Comparative cardiovascular actions of clentiazem, diltiazem, verapamil, nifedipine, and nimodipine in isolated rabbit tissues.

Clentiazem is an 8-chloro-substituted derivative of diltiazem. We compared the relative potency of clentiazem with that of diltiazem, verapamil, nifedipine, and nimodipine in isolated rabbit right atria and vascular smooth muscle removed from various arterial beds. In experiments with isolated right atria, calcium channel blockers were added cumulatively to study relative cardiodepressive potencies (as compared with vascular effects) with the following results: verapamil greater than or equal to diltiazem greater than clentiazem greater than or equal to nimodipine much greater than nifedipine. The aorta, pulmonary, renal, mesenteric, coronary, and basilar arteries were removed, cut helically in strips, and mounted in isolated tissue baths to measure isometric force. Vessels were contracted with either 40 mM KCl (opening voltage-operated calcium channels) or 1 x 10(-5) M norepinephrine (NE, opening receptor-operated calcium channels). Cumulative dose-response curves were generated for relaxation with each calcium channel blocker. All compounds were more potent at relaxing potassium-induced contractions than NE-induced contractions. In vessels precontracted with KCl, neither diltiazem, verapamil, or nifedipine showed selectivity for basilar artery as compared with the mesenteric artery. Both clentiazem and nimodipine were selective (6- and 30-fold, respectively) for basilar artery in blocking potassium-induced contractions. When NE was used to contract the arteries, clentiazem (12-fold), diltiazem (8-fold), verapamil (8-fold), and nifedipine (153-fold) were all more potent in relaxing the contraction in basilar artery than in mesenteric artery. Nimodipine failed to demonstrate selectivity for basilar artery as compared with mesenteric artery contracted with NE.(ABSTRACT TRUNCATED AT 250 WORDS)     

Involvement of specific receptors and calcium mechanisms in serotonergic contractile response of isolated cerebral and peripheral arteries from rats

Studies were performed on isolated cerebral and peripheral arterial segments from the rat to define contractile receptors for 5-hydroxytryptamine (5-HT) and to elucidate the responses to calcium channel blockers in relation to their effects on potassium-induced contractions. 5-HT induced strong contraction of the middle cerebral artery, arteries forming the circle of Willis, basilar artery and tail artery in the mentioned order of relative potency and with an intrinsic activity in the brain vessels approximately 70% of that caused by 124 mM potassium in the buffer solution. Ketanserin inhibited the contraction both in the basilar and tail arteries competitively, with pA2 = 9.35 and 9.09, respectively, suggesting mediation by 5-HT2 receptors. The inhibition by cyproheptadine and methysergide (of the response in the basilar artery) was noncompetitive. High potassium in the buffer solution contracted the basilar and tail arteries biphasically, including prazosin-sensitive alpha-adrenoceptor activation in the latter. Cyproheptadine, nimodipine, verapamil and diltiazem inhibited the 5-HT-induced contraction in the mentioned order of potency. Verapamil was more potent than diltiazem, also in the tail artery, but nimodipine inhibited the contraction only by 35%. Also the (tonic) contraction induced by high potassium concentration was attenuated, with the same relative potency as in the presence of 5-HT except for cyproheptadine, which was less efficient than nimodipine. The transient potassium-induced contraction was inhibited less effectively by the calcium antagonists. The IC50 values were characteristically lower in the basilar than in the tail artery, irrespective of whether the contraction had been produced by 5-HT or high potassium.(ABSTRACT TRUNCATED AT 250 WORDS)     

Verapamil and Nifedipine Inhibition of Contractions Induced by Potassium and Noradrenaline in Human Mesenteric Arteries and Veins

Abstract Ring preparations of human mesenteric arteries and veins were contracted by noradrenaline (1.8×10^-5M) or potassium (127mM). Isometric tension was recorded. In the arterial preparations, the maximum response to noradrenaline was 97±8% (mean±S.E.M.) of that to potassium. In the veins, the corresponding figure was 38±4%. The calcium antagonists verapamil (2.2±10^-7-2.2± 10^-5M) and nifedipine (2.9×10^-8-2.9×10^-6M) relaxed both arteries and veins contracted by noradrenaline or potassium, and reduced the responses to these agents when added 15 min. before stimulation. The time course of relaxation of potassium contracted preparations, induced by verapamil and nifedipine, was more rapid and complete than that produced by a calcium-free, high potassium solution. In contrast to verapamil, nifedipine caused a more pronounced inhibition of the potassium than of the noradrenaline evoked contractions in both arteries and veins. After exposure to a calcium-free medium for 30 min., the arterial response to noradrenaline was significantly (P<0.05) greater than that to potassium. However, the reverse was found in the veins. In both types of vessel, verapamil (2.2×10^-6M) and nifedipine (2.9-10^-7M) were equi-effective in reducing the noradrenaline and potassium induced responses in calcium-free medium. The results suggest that there are differences in reactivity, not only between mesenteric arteries and veins, but also between, e.g. peripheral and mesenteric vessels. The calcium antagonists nifedipine and verapamil do not have an identical mode of action. However, both agents seem to inhibit influx of extracellular calcium, and might also have an inhibitory effect on the release of intracellular calcium.     

Effects of calcium antagonists on intracerebral penetrating arterioles in rats

There is no direct information on the effect of calcium antagonists on intracerebral penetrating arterioles, which are responsible for a significant part of total cerebrovascular resistance. In a study on rats, the effects of four calcium antagonists (diltiazem, verapamil, nifedipine, and nimodipine) on isolated intracerebral penetrating arterioles with mean resting diameters (+/- standard error of the mean) of 52.3 +/- 3.0 micron were investigated. Vessel diameters were monitored in vitro by means of a video microscope dimensional analyzer under constant transmural pressure (60 mm Hg) after cannulation. Each calcium antagonist produced maximal dilation of about 50% (diltiazem 46.4% +/- 5.6%, verapamil 53.1% +/- 6.0%, nifedipine 46.9% +/- 6.1%, and nimodipine 47.1% +/- 5.4%) with varied sensitivity (median effective dose (ED50): diltiazem 1.52 X 10(-6) M, verapamil 1.08 X 10(-7) M, nifedipine 8.65 X 10(-9) M, and nimodipine 1.62 X 10(-9) M). Dilation effects persisted for a significantly longer time after washout with calcium antagonists such as diltiazem (15.5 +/- 1.8 minutes), nifedipine (19.0 +/- 3.9 minutes), and nimodipine (30.0 +/- 1.6 minutes) than after washout with adenosine (8.5 +/- 1.0 minutes). It appeared that the magnitude of vasodilation was greater and the duration of dilation after washout longer in intracerebral penetrating arterioles than that reported for pial arterioles, although sensitivity to each calcium antagonist was quite similar to that reported for larger cerebral arteries. These data provide a possible explanation for the apparent disparity between clinical efficacy and angiographically determined vessel diameter when patients with cerebral vasospasm are treated with calcium antagonists. These agents may have a greater effect on intracerebral penetrating arterioles than on angiographically visible larger arteries.     

Some effects of the calcium promotor BAY K 8644 on feline cerebral arteries

The proposed calcium promotor BAY K 86(44) contracted feline basilar arteries partially depolarized by 10 mmol X 1(-1) potassium in a concentration-dependent manner (EC50 value: (2.1 +/- 1.2) X 10(-9) mol X 1(-1)). The concentration-response curve for prostaglandin (PG) F2 alpha was displaced to the left after pretreatment with BAY K 86(44). PGF2 alpha induced a biphasic contraction in calcium-free medium as has been described previously. The second PGF2 alpha-induced contraction phase in calcium-free medium was abolished by pretreatment with nifedipine or diltiazem. BAY K 86(44) restored the second phase in arteries pretreated with nifedipine, but not in vessels pretreated with diltiazem. The findings suggest that BAY K 86(44) acts as a promotor of calcium-influx, probably by interaction with the 'dihydropyridine receptor' in the cell membrane, and also provide support for the view that PGF2 alpha releases membrane-bound calcium.     

Actions of prostaglandin F2 alpha and noradrenaline on calcium exchange and contraction in rat mesenteric arteries and their sensitivity to calcium entry blockers.

1 The actions of prostaglandin F2 alpha (PGF2 alpha) and noradrenaline on contraction and 45Ca exchange have been studied in rat mesenteric arteries. 2 PGF2 alpha and noradrenaline contracted rat isolated mesenteric artery preparations to about the same extent. The PGF2 alpha-stimulated contractions, unlike those produced by noradrenaline, were completely inhibited in calcium-free physiological solution. 3 The calcium entry blocking drugs, cinnarizine and flunarizine, had little effect on the resting exchange of calcium in the arterial smooth muscle, but inhibited PGF2 alpha-stimulated contractions and 45Ca uptake to a similar extent. 4 Flunarizine was about 7 fold more potent as an inhibitor of noradrenaline- than of PGF2 alpha-mediated contraction and 45Ca uptake and this ratio was about 50 for cinnarizine. 5 EGTA (1.25 mM) produced a relaxation of noradrenaline and PGF2 alpha-induced maximal contractions. Measured over the first 2 min of EGTA contact, the rate of relaxation was much faster in noradrenaline than in PGF2 alpha-stimulated preparations. 6 Turnover of cellular calcium (influx plus efflux) during the first 2 min of noradrenaline contact was much greater than that produced by PGF2 alpha, largely due to a greater effect of noradrenaline on calcium efflux. 7 The results suggest that PGF2 alpha-but not noradrenaline-induced contractions are entirely dependent on the influx of extracellular calcium and that the agonists may stimulate calcium gating mechanisms differently.     

SR 33557, a novel calcium entry blocker. I. In vitro isolated tissue studies.

The effects of SR 33557 on isolated cardiovascular preparations were compared to those of nifedipine, verapamil and diltiazem. In rat aortic strips, SR 33557, like nifedipine, verapamil and diltiazem, caused a significant and simultaneous inhibition of potassium-induced 45Ca++ influx and contractile responses (nifedipine greater than SR 33557 greater than verapamil greater than diltiazem). SR 33557 also antagonized Ca(++)-induced contractions in K(+)-depolarized aorta preparations (pA2:9.08 +/- 0.03) and is the first calcium channel antagonist, structurally not related to 1,4-dihydropyridines, to inhibit competitively contractions induced by BAY K8644. In spike-generating vascular smooth muscle (rat portal vein), contractures evoked by noradrenaline (4 microM) or KCl (100 mM) were reduced by all four antagonists, the pharmacological potency being nifedipine greater than SR 33557 greater than verapamil greater than diltiazem. Unlike SR 33557, nifedipine, verapamil and diltiazem showed a parallel enhancement of frequency of spontaneous contractions in rat portal vein in spite of a concentration-related reduction in amplitude. By using rabbit atrial preparations, spontaneous right atrial rate and electrically stimulated (120/min) basal contractions of left atria were used as indices of chronotropy and inotropy. The potency series for negative chronotropic effects was nifedipine greater than SR 33557 greater than verapamil greater than diltiazem. For negative inotropic effects the potency order was verapamil greater than nifedipine greater than SR 33557 greater than diltiazem, respectively. Thus, SR 33557 should depress heart rate to a greater extent than ventricular contractility. These results suggest that SR 33557 is a potent calcium entry blocker that (unlike verapamil and diltiazem) is particularly selective for vascular smooth muscle and devoid of any potent negative inotropic actions.     

Regulation of the tension of human chorionic vasculature by histamine and prostaglandin F2 alpha.

The calcium dependence of potassium chloride-, prostaglandin F2 alpha (PGF2 alpha)-, and histamine-induced contractions of human chorionic vasculature segments was investigated. In physiologic buffer that contained 1.5 mM calcium chloride, 60 mM potassium chloride induced a rapid and sustained contraction of the vasculature. Potassium chloride-induced contractions were completely inhibited by the calcium channel blockers diltiazem and nifedipine or by excluding Ca2+ from the medium. Histamine (100 microM) induced a rapid increase in vascular tension in physiologic buffer which rapidly faded or desensitized after maximal tension was obtained. The maximal contractile responses to histamine were reduced approximately 50% by diltiazem and nifedipine in physiologic buffer or by suspension in calcium-free medium (OCaPB). Pretreatment of vessels with 20 mM caffeine in OCaPB completely abolished histamine-dependent contractile responses. Prostaglandin F2 alpha (100 nM)-induced increases in vascular tension developed slowly but remained maximal for at least 40 minutes. Contractile responses to PGF2 alpha were reduced 50% to 65% by diltiazem and nifedipine in physiologic buffer or by suspension in OCaPB. Caffeine pretreatment failed to alter the contractile response to PGF2 alpha in OCaPB. The differences in responsiveness of potassium chloride, histamine, and PGF2 alpha under the various conditions used suggest that these agents act by different mechanisms to elicit contractions in chorionic vessels. The potential roles of PGF2 alpha, histamine, and calcium channel blockers in modulating the fetoplacental circulation is discussed.     

Selectivity of phenytoin and dihydropyridine calcium channel blockers for relaxation of the basilar artery

We addressed the questions of whether or not phenytoin is a direct vasodilator and if it is selective for brain blood vessels, by studying the relaxant effects of phenytoin on isolated segments of canine basilar, femoral, and brachial arteries. Two dihydropyridine calcium channel blockers, nifedipine and PY 108-068, were also studied for comparison with phenytoin and to test for cerebral selectivity. Blood vessels were contracted with K+, prostaglandin F2 alpha, or serotonin. Phenytoin relaxed the basilar artery with low potency (pD2, 4.71 +/- 0.14) and moderate selectivity. Phenytoin also antagonized Bay K 8644 contractions of basilar artery in a noncompetitive manner. Basilar arteries contracted with 60 mM K+ were the most sensitive to nifedipine (pD2, 8.72 +/- 0.18), followed by the mesenteric (pD2, 8.24 +/- 0.07), femoral (pD2, 8.04 +/- 0.18), and brachial (pD2, 7.66 +/- 0.23) arteries. A similar pattern was observed in potassium-depolarized arteries relaxed by PY 108-068. The calcium dependence of contraction was studied using intact muscles depolarized in 60 mM K+ as well as chemically skinned basilar artery. Mean pD2 values for Ca2+-induced contractions of intact, depolarized arteries were not different (basilar, 4.15 +/- 0.13; mesenteric, 4.04 +/- 0.07; femoral, 4.24 +/- 0.11). The mean Ca2+ EC50 of chemically skinned basilar arteries was 8.7 X 10(-7) M, which is similar to the Ca2+ sensitivity of other skinned smooth muscles. The beneficial effect of phenytoin in treating cerebral ischemia may be due in part to relaxation of vascular smooth muscle. The dihydropyridines were potent smooth muscle relaxants with selectivity for the basilar artery.(ABSTRACT TRUNCATED AT 250 WORDS)     

Relaxing effects of dilazep and lidoflazine in dog cerebral and renal arteries independent of adenosine

The adenosine potentiating drugs dilazep and lidoflazine were studied for their relaxing ability in isolated dog cerebral and renal arteries contracted under conditions which induce the opening of potential-dependent calcium channels (using K+ at 30, 50 and 100 mmol/l) and under conditions which induce the opening of receptor-operated calcium channels (prostaglandin F2 alpha, PGF2 alpha; 5-hydroxytryptamine, 5-HT) and compared with those of adenosine and a standard calcium entry blocker, diltiazem. Dilazep, lidoflazine and diltiazem exerted concentration-dependent relaxation in cerebral and renal artery ring strips contracted with 30, 50 and 100 mmol/l K+. However, dilazep was slightly more potent at 100 mmol/l K+. In contrast, whereas the high concentration of adenosine (1 X 10(-5)-3.7 X 10(-4) mol/l) relaxes these arteries only at 30 mmol/l K+, it produced a more pronounced concentration-dependent relaxation when PGF2 alpha or 5-HT was used as a contracting agent. The order of relaxing responses of both cerebral and renal arterial ring strips contracted by PGF2 alpha were: diltiazem greater than adenosine greater than lidoflazine greater than dilazep. On the other hand, the relaxing responses on cerebral and renal arteries contracted with 5-HT were: diltiazem greater than lidoflazine greater than adenosine greater than dilazep and diltiazem greater than adenosine greater than lidoflazine greater than dilazep, respectively. Adenosine deaminase reversed the relaxation produced by adenosine, but was unable to reverse the relaxing responses to diltiazem, lidoflazine and dilazep. These findings suggest that dilazep and lidoflazine have a direct relaxing effect independent of adenosine in cerebral and renal artery ring strips possibly through their calcium entry blocking activity. The data suggest that adenosine is more effective on the receptor-operated contractions, whereas dilazep and lidoflazine are more effective on the potential-dependent contractions.     

Cardiac versus vascular effects of a new dihydropyridine derivative, CV-4093. In vitro comparison with other calcium antagonists

The effects of CV-4093, a new dihydropyridine derivative, on isolated cardiovascular tissues were compared with those of several dihydropyridine and non-dihydropyridine calcium antagonists. CV-4093 effectively inhibited the contractions induced in canine femoral arteries by high [K+]0 and Bay K 8644, but incompletely relaxed those induced by norepinephrine. CV-4093, 10(-6) M, abolished the electrically induced slow action potentials in guinea-pig papillary muscles partially depolarized by 25 mM K+ solution and attenuated those induced by isoproterenol, histamine and Bay K 8644. The rank order of potency of dihydropyridine and non-dihydropyridine calcium antagonists in canine femoral arteries and veins precontracted with 120 mM [K+]0 was as follows: nisoldipine greater than nicardipine greater than or equal to nifedipine greater than or equal to CV-4093 greater than verapamil greater than or equal to diltiazem. Nisoldipine was the most potent and CV-4093 was the least potent among these drugs in terms of negative inotropic effect in normally polarized papillary muscles and negative chronotropic effect in right atria of guinea pigs. The rank order of potency for these cardiodepressant actions was nisoldipine greater than or equal to nifedipine greater than nicardipine greater than verapamil greater than diltiazem greater than or equal to CV-4093. The duration of action potential in guinea-pig papillary muscles was shortened by nisoldipine and nifedipine, unchanged by nicardipine and CV-4093 and was slightly prolonged by verapamil and diltiazem. These results suggest that CV-4093 is a calcium antagonist with a highly selective vascular effect and little cardiodepressant action, and could be of value for the treatment of hypertension.     

Differential vasorelaxant effects of milrinone and amrinone on contractile responses of canine coronary, cerebral, and renal arteries

The vasorelaxant effects of milrinone and amrinone in canine coronary, cerebral, and renal arterial rings or strips contracted by either K+-depolarization, U46619 (a thromboxane mimetic), or prostaglandin F2 alpha (PGF 2 alpha) were quantitated. Milrinone was more potent as a vasorelaxant in coronary arteries relative to cerebral or renal arteries regardless of the mode of contraction; amrinone was coronary selective with K+ contraction only. When comparing potency in arteries contracted by different agonists, milrinone was significantly more potent as a vasorelaxant in all three arteries contracted by either U46619 or PGF2 alpha than in arteries contracted by K+ depolarization, whereas amrinone was only selective for U46619-induced contractions in cerebral arteries. This profile of activity for milrinone was similar to that of sodium nitrite and isoproterenol and dissimilar from the calcium entry blocking agents nimodipine and nifedipine. In conclusion, this study shows that coronary vascular selectivity exists for milrinone and amrinone. Moreover, the relaxant profiles of milrinone and amrinone, with different sources of vascular smooth muscle, are unlike those of calcium entry blocking agents and more similar to the profiles of agents that modulate cyclic nucleotide levels.     

Effects of Ca2+ antagonists nifedipine and diltiazem on isolated human chorionic arteries and veins.

5-Hydroxytryptamine (5-HT) and K+ induced contractions in human chorionic arteries and veins. 5-HT-caused responses were blocked by ketanserin (10(-7) and 10(-6) M) and prazosin (10(-5) but not 10(-6) M). K(+)-induced contractions were practically abolished in a Ca2(+)-free medium, whereas those produced by 5-HT were reduced. The efficacy (EC50 values) of diltiazem to produce relaxation in arteries and veins contracted with 40 or 75 mM K+ was similar, but normally greater than that of nifedipine. The potency of nifedipine (IC50 values) to inhibit maximal K+ contractions was greater than to inhibit maximal 5-HT contractions; diltiazem showed an inverse order of potency, which was less than that of nifedipine. The time course of 10(-6) M 5-HT and 75 mM K+ contractions was different, as much in the absence as in the presence of both Ca2+ antagonists; 5-HT contractions were transient, but sustained those elicited by K+. K+ (75 mM) and 5-HT (10(-6) M) produced increases in 45Ca2+ uptake, which were reduced by the Ca2+ antagonists. These results indicate that (a) human chorionic arteries and veins are similarly sensitive to Ca2+ antagonists, (b) 5-HT-induced contractions were largely dependent on extracellular Ca2+ and mainly mediated by 5-HT2 receptors but not by alpha 1-adrenoceptors, and (c) nifedipine was more potent than diltiazem in inhibiting Ca2+ influx through potential- and receptor-dependent Ca2+ channels.     

Effects of prostaglandin F2 alpha and thromboxane A2 analogue on bovine cerebral arterial tone and calcium fluxes

We determined sources of activator calcium for prostanoid-induced cerebrovascular constriction by measuring isometric tension and calcium-45 (45Ca) fluxes in bovine middle cerebral arteries. Constriction induced by prostaglandin F2 alpha or the stable thromboxane A2 analogue SQ-26,655 was near-maximally inhibited in calcium-deficient solutions but only partially inhibited by calcium antagonists (10(-5) M verapamil or 3.3 x 10(-7) M nifedipine). Studies of 45Ca binding at different external Ca2+ concentrations showed that cerebral arteries possess two calcium binding sites, a high-affinity site and a low-affinity site. Each prostanoid significantly increased low-affinity 45Ca uptake (external Ca2+ concentration = 1.2 mmol/l) during 5 minutes of 45Ca loading; for prostaglandin F2 alpha 45Ca uptake increased from 69 to 108 nmol/g and for SQ-26,655, from 78 to 141 nmol/g. The prostanoid-induced increases in low-affinity 45Ca uptake were completely abolished by pretreatment with verapamil or nifedipine. Prostaglandin F2 alpha, SQ-26,655, verapamil, and nifedipine had no effect on high-affinity 45Ca uptake (external Ca2+ concentration = 45 mumol/l) or 45Ca efflux (after 60 minutes' preincubation in calcium-deficient media). Prostaglandin F2 alpha and SQ-26,655 each appear to constrict cerebral arteries by two mechanisms: first, by promoting calcium uptake from low-affinity binding sites through receptor-operated channels sensitive to the calcium antagonists, and second, by releasing calcium from depletable internal stores.     

(+)-S-12967 and (-)-S-12968: 1,4-dihydropyridine stereoisomers with calcium channel agonistic and antagonistic properties in rat resistance arteries.

1. The actions of (+)-S-12967 and (-)-S-12968 two isomers of a new 1,4-dihydropyridine (DHP) derivative, were studied on 125 mM K(+)-, Ca(2+)- and noradrenaline-induced contractions in rat isolated mesenteric resistance arteries and compared to those of nifedipine. 2. The action of (+)-S-12967 and (-)-S-12968 was slow in onset in contrast to nifedipine. Both isomers had a dual contractile and relaxant action in arteries contracted with 125 mM K+; however, the (-)-isomer was about 300 times more potent than the (+)-isomer. The response to 125 mM K+, being depressed by 70%, recovered within 20 to 30 min for all DHP derivatives. All vessels were treated with 1 x 10(-6) M phenoxybenzamine thus excluding the possibility that the contraction is mediated by activation of amine-receptors. 3. Both (+)-S-12967 and (-)-S-12968 at low concentrations potentiated responses induced by Ca2+ in arteries activated by 125 mM K+ and inhibited the responses at higher concentrations. (+)-S-12967 and (-)-S-12968 had no contractile action in arteries kept in normal buffer. Nifedipine had only an inhibitory action on vessel responses to 125 mM K+ and Ca2+. 4. Both isomers and nifedipine depressed the maximal vessel response to noradrenaline by about 20% and 44%, respectively. 5. The results confirm that DHP calcium antagonists selectively inhibit vascular smooth muscle responses induced by high potassium and that the potency of 1,4-DHP isomers may vary considerably.(ABSTRACT TRUNCATED AT 250 WORDS)     

Calcium antagonist-induced relaxation of the prostaglandin-F2 alpha response of isolated calf retinal resistance arteries

The relaxing effect of nitrendipine and D600 on isolated ring segments of calf retinal resistance arteries (i.d. ca. 200 microns) contracted with prostaglandin-F2 alpha (PGF2 alpha) or high potassium solution was studied. The vessel response to both PGF2 alpha and potassium stimulation was dependent on extracellular calcium. Removal of extracellular calcium reduced the vessel response to PGF2 alpha and potassium by 85% and 98%, respectively (P less than 0.01). Both calcium antagonists induced a concentration dependent relaxation of PGF2 alpha pre-contracted vessels with = -log[IC50(M)]-values of 8.01 and 7.13 for nitrendipine and D600 (P less than 0.05), respectively. The calcium antagonists were equieffective in relaxing the vessels amounting to 33% for nitrendipine at 10(-6) M and 37% for D600 at 10(-5) M. Further analysis of the data revealed that the nitrendipine induced relaxation of the PGF2 alpha response was linearly correlated with the internal lumen diameter of the vessels. No correlation was found for the D600 induced relaxation. At the highest concentrations of nitrendipine, 10(-6) M, and D600, 10(-5) M, the potassium induced response was reduced by 94 and 75%, respectively. The results show that calcium antagonists only partially relax retinal resistance arteries contracted with PGF2 alpha although the vessels are dependent on extracellular calcium for active force generation. Calcium must therefore be activated by PGF2 alpha through other pathways than those blocked by calcium antagonists.     

Vasodilatation induced by pinacidil in dogs. Comparison with hydralazine and nifedipine

In helical strips of dog arteries precontracted with prostaglandin (PG) F2 alpha, pinacidil and nifedipine produced a dose-related relaxation. The potencies of pinacidil were in the order of coronary and renal greater than mesenteric greater than basilar and middle cerebral arteries, whereas those of nifedipine were in the order of basilar and renal greater than mesenteric and coronary arteries. Pinacidil caused a greater relaxation in mesenteric veins than in the arteries. Hydralazine consistently relaxed the arteries only at 10(-3) M. In mesenteric artery strips exposed to Ca2+-free, high K+ media, contractions induced by Ca2+ were reduced by 10(-8) M nifedipine, but they were not influenced by 10(-5) M pinacidil or by 10(-4) M hydralazine. In the arteries exposed to Ca2+-free media and stimulated by PGF2 alpha or norepinephrine, tonic contractions induced by Ca2+ were reduced moderately by 10(-5) M pinacidil but only slightly by 10(-8) M nifedipine. In Ca2+-free media, PGF2 alpha-induced contractions were inhibited only by pinacidil. In isolated mesenteric vasculature, perfusion pressure was lowered by pinacidil and hydralazine. It may be concluded that pinacidil produces vasodilatation due to interference with the transmembrane influx of Ca2+ into smooth muscle evoked by receptor stimulation but not that due to inhibition in the Ca2+ influx associated with K+-induced membrane depolarization. Decreased release of Ca2+ from intracellularly stored sites or increased sequestration to the sites may also be involved. Pinacidil appears to dilate arteries and veins as well as resistance vessels, whereas hydralazine appears to act exclusively on resistance vessels.     

Actions of calcium antagonists on pre- and postjunctional effects of neuropeptide Y on human peripheral blood vessels in vitro

The mechanisms underlying the contractile effects of neuropeptide Y (NPY) in relation to those of noradrenaline (NA) on small human blood vessels were studied in vitro. NPY caused contractions of mesenteric veins, renal and skeletal muscle arteries but not of mesenteric arteries. NPY was about 5- to 10-fold more potent than NA. The maximal contractile responses to NPY (5 X 10(-7) M) were 38 +/- 4, 37 +/- 8 and 95 +/- 16% of the response evoked by NA 10(-5) M in the mesenteric vein, renal and skeletal muscle arteries respectively. The NPY effects were resistant to adrenoceptor antagonists. The calcium antagonist nifedipine reduced the effect of NA but not the contractile response to NPY on mesenteric veins. Nifedipine and felodipine reduced the contractile response to both NA and NPY on renal and skeletal muscle arteries. In contrast to the contractile effects of K+, the responses to NPY and NA were largely uninfluenced by changes in extracellular Ca2+ concentrations. Nifedipine still inhibited the NPY contractions in a Ca2+-free medium while high extracellular Ca2+ (7.5 mM) partly reduced the nifedipine effect. NPY reduced the nerve stimulation-evoked [3H]NA overflow from the mesenteric veins via a nifedipine resistant mechanism. The stable analogue alpha, beta-methylene adenosine triphosphate (mATP) was more potent than ATP and had nifedipine-sensitive contractile effects similar to those of NA on the human blood vessels without influencing the nerve-evoked [3H]NA efflux. In conclusion, NPY exerts a potent nifedipine-sensitive vasoconstrictor activity, especially on human skeletal muscle arteries in vitro, although the influx of extracellular calcium may not be a crucial mechanism. The NPY-induced contractions of mesenteric veins and the inhibition of nerve-evoked [3H]NA efflux seem to be mediated via nifedipine resistant messenger systems.     

Nifedipine, diltiazem, bepridil and verapamil uptakes into cardiac and smooth muscles

Vogel et al. (1979; J. Pharmacol. Exp. Ther. 210, 378) reported that one calcium antagonist, bepridil, exerted an effect internally as well as its effect on blocking Ca2+ entry in cardiac muscle. Therefore, the uptakes of tritiated nifedipine, diltiazem, bepridil, and verapamil by cat ileal smooth muscle, chick embryonic ventricular muscle, and rabbit papillary muscle were investigated. It was found that the uptakes of verapamil and bepridil by the muscles were much higher than those of nifedipine and diltiazem. The uptake of bepridil was substantially greater than that of verapamil; thus, the order of uptake was: bepridil greater than verapamil much greater than nifedipine greater than diltiazem. The cardiac muscles accumulated at least 2-fold greater amount of calcium antagonists than the smooth muscle. The amount of a given calcium antagonist accumulated by a muscle was not a function of the ability of that calcium antagonist to inhibit Ca2+ uptake into the muscle, since nifedipine and diltiazem were more potent in depressing Ca2+ uptake, but had the smallest uptakes. The calcium antagonists were more effective in depressing Ca2+ uptake into smooth muscle than into cardiac muscle. Calculation indicates that internal drug concentration at steady state for both cardiac and smooth muscles was either equal to (diltiazem) or much higher than the drug concentration in the medium (bepridil and verapamil). It is concluded that bepridil and verapamil enter and accumulate in the muscle cells, whereas nifedipine and diltiazem permeate more slowly into the muscles. The ability of all four drugs to enter the muscle cells confers the possibility that these calcium antagonists may exert secondary actions on internal sites of the muscle, such as the sarcoplasmic reticulum.     

Uptake of calcium antagonistic drugs into muscles as related to their lipid solubilities

Calcium antagonists, e.g. bepridil and verapamil, block the Ca2+-dependent slow action potentials in frog skeletal muscle [L.M. Kerr and N. Sperelakis, J. Pharmac. exp. Ther. 222, 80 (1982)]. To determine whether the calcium antagonistic drugs may enter the fibers and exert an internal action as well, uptake of tritiated bepridil, verapamil, nitredipine, nifedipine, and diltiazem into rat extensor digitorum longus (EDL) muscles was examined. It was found that the uptake values of verapamil, nitrendipine, and bepridil were much higher than those of nifedipine and diltiazem. The order of uptake was: bepridil greater than nitrendipine greater than verapamil much greater than nifedipine greater than diltiazem. The small uptake values of nifedipine and diltiazem may represent primarily binding to the surface membrane. In frog skeletal muscle (sartorius) also, the uptake of bepridil was greater than that of verapamil, and disruption of the T-tubules by the glycerol method did not change them. The same order of drug uptake values was found for monolayer cultures of vascular smooth muscle cells (rat aorta). The order of uptake in isolated sarcoplasmic reticulum (SR) from rat skeletal muscles was: verapamil greater than nitrendipine greater than bepridil greater than nifedipine greater than diltiazem. The lipid solubility values of the calcium antagonists were measured by their partition coefficients in oil/Ringer, octanol/Ringer, and chloroform/Ringer systems. The order of lipid solubility was: bepridil greater than verapamil greater than nitrendipine greater than nifedipine much greater than diltiazem. Thus, the calcium antagonists with the highest lipid solubilities were taken up more by the muscle cells and SR. It is concluded that verapamil, bepridil, and nitrendipine enter and accumulate inside the muscle cells, whereas nifedipine and diltiazem do not permeate readily.