Tissue selectivity of the novel calcium antagonist sesamodil fumarate in isolated smooth muscles and cardiac muscles

The effects of the novel Ca2+ antagonist sesamodil fumarate (JAN), (+)-3,4-dihydro-2-[5-methoxy-2-[3-[N-methyl-N-[2-[(3,4- methylenedioxy)phenoxy]ethyl]amino]propoxy]phenyl]-4-methyl-3-oxo-2H- 1,4-benzothiazine hydrogen fumarate (SD-3211), on isolated smooth muscles and cardiac muscles were investigated and compared with those of diltiazem, verapamil, nifedipine and nicardipine. Ca2+ antagonistic activity of SD-3211 (pA2 = 8.42) was more potent than that of diltiazem and verapamil, but less potent than that of nifedipine and nicardipine in isolated pig coronary artery. The inhibition of Ca2(+)-induced contraction by SD-3211 was not reversed by drug washout, whereas that by diltiazem was easily reversed by drug washout. SD-3211 produced a concentration-dependent relaxation (EC50 = 5.7 x 10(-8) mol/l) of KCl-contracted pig coronary artery. The order of potency of the various compounds correlated with their respective Ca2+ antagonistic activities. SD-3211 antagonized KCl-induced contraction, but not that induced by A23187, in the rabbit aorta. On the other hand, negative inotropic and chronotropic effects of SD-3211 on the guinea pig right atria approximated those of diltiazem and verapamil. These results suggest that SD-3211 exerts a potent and long-lasting Ca2+ antagonistic effect on isolated arteries, possessing pharmacological properties diverse from those of known Ca2+ antagonists with respect to tissue selectivity, i.e., it is more vasoselective than diltiazem and verapamil, and more cardioselective than nifedipine and nicardipine.     

Effect of S-312, a new calcium channel blocker, on the 1,4-dihydropyridine binding sites in porcine basilar blood vessels and rat aortic smooth muscle cells.

We examined the interaction of two isomers of S-312, a new calcium channel blocker with a bicyclic dihydrothienopyridine structure, with 1,4-dihydropyridine binding sites. Specific bindings of [3H]nitrendipine and (+)-[3H] PN200-110 in membranes prepared from porcine basilar blood vessels were saturable, reversible, and stereoselective, and had high affinities. The binding properties were very similar to those in membranes from other tissues such as the aorta, myocardium, and cerebral cortex. 1,4-Dihydropyridine calcium channel blockers competed for each radioligand binding in the order of: nisoldipine = nicardipine = S-(+)-S-312 greater than nifedipine greater than R-(-)-S-312. S-(+)-S-312 caused a decrease in the Kd values for both radioligands without changing the maximal binding capacity. 1,4-Dihydropyridines inhibited the high K(+)-induced increase in cytosolic free Ca2+ concentration in rat aortic smooth muscle A7r5 cells. S-(+)-S-312 was 3.3-4.9 times more potent than nicardipine or nisoldipine in inhibiting the Ca2+ increase, although S-(+)-S-312 bound to A7r5 cells with almost the same affinity. These and earlier findings show that S-(+)-S-312 exerts effects more potent than expected from the affinity for [3H]nitrendipine or (+)-[3H]PN200-110 binding sites. This was the case with R-(-)-S-312. These dihydrothienopyridine derivatives appear to interact with the Ca2+ channel in a manner slightly different from the conventional 1,4-dihydropyridines.     

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)     

In vitro effects of nicardipine on vascular and cardiac muscle preparations

Nicardipine, a new 1,4-dihydropyridine derivative with Ca++-antagonist activity, was tested on rabbit aortic strips contracted with K+ and NA, and on an isolated guinea pig heart perfused at constant flow. It inhibited K+-contraction in the aortic strips and reduced coronary resistance with the same potency as nifedipine resulting about 10 time less potent than the latter in depressing the myocardial contractility. The effects of nicardipine appear to be related essentially to its Ca++-antagonist activity rather than to any antiphosphodiesterase activity.     

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)     

Pharmacology of BMY 20064, a potent Ca2+ entry blocker and selective alpha 1-adrenoceptor antagonist

BMY 20064 is a dihydropyridine Ca2+ entry blocker with potent and selective alpha 1-adrenoceptor antagonist properties. The drug was equal in potency to nifedipine as a Ca2+ entry blocker in depolarized smooth muscle preparations. It was less active than nifedipine in antagonizing Ca2+-induced contractions of isolated guinea pig papillary muscles paced at 0.2, 1.0, or 2 Hz. BMY 20064 was a potent (0.1-0.2 X prazosin) and selective alpha 1-adrenoceptor antagonist in radioligand binding assays and in ganglion-blocked anesthetized rats challenged with phenylephrine. BMY 20064 blocked both the K+ and alpha 1-adrenergic agonist-induced increases in 45Ca uptake into rabbit aortic rings. The drug was more effective than nifedipine, prazosin, or combinations of the drugs in preventing ATP depletion of the rat heart during global ischemia. BMY 20064 was a potent antihypertensive agent in normotensive rats and in SHR. BMY 20064 administered intraarterially (i.a.) dilated both femoral and coronary arterial beds of the dog. Hemodynamic changes elicited by BMY 20064 in anesthetized dogs were similar to those induced by nifedipine. BMY 20064 appears to be a more effective myocardial antiischemic agent than nifedipine, prazosin, or combinations of nifedipine and prazosin. A drug of this type may be more efficacious than dihydropyridines in the management of ischemic episodes.     

Effects of felodipine on vascular smooth muscle in comparison with nifedipine.

Felodipine (ethylmethyl 4-(2,3-dichlorophenyl)-1,4-dihydro-2,6-dimethyl- 3,5-pyridine dicarboxylate, CAS 72509-76-3), a vasoselective calcium antagonist, has a slow onset inhibitory effect on high K(+)-induced contractions in vascular smooth muscle and a longer duration than that of nifedipine. In addition it non-competitively inhibits Ca(++)-induced contraction in the depolarized aorta or femoral artery in the rat. Felodipine's inhibitory effect on caffeine or norepinephrine-induced contraction was observed at a micromolar range. This result suggests that felodipine may inhibity Ca++ release from intracellular Ca++ stores through a mechanism of Ca++ or inositol 1,4,5-triphosphate induced Ca++ release in addition to a blockade of Ca++ influx through the sarcolemma.     

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.     

Vascular and myocardial effects of mecinarone

The effects of mecinarone on the contractions induced by noradrenaline, potassium and calcium were studied with the isolated aorta and central artery of the rabbit ear. In our experimental conditions mecinarone was found to act as a competitive antagonist of calcium. It preferentially inhibited the noradrenaline-evoked contraction induced in the rabbit ear artery and had a smaller effect on the extracellular calcium-induced contraction of depolarized arterial smooth muscle. In the isolated guinea pig heart, mecinarone produced a reversible decrease in the rate of rise of the ascendant phase of the action potential without significantly changing the resting potential. In hearts inactivated by depolarisation, mecinarone inhibited the contractions restored by isoproterenol without suppressing the calcium action potential. These results suggest that the mode of action of mecinarone is different from that of verapamil or nifedipine but is comparable to that of chlorpromazine, although mecinarone has no sedative effect.     

Calcium antagonistic properties of nicardipine, a dihydropyridine derivative assessed in isolated cerebral arteries and cardiac muscle

Calcium antagonistic properties of nicardipine (YC-93), a 1,4-dihydropyridine derivative, were studied in isolated cerebral artery and cardiac tissues of rabbits and dogs. Calcium antagonism was assessed in electrically stimulated rabbit basilar artery. Nicardipine at 10(-7) mol/l shifted to the right the dose-response curve for Ca of the phasic contraction evoked by electrical stimulation with an alternating current, and at higher concentration it reduced the maximum tension and slope of the dose-response curve. Nicardipine inhibited the augmented contraction produced by high K, k-strophanthin and 5-hydroxytryptamine. It also caused dose-dependent inhibition of 45Ca uptake enhanced by 80 mmol/l K in dog basilar artery, and had a slight effect on the resting 45Ca uptake. In rabbit basilar artery treated with saponin, nicardipine in a dose of 10(-5) mol/l had no apparent effect on the increase in tension induced by excess Ca. Nicardipine had negative inotropic and chronotropic actions. These results suggest that nicardipine, like nifedipine, has a relative high vascular selectivity, and primarily acts by inhibiting Ca influx through the plasma membrane of vascular and cardiac tissues.     

The activity of nifedipine,diltiazem, verapamil,and lidoflazine in isolated tissues: An approach to the determination of calcium channel blocking activity

A characteristic profile of activity was obtained in six isolated tissues for the calcium channel antagonists nifedipine, diltiazem, verapamil, and lidoflazine. All drugs produced relaxation of K⁺ depolarized guinea pig ileal longitudinal muscle strips and K⁺ depolarized canine coronary artery, depression of electrically stimulated basal contractions of guinea pig left atria, and depression of guinea pig right atrial rate. Also, all drugs produced parallel dextral displacement of concentration-response curves to calcium in guinea pig depolarized taenia caeci. The potency for this effect was quantified by Schild analysis yielding the following pA₂ estimates: nifedipine 9.5, diltiazem 7.65, verapamil 7.8, and lidoflazine 7.0. Nifedipine, diltiazem, and lidoflazine produced no relaxation of methoxamine-contracted rabbit aortae while weak effects were observed with verapamil at concentrations 100 times greater than those required to reverse calcium effects in other tissues. In general, nifedipine and diltiazem displayed selectivity for smooth muscle over cardiac muscle while verapamil showed the least selectivity in this regard.     

Inhibition of thromboxane A2-induced vasocontraction by KF4939, a new anti-platelet agent, in rabbit mesenteric and dog coronary arteries

The effect of a new anti-platelet agent, KF4939, on thromboxane A2 (TXA2)-induced vasocontraction was studied in superfused rabbit mesenteric and dog coronary arteries, in comparison with the effects on the contractions evoked by KCl, noradrenaline, serotonin, angiotensin II and histamine. The calcium sources involved in the TXA2-induced vasocontraction were also examined. The TXA2-induced contraction of the rabbit mesenteric artery was partly attenuated after exposure to the calcium-free medium, but was not attenuated by nifedipine. The TXA2-induced contraction of the dog coronary artery was markedly attenuated by nifedipine. These results indicate that TXA2 utilizes both intracellularly stored calcium and an extracellular source of calcium for its vasocontraction, and the voltage-dependent calcium channel plays an important role in the dog coronary artery, but in the rabbit mesenteric artery. KF4939 inhibited the TXA2-induced contraction in both arteries. In the rabbit mesenteric artery, three times and more higher concentration than that to inhibit TXA2-induced one were required to inhibit other agonist induced contractions, KF4939 caused no alteration in the KCl-induced contraction of both arteries. Thus, KF4939 seems to be a selective inhibitor of TXA2-induced vasocontraction, and the receptor-linked mechanism may be a possible site of the TXA2 antagonistic action of KF4939.     

Selective opioid receptor agonist and antagonist displacement of [3H]naloxone binding in amphibian brain

Vascular responses to endothelin peptides have been proposed to be mainly mediated via subtypes of the endothelin receptor, endothelin ET_A1, endothelin ET_B1, and endothelin ET_B2. The antagonist activity of 27-O-3-[2-(3-carboxy-acryloylamino)-5-hydroxyphenyl]acryloyloxy myricerone, sodium salt (S-0139) at these endothelin receptor subtypes was evaluated using isolated rabbit femoral, pulmonary, and mesenteric arteries. S-0139 competitively antagonized the endothelin-1-induced contraction mediated by the endothelin ET_A1 receptor in endothelium-denuded rabbit femoral arteries with a pA₂ value of 8.6±0.1. Endothelin ET_B2 receptor-mediated contraction induced by sarafotoxin S6c in endothelium-denuded rabbit pulmonary arteries was also inhibited by S-0139 with a pA₂ value of 5.6±0.1. The pA₂ value of S-0139 for the endothelin ET_B1 receptor, evaluated from the endothelin-3-induced relaxant response in endothelium-intact rabbit mesenteric arteries, was 6.2±0.2. In isolated canine basilar, coronary, mesenteric and renal arteries, endothelin-1 caused concentration-dependent contractions with EC₅₀ values of 0.49±0.07, 0.61±0.25, 0.92±0.21 and 1.18±0.24 nM, respectively. S-0139 antagonized the endothelin-1-induced contraction in these arteries with pA₂ values of 8.0±0.1, 7.6±0.2, 7.6±0.2 and 7.6±0.1, respectively. These results suggest that S-0139 is a potent and selective endothelin ET_A1 receptor antagonist, and that the contractions induced by endothelin-1 in canine basilar, coronary, mesenteric and renal arteries are mediated mainly via the endothelin ET_A1 receptor subtype.     

Comparison of vasoconstrictor actions of endothelin-1 in cerebral, coronary, and mesenteric arteries of the dog.

Vasoconstrictor actions of endothelin-1 (ET) were compared between endothelium-removed strips of cerebral (basilar, posterior cerebral, and middle cerebral) and peripheral (coronary and mesenteric) arteries of the dog. ET produced a concentration-dependent contraction in these arteries. A threshold concentration and EC50 value for ET were significantly lower in the basilar, posterior cerebral, middle cerebral, and coronary arteries than in the mesenteric artery. In the basilar artery, nifedipine caused a rightward displacement of the concentration-response curve for ET with a significant reduction in the maximum response to ET. On the other hand, nifedipine showed a typical noncompetitive antagonism against ET in the mesenteric artery. Contractile responses of the mesenteric artery to ET determined under an elevation of extracellular K+ concentration were comparable to the responses of the basilar artery to this peptide determined under normal K+ concentrations. The cerebral and coronary arteries, but not the mesenteric artery, relaxed significantly from the resting level when placed in a Ca(2+)-free solution containing 0.1 mM EGTA (0-Ca solution). The readdition of Ca2+ to the cerebral and coronary arteries soaked in the 0-Ca solution caused a biphasic contraction that was susceptible to inhibition by nifedipine. When ET in concentrations below 10(-9) M was introduced before the Ca(2+)-induced contraction, this peptide produced no detectable contraction, but potentiated the Ca(2+)-induced contraction. The extent of potentiation induced by ET was much greater in the cerebral and coronary arteries than in the mesenteric artery. Even in the 0-Ca solution, higher concentrations of ET (1 x 10(-8) and 3 x 10(-8) M) produced a contraction that was weaker in the basilar artery than in the mesenteric artery. These results indicate that the cerebral and coronary arteries exhibited more potent contractions in response to lower concentrations (below 10(-9) M) of ET than the mesenteric artery. A likely possibility for these enhanced responses to ET in the cerebral and coronary arteries appears to be that the voltage-dependent Ca2+ channels in these arteries are more activated in the resting state than those in the mesenteric artery.     

Calcium antagonistic and spasmolytic activities of a new 1,5-benzothiazepine derivative in isolated canine and monkey arteries

Effects of TA-3090 ((+) (2S,3S)-3-acetoxy-8-chloro-5-(2-(dimethylamino)ethyl)-2, 3-dihydro-2-(4-methoxyphenyl)-1,5-benzothiazepin-4-(5H)-one maleate)and diltiazem on contractions induced by different spasmogens were investigated in isolated canine and monkey arteries. Ca2+-antagonistic action in canine arteries, assessed by suppression of Ca2+-induced contraction in Ca2+-free, K+-depolarizing solution, was as follows; basilar (pA2 = 8.34) greater than coronary (pA2 = 7.95) greater than renal (pA2 = 7.46) = mesenteric artery (pA2 = 7.36). The potency of TA-3090 was 10 times greater in basilar artery and 2 to 3 times greater in the other arteries than that of diltiazem. The effect of TA-3090 on the arterial segment was more persistent than that of diltiazem. Relative vasorelaxing potency of TA-3090 to diltiazem in K+-induced contractions was greatest in the basilar artery among the tested arteries of both monkeys and dogs. Spasmolytic activities of TA-3090 on 5-HT-, PGF2 alpha-, U-46619 (thromboxane A2/prostaglandin H2 agonist) and oxyhemoglobin-induced contractions in canine basilar arteries were more potent than those of diltiazem, especially on 5-HT-induced contraction. In addition, TA-3090 suppressed 3,4-diaminopyridine-induced rhythmic contraction in the canine coronary artery. These results indicate that TA-3090 has potent Ca2+-antagonistic and spasmolytic activities, and these actions are most selective for basilar artery.     

Calcium channel blocking properties of amlodipine in vascular smooth muscle and cardiac muscle in vitro: evidence for voltage modulation of vascular dihydropyridine receptors

Amlodipine was twice as potent as nifedipine at inhibiting Ca2+-induced contractions in depolarised rat aorta (IC50 1.9 nM vs. 4.1 nM) but, unlike nifedipine, displayed a very slow onset of action. Contractions induced by depolarising steps with 45 mM K+ were much less potently blocked by amlodipine (IC50 19.4 nM), whereas the potency of nifedipine was little changed (IC50 7.1 nM). This difference may be explained by a modulated receptor hypothesis, similar to that described for cardiac muscle, in which block of vascular calcium channels by dihydropyridines is enhanced at depolarized membrane potentials, such voltage-dependence only being apparent with a slow-acting drug such as amlodipine. Recovery from amlodipine block of K+-responses in rat portal vein after drug washout was also very slow. Amlodipine and nifedipine blocked phenylephrine-induced contractions of the rat aorta with potencies similar to those against depolarisation-induced responses. Negative inotropic potencies of amlodipine and nifedipine in perfused guinea pig hearts were approximately one-tenth those against Ca2+-induced contractions in rat aorta. Amlodipine caused complete block of guinea pig papillary muscle single-cell slow action potentials at a concentration (5 microM) that had no effect on upstroke velocity of normal, fast potentials but reduced the duration of the plateau phase.     

Antihypertensive activity of OPC-13340, a new potent and long-acting dihydropyridine calcium antagonist, in rats.

The antihypertensive action of OPC-13340, a new dihydropyridine, was studied in rats and compared with the action of nicardipine and other dihydropyridines. OPC-13340 showed more potent and longer hypotensive action than nicardipine when administered either intravenously (i.v.) or orally in normotensive and hypertensive rats. Among 6 compounds tested, (OPC-13340, nifedipine, nitrendipine, nisoldipine, nicardipine and diltiazem), OPC-13340 was the most potent and long-acting when administered orally to spontaneously hypertensive rats (SHR). Tachycardia after administration of OPC-13340 was less or diminished earlier than that of nicardipine. Oral administration of OPC-13340 (3 mg/kg) once daily for 13 days did not cause any rebound phenomena in SHR. The compound inhibited Ca- or K-induced contractions in isolated rat aorta and shortened action potential duration in guinea pig papillary muscle, suggesting Ca channel blocking action. OPC-13340 might be useful as a drug for once-daily therapy of essential hypertension.     

Receptors for substance P. I. The pharmacological preparations

The undecapeptide substance P (SP) is both a potent stimulant of intestinal (guinea pig ileum), vascular venous (rabbit mesenteric vein) and tracheal (guinea pig trachea) smooth muscles, and a relaxant of large arteries (dog common carotid artery, rabbit aorta). SP also decreases the peripheral vascular resistance in the coronary vessels of rabbit isolated hearts. The contractions of the guinea pig ileum in response to SP are partially antagonised by atropine and significantly depressed by indomethacin, while the contractions of the guinea pig trachea are strongly potentiated by indomethacin. Large arteries, but not the veins, are insensitive to SP when the endothelium is removed, and slightly less responsive to the peptide when this is applied in the presence of mepacrine, eicosatetraynoid acid or BW 755C. These results indicate that all myotropic actions of SP, except that on venous smooth muscle, are partially or completely indirect and most of them are due to the release of endogenous agents: these are acetylcholine, prostaglandins and possibly histamine in the guinea pig ileum, prostaglandins and possibly a leukotriene in the guinea pig trachea and a still unidentified factor of endothelial origin in dog and rabbit large arteries. The effect of substance P on the rabbit isolated anterior mesenteric vein appears to be a direct one.     

A study on the hypotensive mechanism of pinacidil: relationship between its vasodilating effect and intracellular Ca2+ levels

The direct effect of pinacidil on contractile protein systems and the relationship between its vasodilating effects and intracellular Ca2+ concentration were studied in isolated smooth muscle of guinea pigs. Pinacidil, nifedipine and hydralazine did not inhibit Ca2+-induced contracture in saponin-treated taenia caecum of guinea pig, while trifluoperazine and W-7 markedly suppressed it. Simultaneous determination of the intracellular Ca2+ concentration (fura 2 method) and tension development of the isolated femoral artery of guinea pigs showed that the intracellular Ca2+ level always decreased before vasorelaxation after pinacidil or nifedipine administration. Pinacidil showed no effect on cyclic AMP or cyclic GMP contents in rabbit aortic strips. These results indicate that pinacidil, like nifedipine and hydralazine, do not seem to directly act on the process from the formation of Ca2+-calmodulin to the actinmyosin interaction in the contracting mechanism of vascular smooth muscle, and that pinacidil relaxes the vascular smooth muscle by decreasing intracellular Ca2+-levels without elevating cyclic nucleotides.     

Calcium antagonist properties of diclofurime isomers. I. Functional aspects

Trans-diclofurime has been shown to be a potent calcium antagonist which resembles verapamil in vitro and in vivo. Trans-diclofurime was a potent antagonist of Ca2+-induced contractions in K+-depolarized taenia preparations from the guinea pig caecum (pA2 = 8.3 +/- 0.2), whereas the cis isomer was 50 times less active (pA2 = 6.6 +/- 0.1); the inhibitory effects of trans-diclofurime were reversed noncompetitively by the Ca2+ channel activator Bay K 8644. Trans-diclofurime and verapamil were equipotent inhibitors of electrically evoked contractions of guinea pig left atria preparations; the inhibitory effects were frequency dependent and cis-diclofurime was 10 times less effective. Both diclofurime isomers prolonged the effective refractory period at high concentrations, indicating that they also possess local anaesthetic properties. Trans-diclofurime and verapamil reduced blood pressure in pithed rats infused with angiotensin II. Hypotensive effects were accompanied by bradycardia and prolongation of PR intervals, leading to second-degree atrioventricular block. The cis isomer was less potent. Diclofurime is thus a very potent calcium antagonist in heart and smooth muscle and has some additional membrane-stabilizing properties.