Contractile actions of racemic and d-propranolol on isolated canine mesenteric and coronary arteries

This investigation was undertaken to determine whether propranolol exerts a direct vasoconstriction action which contributes to the increased systemic and regional vascular resistance observed after its acute administration. Helically cut strips of canine mesenteric and coronary arteries were exposed to cumulative concentrations of racemic propranolol, d-propranolol, metoprolol and sotalol. Racemic and d-propranolol were equipotent in eliciting concentration-related increments in tension in the mesenteric (3 x 10(-6)-3 x 10(-5) M) and coronary (3 x 10(-7)-3 x 10(-5) M) arterial strips. Metoprolol and sotalol did not cause contractions in concentrations up to 10(-4) M. Phenoxybenzamine, 10-(-6) M, did not alter the contractile responses elicited by racemic and d-propranolol. Upon exposure of mesenteric strips to calcium-free media or 10(-6) M verapamil, the contractile responses to propranolol (3 x 10(-5) M) and KCl (30 mM) were markedly reduced (not significantly different), whereas norepinephrine (10(-6) M)-induced responses were inhibited to a significantly lesser degree. In coronary arteries exposed to calcium-free media or 10(-6) M verapamil, the responses to propranolol, KCl and methoxamine (10(-5) M) were all extensively decreased (not significantly different). These results indicate that propranolol exerts a direct contractile effect on canine mesenteric and coronary arteries, which is unrelated to its beta adrenergic blocking activity and is not mediated through action on alpha adrenergic receptors. The propranolol-induced contraction appears to be associated predominantly with an influx of calcium ion.     

Nicotine-induced relaxation in isolated canine cerebral arteries.

Nicotine in concentrations ranging from 5 times 10- minus 6 to 10- minus 4 M caused a transient relaxation in strips of isolated canine basilar, middle cerebral and posterior cerebral arteries contracted with prostaglandin F2a (PGF). Transmural neural stimulation did not produce changes in the tension of these arteries. Tyramine contracted the arterial strips. Relaxation induced by nicotine was markedly attenuated by hexamethonium (10- minus 5 M), cocaine (10- minus 5M), bretylium (2 times 10- minus 5 M), sotalol (5 times 10- minus 5 M), propranolol (10- minus 6 M) and removal of Ca-++ from the bathing media. Relaxation was not significantly influenced by atropine, physostigmine, ouabain, tetrodotoxin, aminophylline and sotalol (10- minus 5 M). In cerebral arterial strips, the addition of K-+ also elicited a transient relaxation which was abolished by treatment with ouabain but not with other blocking agents. In mesenteric arterial strips, nicotine caused a transient contraction as did transmural stimulation. The nicotine-induced contraction was markedly attenuated or completely abolished by hexamethonium, cocaine, bretylium and phentolamine and also high concentration of sotalol (5 times 10- minus 5 M), and propranolol. Tetrodotoxin did not affect the contractile response to nicotine but did abolish the response to transmural stimulation. It is strongly suggested that relaxation induced by nicotine in cerebral arterial strips is the result of a specific action on nicotinic receptors as is the contraction observed in mesenteric arteries. It appears that beta adrenergic and cholinergic mechanisms do not play major roles in the genesis of nicotine-induced relaxation nor does it appear that an electrogenic Na-+ pump mechanism is involved.     

Reduced function of the stimulatory GTP-binding protein in beta adrenoceptor-adenylate cyclase system of femoral arteries isolated from spontaneously hypertensive rats

Arterial relaxant responses to beta adrenoceptor agonists are decreased in spontaneously hypertensive rats (SHR) when compared with normotensive Wistar-Kyoto rats (WKY). To determine which component of the beta adrenoceptor-adenylate cyclase (AC) system is involved in the decreased beta adrenoceptor responses, effects of two activators of AC-forskolin and cholera toxin and of dibutyryl cyclic AMP (DBcAMP) were compared between the strips of femoral arteries isolated from 13-week-old SHR and age-matched WKY. Arterial relaxant responses to either forskolin, an activator of AC or DBcAMP were not significantly different between the SHR and WKY, whereas the relaxant responses to norepinephrine (NE) via beta adrenoceptors were significantly weaker in the SHR than in the WKY. In the absence of timolol, a beta adrenoceptor antagonist, contractile responses to NE were significantly greater in the SHR than in the WKY. Timolol augmented the contractile responses to NE to a greater extent in the WKY than in the SHR. After the blockade by timolol of beta adrenoceptors, contractile responses to alpha adrenoceptor stimulation with NE were not significantly different between the two strains. The pretreatment of the strips with cholera toxin, an activator of stimulatory GTP-binding protein (Gs), antagonized the alpha adrenoceptor-mediated contractions much greater in the WKY than in the SHR. The alpha adrenoceptor-mediated contractions after the pretreatment with cholera toxin were comparable to the contractile responses to NE determined in the absence of timolol in either the SHR or the WKY. Forskolin and DBcAMP also antagonized the alpha adrenoceptor-mediated contractions. However, these antagonisms were not significantly different between the two strains. The cellular cAMP content in arterial strips after the stimulation with NE was significantly less in the SHR than in the WKY, whereas the cAMP contents were similar in arterial strips from both strains which were stimulated with forskolin. These results suggest that the reduced function of Gs is involved in the abnormality of beta adrenoceptor-AC system in the SHR femoral artery.     

The influence of mecamylamine on contractions induced by different agonists and on the role of calcium ions in the isolated rabbit aorta.

The exposure of helically cut strips of rabbit aorta to mecamylamine (1 mM) for 5 minutes blocked the histamine (3.25 muM)-induced contractions completely and reduced those induced by potassium (25mM, 68%) without affecting contractions induced by norepinephrine (3.0 muM, NE) or acetylcholine (10 muM). Mecamylamine, by itself, did not exhibit agonist activity, but it increased the contractile tension of the muscle which was contracted by NE. The responses to NE were either enhanced or not affected by small doses of mecamylamine (1 mM), but were partially blocked by large doses of mecamylamine (10 mM). The antagonism exhibited by mecamylamine in low doses (1 mM) against the above agonists was reversible. The shape of the response to NE on the muscle, which was treated with a high dose of mecamylamine (20 mM) and washed, was significantly different from that of the control. Our results are consistent with the following conclusions: 1) Mecamylamine in low doses (1 mM) blocks the histamine receptor and therefore the histamine-induced contractions. Two molecules of mecamylamine are competitive with 1 molecule of histamine. 2) Mecamylamine blocks the potassium-induced contractions, but does not block completely the calcium influx in the potassium-depolarized muscles. Therefore, it may interfere partially with the utilization of extracellular or loosely bound calcium during potassium-induced contractions. 3) Mecamylamine in high doses (10 mM) partially blocks norepinephrine-induced contractions by affecting firmly bound calcium stores. 4) Procaine antagonizes the effects of NE, histamine and acetylcholine by affecting, at least partially, the firmly bound calcium stores.     

Guanine nucleotide binding proteins may modulate gating of calcium channels in vascular smooth muscle. II. Studies with guanosine 5'-(gamma)triphosphate

Our previous studies with fluoride have indicated that G-proteins may mediate the gating of Ca++ channels in vascular smooth muscle (VSM). We now present further studies on the relationship between G-proteins and Ca++ channels in VSM using guanosine-5'-(gamma-thio)triphosphate (GTP gamma S), a hydrolysis-resistant analog of GTP. Rat tail artery helical strips pretreated with GTP gamma S in a cytosol-like solution contracted in a Ca++-dependent manner in the absence of a depolarizing concentration of K+, hormones or any other Ca++ agonists. Contraction was dependent on the concentrations of applied GTP gamma S. The ability of strips pretreated with GTP gamma S to contract in response to Ca++ was not reversed by repeated washing. Incubation with 1 mM GTP applied extracellularly did not induce tension development. Treatment with a subthreshold concentration of GTP gamma S shifted the K+ concentration-related tension curve to the left but did not alter the maximum response. The contractions induced by GTP gamma S pretreatment and by submaximal (60 mM) KCI were additive at all levels of Ca++ tested. Extra tension development could be evoked from tissue maximally contracted with GTP gamma S by adding maximal K+ and norepinephrine. The relaxing sensitivity of the GTP gamma S-related contraction to reversal by nifedipine was between those for K+ depolarization and norepinephrine, and the GTP gamma S-induced rise in tension was partially inhibited by the Ca++ channel blocker nifedipine. Ca++-elicited contraction of the GTP gamma S-pretreated strips was relaxed by forskolin, an adenylate cyclase activator, 3-isobutyl-l-methyl-xanthanine, a cyclic nucleotide phosphodiesterase inhibitor, and dibutyryl cyclic AMP.(ABSTRACT TRUNCATED AT 250 WORDS)     

Phorbol diesters alter the contractile responses of porcine coronary artery

We have studied the effects of activators of the Ca++- and phospholipid-dependent enzyme protein kinase C on isometric tension development by both intact and skinned coronary artery strips. The intact strips contracted upon incubation with 12-O-tetradecanoylphorbol-13-acetate. 12-O-tetradecanoylphorbol-13-acetate produced a leftward shift in the concentration-response relationship for contraction of the tissues by K+, histamine and norepinephrine. Phorbol-12,13-dibutyrate elicited contraction of detergent-skinned artery strips when the free Ca++ concentration in the bathing media was 0.1 microM or greater. This effect was diminished greatly in the presence of polymyxin B, a putative inhibitor of protein kinase C. Phorbol-12,13-dibutyrate shifted the Ca++ concentration-tension response relationship for the skinned tissue to the left. These results are consistent with a role for protein kinase C in regulating the contractile responses of coronary arterial smooth muscle to a variety of stimuli, at least in part by increasing the sensitivity of the contractile apparatus to Ca++.     

Effects of Bay k 8644 and nifedipine on isolated dog cerebral, coronary and mesenteric arteries

Vasoconstrictor effects of Bay k 8644, a dihydropyridine Ca++ agonist, and vasorelaxant effects of nifedipine were investigated in helical strips of dog cerebral (basilar, posterior cerebral and middle cerebral) and peripheral (coronary and mesenteric) arteries. The addition of Bay k 8644 produced a dose-dependent contraction in the absence of any contractile agent in the basilar artery with a pD2 value of 8.53. Similar sensitivity to Bay k 8644 was observed in the posterior cerebral, middle cerebral or coronary artery. Bay k 8644 was much less effective in producing a contraction in the mesenteric artery. An elevation of the concentration of extracellular K+ eliminated the difference between the responses to Bay k 8644 in the basilar and mesenteric artery. Contractile responses of the basilar artery to Bay k 8644 were antagonized competitively by nifedipine (pA2 = 8.17), but non-competitively by diltiazem. The pA2 values for nifedipine antagonism of Bay k 8644 responses with the elevated K+ were the same between the basilar and mesenteric arteries. Increased sensitivity to exogenously added K+ also was observed in cerebral and coronary arteries when compared with the mesenteric artery. The addition of nifedipine to an unstimulated strip produced a dose-dependent relaxation in cerebral and coronary arteries, but not in the mesenteric artery. When the cerebral and peripheral arteries were contracted with K+ to the same magnitude, nifedipine produced similar relaxations among these arteries. Nifedipine was less efficacious in antagonizing the contractile response to Bay k 8644 compared with the contractile response to K+ in cerebral arteries. These results suggest that 1) the voltage-dependent Ca++ channels in the cerebral and coronary arteries are in different states of activation from those in the mesenteric artery, 2) Bay k 8644 contracts the cerebral and coronary arteries by acting primarily on the same site, presumably dihydropyridine receptors of the voltage-dependent Ca++ channels at which nifedipine acts, 3) the dihydropyridine receptors were the same between the basilar and mesenteric arteries and 4) there may be a difference in the state of the Ca++ channel in the arteries between the stimulation with Bay k 8644 and K+-depolarization.     

Calcium dependence of phorbol 12,13-dibutyrate-induced force and myosin light chain phosphorylation in arterial smooth muscle

Phorbol dibutyrate (PDB) is an activator of protein kinase C and has been observed to cause a slow developing contraction in vascular smooth muscle. The mechanism of phorbol ester-induced contraction is unknown. We studied the Ca++-dependence of, and the degree of myosin light chain phosphorylation (MLC-P), during PDB-induced contractions in rabbit aortic rings. PDB elicited concentration-dependent contractions (3 X 10(-8) to 10(-6) M) in rabbit aortic rings incubated in normal (1.6 mM Ca++) physiologic salt solution (PSS). Addition of the Ca++-channel blocker nifedipine (0.1 microM) to PSS or removal or Ca++ from PSS significantly reduced the contractile responses to PDB. Depletion of Ca++ by repeated washes in O Ca++-PSS containing 10(-3) M ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid reduced, but did not eliminate, the responses to PDB. In PSS, PDB significantly increased the fraction of phosphorylated MLC/total MLC to 0.33 from a resting value of 0.20. Ca++ depletion reduced the resting fraction (MLC-P/MLC) to 0.14. PDB-stimulated contractions in Ca++-depleted tissues occurred in the absence of significant increases in MLC-P. Sodium nitroprusside partially relaxed PDB-induced contractions by approximately 50% whether elicited in the presence of 1.6 mM Ca++ or after Ca++ depletion. In both cases relaxation occurred in the absence of statistically significant decreases in MLC phosphorylation. Ca++-dependent MLC phosphorylation may account for a component of the PDB contractile response in rabbit aorta. Studies in the absence of Ca++ suggest that PDB may activate contraction without concomitant MLC-P.     

Effects of pimobendan, a cardiotonic and vasodilating agent with phosphodiesterase inhibiting properties, on isolated arteries and veins of rats.

Effects of pimobendan (PBD) were investigated on isolated rat blood vessels. PBD dose-dependently relaxed aortic, femoral arterial and mesenteric venous preparations precontracted with KCl and reduced the amplitude of spontaneous contractions of portal venous preparations; the sensitivity to PBD was femoral greater than portal greater than mesenteric greater than aorta. Relaxation response of the femoral artery to PBD was not changed by propranolol and aminophylline. Glyceryl trinitrate (GTN), isoproterenol (ISO), forskolin and adenosine also elicited dose-dependent relaxations of femoral arteries; the rank order of potency (mean negative log EC50 value) was GTN greater than ISO greater than PBD = forskolin greater than adenosine. The relaxation responses to PBD and isobutyl methylaxanthine (IBMX) were not attenuated with removal of endothelial cells. In the femoral artery, methylene blue diminished GTN-induced relaxation but not PBD-induced relaxation. PBD and IBMX increased the relaxation responses of the artery to cyclic AMP-forming drugs (ISO, forskolin and adenosine) but not a cyclic GMP-forming drug (GTN). PBD and IBMX increased the relaxation response of mesenteric veins to ISO. The drugs noncompetitively inhibited arterial contractions accompanied by voltage-dependent and alpha-adrenoceptor-operated Ca2+ influxes. In the absence of extracellular Ca2+, PBD and IBMX reduced contractile responses of arteries to norepinephrine but not caffeine. The present results suggested that PBD relaxed the blood vessels, at least in part, through an intracellular accumulation of cyclic AMP.     

Involvement of endogenous prostaglandin I2 in the vascular action of histamine in dogs

In helical strips of dog mesenteric and gastroepiploic arteries contracted with prostaglandin (PG) F2 alpha, the addition of histamine (10(-6) M) caused a relaxation, which was markedly attenuated by treatment with aspirin, indomethacin or tranylcypromine. Treatment with chlorpheniramine prevented the inhibitory effect of aspirin or tranylcypromine. Combined treatment with chlorpheniramine plus aspirin or tranylcypromine slowed the development of histamine-induced relaxations as did the treatment with chlorpheniramine alone. Relaxations of mesenteric and gastroepiploic arteries induced by histamine were markedly attenuated or abolished by combined treatment with chlorpheniramine and cimetidine. Histamine-induced relaxations of coronary and renal arterial strips were suppressed by cimetidine alone but were unaffected by aspirin. Contractile responses of cerebral arterial strips to histamine were attenuated by chlorpheniramine and potentiated by aspirin. The collagen-induced platelet aggregation was inhibited by treatment with bathing media in which mesenteric arteries were stimulated by histamine; the inhibition was prevented by treatment of the arteries with aspirin. It may be concluded that relaxation of mesenteric and gastroepiploic arteries induced by histamine is mainly associated with the release of prostaglandin I2 from the arterial wall, which results from an activation of histaminergic H1 receptors. Histamine-induced cerebroarterial contractions mediated via H1 receptors appear to be partly counteracted by prostaglandin I2 released.     

Mechanism of vascular smooth muscle contraction by sodium fluoride in the isolated aorta of rat and rabbit.

The purpose of this study was to determine the cellular basis for fluoride ion (F-)-induced contractions of isolated aortic rings from both the rat and the rabbit. The F- contractions were not affected by endothelial denudation but were enhanced in the presence of A (0.1 or 1.0 mM) added to the bathing Krebs' solution. The contractile effect of F- also was not modified by bathing with Ca(++)-free + ethylene glycol bis(b-aminoethylether)-N,N-tetracetic acid Krebs' solution or nifedipine (10 microM), but was attenuated by inorganic (Cd++, Co++ and Ni++) Ca++ antagonists in normal and Ca(++)-free Krebs' media. Bis(o-aminophenoxy)-ethane-N-N-N'-N'-tetraacetic acid, ryanodine and intracellular Ca++ modulators, respectively, caused 36.1 +/- 6.1%, 16.4 +/- 6.8% and 52.3 +/- 7.3% inhibition of the contractile response to F- in a Ca(++)-free media while causing near complete inhibition of norepinephrine-induced contractions. F- contractions were also inhibited by the calmodulin antagonists W-7 and calmidazolium (IC50 = 23.0 +/- 7.0 and 45.0 +/- 10.0 microM, respectively). On the other hand, the protein kinase C antagonists staurosporine and H-7 potently (IC50 = 0.016 +/- 0.007 and 1.1 +/- 0.5 microM, respectively) inhibited the fluoride-induced contractions. Aortic rings from the rabbit were similarly potently antagonized by the protein kinase C inhibitors, however, K(+)-induced contractions were also equally sensitive to these agents in both rat and rabbit tissues. The putative phospholipase C inhibitor neomycin was significantly less effective (IC50 = 13.0 +/- 5.0, 0.44 +/- 0.09 and 0.89 +/- 0.40 mM) at inhibiting F- than norepinephrine and KCl contractile effects.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sources of activator calcium for extrinsic vascular tone and nimodipine inhibition of that tone in proximal vs. distal rabbit ear arteries

The contractile responses of three artery segments of diminishing diameter in the rabbit ear (i.e., unstretched lumen diameter approximately 300 mu in central ear artery, unstretched lumen diameter approximately 150 mu in main side branch off the central ear artery and unstretched lumen diameter approximately 75 mu in terminal branch off the main side branch) to high K+, norepinephrine (NE) and 5-hydroxytryptamine (5-HT) were tested before and after their incubation in Ca++-free physiologic salt solution for times varying from 3 to 60 min. The time course of reduction of the contractile responses to K+ with Ca++-free conditions in all classes of vessels could be represented by monoexponential curves that were not significantly different from each other. The contractile response of all the ear arteries to NE and 5-HT was biphasic. The first rapid transient phase (phase I) was more resistant to change upon the removal of exogenous Ca++ than the second usually equilibrium-like component (phase II), which was dramatically and rapidly reduced (but not necessarily eliminated) by this procedure. The extent of decline upon Ca++ removal in most instances was greater for 5-HT than for NE. The rate of falloff of both phases of contraction to NE and 5-HT was faster in the smaller compared with the larger arteries. These results suggest that, as vessels in the rabbit ear arterial tree get smaller, the contribution of a tightly bound or intracellular Ca++ pool to both phases of amine-induced contraction becomes smaller, but this contribution is greater to the transient compared with the equilibrium phase.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mode of vasorelaxing action of 5-[3-[[2-(3,4-dimethoxyphenyl)-ethyl]amino]-1-oxopropyl]-2,3,4,5- tetrahydro-1,5-benzothiazepine fumarate (KT-362), a new intracellular calcium antagonist

In rabbit aorta, pretreatment with KT-362 (KT; 10(-6) and 10(-5) M) inhibited contractile responses to norepinephrine (NE; 3 X 10(-9)-10(-5) M) and methoxamine (10(-7)-10(-4) M) but failed to affect responses to potassium (10-70 mM). KT (10(-5) M) partially inhibited Ca++-induced contractions in K+-depolarized aorta pre-equilibrated in a Ca++-free medium. After incubation of tissues for 30 min in a Ca++-free medium containing EGTA (0.2 mM), residual responses to NE and methoxamine were inhibited by KT (10(-6)-10(-4) M) and nitroglycerin (10(-5) M), but not by nifedipine, verapamil or diltiazem (all 10(-5) M). The inhibitory action of a combined treatment with KT and nitroglycerin (both 10(-5) M) on the residual response to NE was also much greater than that of either agent alone. In a Ca++-free medium, the residual caffeine-induced contraction of rabbit iliac artery was inhibited by KT (10(-5)-10(-4) M) but not by nifedipine (10(-5) M). The inhibitory action of KT on the residual responses to methoxamine and caffeine in a Ca+-free medium was much greater than that of nitroglycerin. In a Ca++-free medium with low EGTA (0.01 mM), D600 (10(-5) M) and NE (3 X 10(-7) M), the addition of Ca++ (2 mM) resulted in a tonic contraction.(ABSTRACT TRUNCATED AT 250 WORDS)     

Analysis of the effect of 5-hydroxykynurenamine,, a serotonin metabolite, on isolated cerebral arteries, aortas and atria.

In helically cut strips of dog and human cerebral arteries, the addition of 5-hydroxykynurenamine (5-HK) caused dose-related contractions, and this contractile response was attenuated by methysergide and lysergic acid diethylamide (LSD). Treatment of the strips with 5-HK in concentrations higher than 2 X 10- minus 6 M shifted the dose-response curve of serotonin to the right and downward. The inhibitory effect of 5-HK was reversed only partially by repeated washings, whereas the inhibition caused by methysergide and LSD was completely reversed. The contractile response to K-+ (30 mM) was reduced only slightly even at the highest concentration of 5-HK used (5 X 10-minus 5 M). In strips of rabbit aortas, the contractile effect of serotonin was markedly attenuated by 5-HK, whereas that of norepinephrine was reduced only slightly. In isolated rabbit right atria, 5-HK produced a transient increase in the rate as did serotonin, 5-HK being approximately 1/20 as potent as serotonin. The positive effect was not influenced by methysergide but was markedly attenuated or abolished by 10-minus 6 M propranolol and 3 X 10-minus 6 M cocaine. Treatment with 5-HK (5 X 10-minus 5 M) did not modify the chronotropic response to isoproterenol. It appears that 5-HK produces vascular contractions via serotonergic receptors, inhibits specifically the contractile response of arteries and aortas to serotonin and causes a tachycardia in rabbit atria by liberating catecholamines from adrenergic nerves.     

Specific actions of gallium on norepinephrine-induced tension and associated 45Ca movements in rabbit aortic smooth muscle

Gallium ion (Ga) dose-dependently (60-360 microM) inhibited contractions induced by norepinephrine (NE, 1 microM) in rabbit aortic (and media intimal) strips, but did not affect contractions elicited with high K+ (80 mM) solution. The initial phasic portion of the NE-induced response was either unaffected or only slightly (less than 10%) reduced, but the tonic portion of the response was inhibited completely by higher concentrations (greater than or equal to 300 microM) of Ga . In resting muscles, the equilibrated (90 min) 45Ca uptake was not altered by Ga (360 microM). Also, 45Ca efflux from either high- or low-affinity Ca++ binding sites was unaltered by Ga . The effects of Ga (360 microM) on 45Ca retained after a subsequent 60-min washout at 0.5 degrees C in an isosmotic (80.8 mM) La solution were also examined. High affinity La -resistant 45Ca released by NE (1 microM) was not altered by Ga . Under conditions favoring low affinity Ca++ uptake, 45Ca retention in control and K+-treated muscles was not changed by Ga , but the additional incremental 45Ca uptake associated with NE (in the presence of high K+) was blocked. Thus, Ga appears to have a selective inhibitory action on NE-associated 45Ca uptake without affecting either resting and high K+-induced 45Ca uptake or that 45Ca fraction released by NE. This action may result from a selective blockade by Ga of receptor-linked Ca++ channels in rabbit aortic smooth muscle.     

EFFECTS OF CICLETANINE ON HISTAMINE-INDUCED CONTRACTIONS OF ISOLATED RABBIT MESENTERIC ARTERIES

The antagonism by cicletanine of contractile responses to histamine has been examined in vitro on ring preparations of rabbit mesenteric arteries. Cicletanine (10(-8)-10(-6) M) caused a parallel rightward shift of histamine concentration response curve, with a pA2 value of 7.48 (slope = 0.89 +/- 0.19, not significantly different from unity). Histamine-induced contractions were nifedipine-sensitive and associated with cicletanine-sensitive increased 45Ca uptake. Endothelium removal resulted in enhanced contractile responses to histamine, but did not significantly modify cicletanine-induced antagonism: KB (dissociation constant) values for cicletanine antagonism in the presence or absence of endothelium were: 3.7 (+/- 0.1) X 10(-8) M and 3.6 (+/- 0.3) X 10(-8) M, respectively. Cicletanine (greater than 10(-4) M) also significantly attenuated 10 mM caffeine-induced contractions in rings exposed to Ca-free 100 mM K+ depolarizing medium. The results suggest that cicletanine-induced antagonisms of histamine H1 receptor-mediated contractions of rabbit mesenteric arteries is associated with interference with calcium entry as well as at high concentrations, release from intracellular stores.     

Diabetes and reactivity of isolated human saphenous vein

Summary. Helical strips of saphenous veins from diabetic (n=8) and non-diabetic (n= 18) humans were studied in vivo for their responsiveness to several vasoactive agents. Following application of passive force (?20·0 mN), venous strips from non-diabetic humans often developed spontaneous phasic contractile activity (12 out of 18 patients; 2–5 contractions/min). These intrinsic changes in force were seen in venous strips from only one diabetic patient. The phasic contractions were not altered by treatment with phentolamine, whereas the calcium channel blocker, D-600, and calcium-free solution (1·0 mM EGTA) inhibited the phasic contractions. Saphenous veins from diabetic patients developed less maximal, active tension in response to norepinephrine than those from non-diabetic patients. Contractile responses to serotonin, angiotensin II, and elevated potassium concentration in saphenous veins from diabetic patients were not different from those in veins from non-diabetic patients. These observations demonstrate attenuated development of active tension in response to alpha-adrenergic receptor activation and reduced spontaneous contractile activity in venous smooth muscle from diabetic patients.     

Mechanism of inotropic action of xestoquinone, a novel cardiotonic agent isolated from a sea sponge.

Xestoquinone (XQN) isolated from the sea sponge Xestospongia sapra produced dose-dependent cardiotonic effects on guinea pig left and right atria. A direct action of XQN (1-30 microM) on the contractile machinery of cardiac myofilaments was demonstrated in chemically skinned fiber preparations from guinea pig papillary muscles. In atrial preparations, the XQN-induced inotropic effect was markedly inhibited by verapamil or nifedipine, but was not affected by practolol, chlorpheniramine, cimetidine, tetrodotoxin or reserpine. The Ca++ dependence curve for the contractile response of the atria was substantially shifted to the left by XQN (10 microM), and this XQN-induced shift was reversed by verapamil. The time-to-peak tension and relaxation times of the atrial contractions were shortened by XQN, and the action potential duration was markedly prolonged. Whole-cell patch clamp recordings in left atrial strips confirmed that XQN (30 microM) increased the slow inward current. However, there was a temporal dissociation between altered tension development and prolongation of the action potential duration. Cyclic AMP phosphodiesterase activity was inhibited and tissue cyclic AMP content of guinea pig left atria was increased by XQN (0.3-10 microM) in a concentration-dependent manner, but increases in cyclic AMP content did not occur in parallel with increases in contractile response. These observations suggest that an enhancement of intracellular cyclic AMP content and Ca++ influx across the cell membrane contribute to the late phase of XQN-caused cardiotonic responses, whereas the early phase may largely be elicited through direct activation of contractile elements. XQN may provide a novel leading compound for valuable cardiotonic agents.     

Spontaneously hypertensive rat resistance artery structure related to myogenic and mechanical properties

This investigation related arterial structure to myogenic (pressure-dependent) contractile responses in resistance arteries from spontaneously hypertensive rats (SHRs) and Wistar-Kyoto (WKY) normotensive control rats under pressurized conditions in vitro. Femoral and mesenteric resistance arteries from either strain were cannulated and pressurized in an arteriograph for the determination of pressure-diameter relationships under passive and active conditions in the range 5-200 mmHg transmural pressure. Arterial geometrical measurements were made under relaxed conditions at 100 mmHg. Media thickness/lumen diameter (M/L) ratios were significantly increased in SHR femoral (5.00+/-0.44% compared with 3.63+/-0.34%; P<0.05) and mesenteric (4.40+/-0.29% compared with 2.62+/-0.23%; P<0.001) arteries compared with those from WKY rats. Maximum myogenic contractions, assessed as minimum normalized diameters, were not significantly different in SHR and WKY rat femoral (0.41+/-0.03 and 0.40+/-0.02 respectively) or mesenteric (0.56+/-0.02 and 0.63+/-0.03 respectively) arteries. Arterial mechanical analyses demonstrated that incremental elastic modulus is reduced in SHR mesenteric arteries, but is not significantly different in SHR femoral arteries, compared with those from WKY rats. Additionally, wall stress at estimated in vivo pressures under passive and active conditions are similar in SHR and WKY rat arteries. These data demonstrate that increased M/L ratios in resistance arteries from SHRs are not associated with increased maximum pressure-dependent contractile responses. Increased M/L ratios in resistance arteries from SHRs are not accounted for by increased vessel wall stiffness, but the hypertension-associated arterial geometrical abnormalities act to normalize wall stress in the face of increased arterial pressure.     

Mechanism of action of a novel antivasospasm drug, HA1077

HA1077, but not the potent calcium entry blocker nicardipine reversed experimental chronic cerebral vasospasm induced in a two-hemorrhage canine model. The i.a. administration of HA1077 produced significant increases in vertebral blood flow in dogs. The effects of HA1077 on the vascular responses were studied in vitro. Spiral strips of rabbit aorta were mounted for isometric tension recording in physiological salt solution. HA1077 produced a competitive inhibition of the Ca++-induced contraction of the depolarized rabbit aorta. The pA2 of HA1077 for the Ca@-induced contraction was 6.71. The inhibitory effect of HA1077 on the KCl-induced contraction was not altered by atropine, propranolol, theophylline or indomethacin. HA1077 (10(-8) to 10(-4) M) inhibited contractile responses to KCl, phenylephrine (PHE) and prostaglandin (PG) F2 alpha similarly, whereas verapamil, diltiazem and nicardipine were much less effective in blocking the contractions induced by PHE or PG. Even in Ca++-free physiological salt solution, both PHE and PG were capable of contracting the aorta. These Ca++-free contractile responses to PHE and PG were antagonized effectively by HA1077. Verapamil failed to inhibit these contractions. We also investigated the effects of HA1077 on guinea pig heart contractility. In contrast to calcium entry blockers (which are known to have a direct negative inotropic effect), HA1077 did not change the developed tension in the left atrium at concentrations up to 3 X 10(-4) M. The present evidence demonstrates that the novel antivasospasm drug HA1077 is a class of calcium antagonists different from the calcium entry blockers.