCh or thapsigargin, La3+ inhibited the extrusion of Ca2+ at the plasma membrane. This phenomenon was exploited in protocols designed to investigate the pathways for filling agonist-sensitive Ca2+ stores in lacrimal cells. The results show that, in contrast to previous suggestions that external Ca2+ is required to replenish agonist-regulated Ca2+ stores, the inhibition of Ca2+ extrusion permits recycling of Ca2+ released by MeCh back into an MeCh- and thapsigargin-sensitive pool. Thus, although extracellular Ca2+ is the major source for refilling the intracellular Ca2+ stores under physiological conditions, the pathway by which this Ca2+ enters the pool need not be a direct one. These results are consistent with the recently revised capacitative model for the refilling of intracellular Ca2+ stores through Ca2+ influx subsequent to Ca2+ depletion, according to which refilling of intracellular Ca2+ stores occurs via a cytoplasmic route rather than a direct channel between intracellular Ca2+ stores and the extracellular space.

Animals↗
PubMed1990

Inhibition of vasoconstriction by tetramethylpyrazine: does it act by blocking the voltage-dependent Ca channel?

Using dog mesenteric arterial ring preparations, we studied the in vitro vascular effects of tetramethylpyrazine (TMP, also known as Ligustrazine), a vasoactive component derived from Ligustium Wollichii Franchat; a widely used ingredient in Chinese herbal medicine for the treatment of cardiovascular diseases. TMP had no effect on the resting force of the vascular muscle, but it caused a dose-dependent inhibition of vascular contractile responses to KCl and phenylephrine (PHE). These inhibitory effects of TMP were reversible and more prominent at lower agonist concentrations. TMP also inhibited the responses to PHE in Ca-free medium containing 50 microM EGTA suggesting the action on the release of intracellular Ca2+. Therefore, TMP is not a specific Ca-channel blocker as claimed previously. Its vasodilatory effect is mediated by the inhibition of Ca influx, as well as the release of intracellular Ca2+.

Animals↗
PubMed1990

Inhibitory effects of amiloride on alpha adrenoceptors in canine vascular smooth muscle.

Amiloride inhibits vascular smooth muscle contractions from canine aorta and saphenous vein. The mechanisms were studied using radioligand binding and functional techniques. Amiloride inhibited [3H]prazosin and [3H]rauwolscine binding to alpha-1 and alpha-2 adrenoceptors in a concentration-dependent manner. Amiloride increased Kd values for [3H]rauwolscine without affecting the maximum binding of [3H]prazosin. These results suggest that the drug interacts with the alpha-1 adrenoceptor binding sites in a competitive manner and with the alpha-2 adrenoceptor binding sites in a noncompetitive manner. Amiloride reduced maximal contractile responses to agonists selective for both alpha adrenoceptors and to elevated K+, the EC50 values were increased by about 10-fold in the presence of amiloride. In Ca+(+)-free Krebs' solution, contractions induced in saphenous vein after addition of Ca++ in saphenous vein in the presence of adrenoceptor agonists were inhibited by amiloride. Our results suggest that amiloride reduced alpha-1 and alpha-2 adrenoceptor-mediated responses and inhibited Ca++ influx.

Adrenergic alpha-Antagonists↗
PubMed1989

Presynaptic and postjunctional muscarinic receptors in dog ileum: binding studies.

[3H]N-Methylscopolamine identified two distinct populations of muscarinic receptors in membranes derived from the longitudinal smooth muscle/myenteric plexus of dog ileum. In isolated axonal varicosities, the half-maximal saturation of binding sites occurred at 2.38 +/- 0.39 nM [3H]N-methylscopolamine, with maximal binding capacity 140 +/- 35 fmol/mg protein (mean +/- S.D., n = 8). In purified smooth muscle plasma membranes, the Kd value was 16 +/- 3 nM with Bmax 1960 +/- 494 fmol/mg. The displacement potencies of subtype-selective muscarinic antagonists in the fraction of axonal varicosities followed the order 4-diphenylacetoxy-N-methylpiperidine (4-DAMP) methiodide much greater than pirenzepine = methoctramine greater than AF-DX 116 with pKi values 7.38, 5.67, 5.70 and 5.13, respectively. Both 4-DAMP methiodide and pirenzepine were approximately 4-fold less potent in displacing the ligand from the receptors in smooth muscle plasma membranes as compared to varicose receptors. The potency ratios of cardioselective antagonists methoctramine and AF-DX 116 on varicose and smooth muscle receptors were 1 and 1.7. It is concluded that presynaptic receptors located on isolated axonal varicosities have pharmacological properties similar to glandular (M3) subtype of muscarinic receptors. The binding properties of receptors present in smooth muscle plasma membranes were found incompatible with those of any of the M1, M2 or M3 subtypes.

Animals↗
PubMed1989

Abnormal contractile response of aortas from spontaneously hypertensive rats to Ca2+ after depletion of Ca2+ in Ca2+-free medium.

Vascular responses of aortic rings from spontaneously hypertensive rats (SHR) were compared to those of the normotensive Wistar-Kyoto rats (WKY) in three sets of experimental protocols. The responses to cumulative doses of KCl indicated that SHR aortic rings were hyperresponsive to low but not high doses of KCl compared to WKY aortic rings. After Ca depletion by prolonged incubation of the rat aortic rings with Ca2+-free, EGTA containing solution, Ca repletion resulted in contraction. The magnitude of such a contraction was dependent on the period of Ca depletion and was highly sensitive to dihydropyridine Ca channel blocker, nifedipine. Although the Ca-depleted aortic rings eventually developed to the same level of maximum tension development upon Ca repletion, it took a considerably shorter period of Ca depletion for SHR than for WKY aortic rings to reach the maximum contraction upon Ca repletion. Our findings support the view that cell membranes of vascular smooth muscle in hypertension are more excitable and more susceptible to membrane destabilization by Ca removal.

Animals↗
PubMed1989

Characterization of alpha-adrenoceptor subtypes by [3H]prazosin and [3H]rauwolscine binding to canine venous smooth muscle membranes.

Postsynaptic alpha-adrenoceptor subtypes were studied using [3H]prazosin and [3H]rauwolscine binding to plasmalemma-enriched microsomal fractions isolated from dog saphenous veins and mesenteric veins. Both radioligands showed saturable binding consistent with the presence of a single homogeneous binding site in each case, based on Scatchard analysis. The Kd values of [3H]prazosin and [3H]rauwolscine, calculated from kinetic studies were similar to those from equilibrium binding data in both venous muscle membranes. The microsomal membranes of dog saphenous vein and mesenteric vein contained about a fourfold higher density of the high affinity [3H]rauwolscine binding sites than those for [3H]prazosin binding. In competition studies, IC50 values for displacement of rauwolscine or prazosin suggested that the sites of interaction for the antagonists prazosin and rauwolscine were independent. Phenylephrine, a functionally selective alpha-adrenoceptor agonist, competed with a similar IC50 value for the specific binding sites of [3H]prazosin and [3H]rauwolscine; but B-HT 920, a functionally selective alpha 2-adrenoceptor agonist, competed for [3H]rauwolscine and [3H]prazosin binding with distinctly different IC50 values. Our data show the existence of two populations of alpha-adrenoceptor antagonist binding sites in the plasma membranes of dog saphenous vein and mesenteric vein, and raise the question whether agonist selectively depends on different affinities or on differential efficacies at one or two sites.

Animals↗
PubMed1989

Does in vitro activation of postsynaptic alpha 2-adrenoceptor utilize intracellular Ca2+ for contraction in dog saphenous vein?

Dog saphenous vein spiral strips were employed to determine whether an intracellular source of Ca2+ is used for contraction upon activation of the alpha 2-adrenoceptor by B-HT 920 in Ca2+-free Krebs solution containing 50 microM EGTA. The studies were carried out in parallel with the activation of the alpha 1-adrenoceptor by phenylephrine (Phe) under the condition that B-HT 920 (10(-5) M) and Phe (2 x 10(-6) M) gave rise to a similar level of responses in Ca2+-containing Krebs solution. A similar level of responses to these agonists at equieffective concentrations in Ca2+-free medium were also observed. Such responses to Phe and B-HT 920 were inhibited by 10(-7) M rauwolscine and 10(-7) M prazosin, respectively, and were not affected by 10(-7) M nifedipine or 5 mM Mn2+. The responses to B-HT 920 (10(-5) M) and submaximal concentration of Phe (2 x 10(-6) M) in Ca2+-free medium were additive. However, if the vascular strips were first contracted maximally with 10(-4) M Phe in Ca2+-free medium to deplete the intracellular Ca store, subsequent addition of B-HT 920 failed to induce additional response. Our results strongly suggest that activation of alpha 2-adrenoceptor in dog saphenous vein in Ca2+-free medium indeed utilizes intracellular Ca2+ for contraction. We also found that the failure of earlier studies to demonstrate the contractile effects of B-HT 920 in dog saphenous vein was due to experimental artifacts derived from the use of high concentration of EGTA and artificial pH-buffering reagent.

Animals↗
PubMed1989

Distribution of opioid receptors in canine small intestine: implications for function.

Distribution of the binding sites for [3H]diprenorphine, a non-selective opiate ligand, was studied in membrane fractions from longitudinal muscle/myenteric plexus and circular muscle containing deep muscular plexus. [3H]saxitoxin was used as a marker for neuronal plasma membranes and 5'-nucleotidase as a marker for smooth muscle plasma membranes. Saxitoxin binding correlated strongly with diprenorphine binding, but 5'-nucleotidase correlated poorly with diprenorphine or saxitoxin binding in these fractions. Opiate binding sites in membranes of myenteric and deep muscular plexus were of high affinity (Kd = 0.12 and 0.18 nM, respectively) with maximum binding capacity of 400 and 500 fmol/mg protein, respectively. Competition experiments using subtype-selective opiate ligands indicated that all three subtypes of opiate receptors were present in the same ratio of 40-45% mu-subtypes, 40-45% delta-subtypes, and 10-15% kappa-subtypes on both plexuses. Opiate receptors of canine small intestine, therefore, are located primarily or exclusively on nerves with similar distributions in nerve membranes containing only axonal varicosities (deep muscular plexus) as in those containing neurons, dendrites, and varicosities (myenteric plexus).

Animals↗
PubMed1989

Epinephrine facilitates neurogenic responses in isolated segments of dog mesenteric arteries.

Epinephrine (EPI) bitartrate (10(-9)-10(-8) M) significantly enhanced tension development in response to electrical field stimulation in isolated segments of dog mesenteric arteries. Responses to exogenous norepinephrine (NE) were generally unaffected, indicating that the EPI-induced increases in response to field stimulation are not explicable in terms of changes in postjunctional sensitivity. The facilitatory effects of EPI (5 X 10(-9) M) were unaffected by atenolol (10(-6) M) but were completely abolished by timolol (2 X 10(-7) M), suggesting an involvement of beta 2-adrenoreceptors in mediating the EPI-induced facilitation of neurogenic responses. Responses to exogenous NE were usually unaffected by either beta-adrenoreceptor antagonist. The results suggest that release of endogenous sympathetic neurotransmitter, measured in terms of postjunctional effects on vascular smooth muscle tone, appears to be modulated by prejunctional facilitatory beta 2-adrenoreceptors and that these receptors may be a physiologic site of action of EPI.

Animals↗
PubMed1989

Relation between density (maximum binding) of alpha adrenoceptor binding sites and contractile response in four canine vascular tissues.

In microsomal fractions from dog aorta, saphenous veins, mesenteric arteries and veins, both [3H]prazosin and [3H]rauwolscine displayed monophasic saturation in binding. The Kd for [3H] rauwolscine binding was similar for all these blood vessels, but the maximum number of [3H]rauwolscine binding sites was 3 to 7 times higher in veins compared to arteries. The Kd for [3H] prazosin was higher in saphenous vein than that in the arteries. The maximum number of binding sites for [3H]prazosin was similar, except for that in aorta, which was 3 times greater. Phenylephrine (alpha-1 adrenoceptor selective agonist) or norepinephrine (nonselective adrenoceptor agonist) produced similar maximal responses in all vessels. The alpha-2 adrenoceptor selective agonist, B-HT 920 (2-amino-6-allyl-3,4,7,8-tetrahydro-6H-thiazolo[5,4-d]-azepine)-induced contraction in veins but not in arteries. Prazosin (10(-6) M) inhibited completely the contractions to norepinephrine (3 x 10(-6) M) in mesenteric arteries and to phenylephrine (3 x 10(-6) M) in arteries and veins. Contractile responses of mesenteric artery were unaffected by rauwolscine. Rauwolscine (10(-7) M) caused a greater parallel rightward shift of the concentration-response curve to norepinephrine than did prazosin (10(-7) M) in saphenous veins, and a further rightward shift of responses to norepinephrine after 10(-7) M prazosin in mesenteric vein and saphenous vein and abolished B-HT 920-induced responses at alpha-2 adrenoceptors. The tissues responding to B-HT-920 correspond to those having the highest alpha-2 receptor density as measured by [3H]rauwolscine binding. The density of such sites required for contraction to be initiated in veins was much higher than with alpha-1 adrenoceptor sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1989

Identification and characterization of presynaptic and postsynaptic beta adrenoreceptors in the longitudinal smooth muscle/myenteric plexus of dog ileum.

The subcellular distribution of [125I]cyanopindolol (125I-CYP) binding in membrane fractions derived from the longitudinal smooth muscle/myenteric plexus (LSM/MP) of dog ileum followed a bimodal pattern with selective enrichments in the purified synaptosomal (S2) fraction and the purified smooth muscle plasma membrane (M2) fraction. The half-maximal saturation of binding sites in both fractions occurred at 10 pM [125I]CYP with maximal binding capacities, in femtomoles per milligram of protein, 70 +/- 10 (S2 fraction) and 110 +/- 25 (M2 fraction). Approximately 80% of binding sites in either membrane fraction exhibited a high affinity for beta-2 selective antagonist ICI 118,551 (Ki = 1.4 +/- 0.4 nM in S2 and 1.8 +/- 0.6 nM in M2; mean +/- S.D. n = 3), whereas remaining 20% of binding sites had a low affinity for this agent (Ki = 2.6 +/- 1.5 microM in S2 and 0.55 +/- 0.2 microM in M2). The beta-1-selective antagonist ICI 89,406 had a high affinity for approximately 20% of binding sites (Ki = 12 +/- 7 nM in S2 and 4 +/- 2 nM in M2) and a low affinity for approximately 80% of binding sites (Ki = 0.34 +/- 0.16 microM in S2 and 0.42 +/- 0.24 microM in M2). The displacement potencies of beta-adrenergic agonists followed the order (-)-isoproterenol greater than salbutamol (-)-epinephrine greater than (-)-norepinephrine in each membrane preparation. The inclusion of 0.1 mM 5'-guanylylimidodiphosphate into assay medium resulted in a decreased affinity of receptors for agonists and increased values of Hill coefficients.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenylyl Cyclases↗
PubMed1989

Modulation of pyloric motor activity via adrenergic receptors.

The influences of adrenergic nerves on pyloric motor function and the locus and types of receptors involved were examined. Using glyoxylic acid fluorescence a dense adrenergic innervation of the inner pyloric muscle ring was demonstrated. Pyloric motor activity was monitored while close i.a. injections of a selective alpha-1 adrenoceptor agonist, phenylephrine, or a selective alpha-2 adrenoceptor agonist, B-HT-920, were given. Neither agonist affected basal pyloric motor activity, but both inhibited pyloric activity when it was stimulated by duodenal field stimulation or by intraduodenal acid infusion. The actions of each of the inhibitory agonists, phenylephrine or B-HT-920, were blocked selectively by prazosin or rauwolscine, respectively. Injection of isoproterenol usually had no effect or excited basal pyloric activity. This excitation could be antagonized selectively by propranolol or by atropine. Injection of isoproterenol after neural blockade by tetrodotoxin inhibited pyloric motor activity. Receptor binding studies carried out with subcellular nerve or muscle enriched membrane fractions of canine pyloric muscle with [3H]prazosin, [3H]rauwolscine and [125I]cyanopindolol revealed a dual location of alpha-1, alpha-2 and beta receptors on both nerve membranes and smooth muscle membranes. These results suggest that adrenergic effects on the pyloric muscle can be exerted by pre- and postsynaptic beta receptors which, respectively, excite by releasing acetylcholine and inhibit by acting on receptors on the pyloric muscle. Also inhibitory alpha-1 and alpha-2 receptors are present on cholinergic nerves. The functions of postsynaptic alpha-1 and alpha-2 adrenoceptor binding sites on smooth muscle are so far unknown.

Animals↗
PubMed1988

Alterations of elastin and elastase-like activities in aortae of diabetic rats.

The elastin content of the aortic muscle and the elastase-like activity of the extracts of aortic muscle were studied in spontaneously diabetic BB rats and in rats made diabetic by a single bolus i.v. injection of alloxan. In both modes of diabetes, the total alkaline-insoluble aortic elastin content was significantly reduced in diabetic rats compared to that in the corresponding control rats. This reduction in aortic elastin was also accompanied by a consistent increase in the elastase-like activities of the aortic extracts prepared from the same tissues. Such a reciprocal relationship between aortic elastin content and elastase-like activity has previously been reported in rats with malignant hypertension. Since the rats used in this study were not hypertensive, the altered elastin metabolism observed in this work is likely to be a manifestation of diabetic disease and may in part account for the vascular changes associated with diabetes mellitus.

Animals↗
PubMed1988

The plasma-membrane component is the primary site of action of alloxan on ATP-driven Ca2+ transport in vascular-muscle microsomal fractions.

The direct effects of alloxan in vitro on the handling of Ca2+ by microsomal fractions from dog aortic smooth muscle were investigated. Preincubation of the vascular-muscle microsomal membranes with alloxan showed a suppression of both binding of Ca2+ (in the absence of ATP) and ATP-driven Ca2+ transport. Alloxan substantially inhibited the microsomal ATP-driven Ca2+ transport stimulated by Pi, but not that stimulated by oxalate. Studies using subfractions isolated from the microsomal membranes on a sucrose density gradient indicated that plasma membrane is the primary site of action of alloxan on the ATP-driven Ca2+ transport.

Adenosine Triphosphate↗