A comparison of the effects of verapamil and cinnarizine upon responses elicited by selective alpha 1- and alpha 2-adrenoceptor agonists in the autoperfused canine hindlimb

A difference in responsiveness of isolated hepatic artery, aorta, portal vein and vena cava of pig to flavin adenine dinucleotide including liver extract

1. Tension of isolated hepatic artery, aorta, portal vein and vena cava of pigs was measured isometrically to study the mode of action of flavin adenine dinucleotide, including liver extract (FADLE). 2. FADLE showed concentration-dependent relaxation of norepinephrine contraction in the hepatic artery, but FADLE had little influence on the aorta, portal vein and vena cava contracted by norepinephrine (NE). 3. In Ca(2+)-free solution, FADLE-induced relaxation in both the hepatic artery and aorta contracted by NE was significantly reduced, and its reduction was greater in the hepatic artery than in the aorta. Furthermore, extracellular Ca2+ dependence of norepinephrine-induced contraction was much larger in the hepatic artery than in the aorta. 4. Results suggest that FADLE shows regional variations in vasorelaxation through the alteration of Ca2+ movement by the cell, and that this may reflect one of the mechanisms in its ability to increase blood flow in hepatic tissue.

Nisoldipine inhibits adrenergic responses in the hindquarters vascular bed of the cat

The effects of the calcium entry blocking agent nisoldipine on adrenergic vasoconstrictor responses were investigated in the hindquarters vascular bed of the cat under conditions of controlled blood flow. Nisoldipine dilated the hindquarters vascular bed and inhibited vasoconstrictor responses to Bay K 8644, a nifedipine analog which promotes calcium entry. During infusion of nisoldipine, vasoconstrictor responses to sympathetic nerve stimulation, norepinephrine, and tyramine were inhibited in a reversible manner. In addition to blocking responses to nerve-released and exogenous norepinephrine, the calcium entry antagonist decreased responses to methoxamine and BHT 933, alpha 1- and alpha 2-adrenoceptor agonists. Responses to methoxamine were reduced by prazosin, an alpha 1-adrenoceptor antagonist, but not by yohimbine, an alpha 2-adrenoceptor blocking agent, whereas responses to BHT 933 were decreased by yohimbine but not by prazosin. The results of these studies suggest that vasoconstrictor responses to neuronally released and exogenous norepinephrine, as well as to selective alpha 1- and alpha 2-adrenoceptor agonists, are dependent in part on an extracellular source of calcium in resistance vessels of the feline hindquarters vascular bed. The inhibitory effect of nisoldipine on vasoconstrictor responses to neuronally released norepinephrine may be important in the antihypertensive actions of calcium entry blocking agents.

Inhibition of vasoconstriction to cirazoline by calcium-entry blockade after phenoxybenzamine in rat perfused hindquarters

The effects of phenoxybenzamine and benextramine were assessed with respect to the vasoconstriction to cirazoline in rat perfused hindquarters. Experiments were performed with and without restriction of inward calcium flux by addition of nifedipine (10(-9)-10(-6) M) to the standard physiological solution (PS), or by omission of calcium chloride from the PS. Addition of nifedipine or omission of Ca2+ did not affect the maximal response or potency of the selective but partial alpha 1-adrenoceptor agonist, cirazoline in rat perfused hindquarters. Upon pretreatment with phenoxybenzamine (0.03-30 micrograms/kg, i.v. at -1 h) or benextramine (1 mg/kg, i.v. at -2 h) both the slope and the maximal response to cirazoline were depressed. After phenoxybenzamine but not after benextramine the maximal response to cirazoline was depressed further upon addition of nifedipine or omission of Ca2+ from the PS. It is concluded that phenoxybenzamine selectively inhibits that part of the alpha 1-adrenoceptor mediated vasoconstriction that is independent of extracellular calcium, thereby unmasking a calcium-entry sensitive mechanism of vasoconstriction to cirazoline.

Influence of calcium entry blockade on alpha 1- and alpha 2-adrenoceptor mediated vasoconstriction in the forearm of hypertensive patients

The influence of treatment with the calcium entry blockers PY 108-068 (PY) and PN 200-110 (PN) on alpha 1- and alpha 2-adrenoceptor mediated vasoconstriction has been investigated in the forearms of hypertensive patients. Changes in forearm vascular resistance (FVR) in response to the intra-arterial infusion of drugs were determined at the end of a placebo period and after 2-4 weeks of treatment with PY or PN. The drugs used were the selective agonists methoxamine (alpha 1) and B-HT 933 (alpha 2). During placebo, basal FVR was dose-dependently increased by methoxamine and B-HT 933. Basal blood pressure was lowered during PN but not during PY. Treatment with the calcium entry blockers did not influence the effect of methoxamine, but the vasoconstriction induced by B-HT 933 was attenuated by both of the calcium entry blockers. These results confirm the findings in animal studies that calcium entry blockers preferentially inhibit the alpha 2-adrenoceptor mediated vasoconstriction induced by selective agonists.

A comparison of the ability of various alpha-adrenoreceptor agonists to induce EGTA-resistant contractions of rat aorta

Phenylephrine (PE) and Guanfacine (GU) produced maximum contractions of rat isolated aorta which were 96.8 +/- 1.3% and 96.0 +/- 2.8% respectively of the maximum responses induced by noradrenaline (NA). The responses produced by maximum concentrations of NA following 2 min incubation of the preparations with 3.0 mM EGTA were 38.1 +/- 1.2% of the maxima achieved in normal Krebs. PE produced maximum EGTA-resistant responses which were significantly less than those seen with NA; GU produced EGTA-resistant responses which were significantly less than those to PE. The concentration response curves to NA and PE were shifted to the right by incubation with EGTA. The curve for PE was shifted by a significantly greater degree than that for NA. Amidephrine, UK-14,304 and St-587 produced only nominal EGTA-resistant responses even though one of these (UK-14,304) produced contractions in normal Krebs which were almost 70% of the maximum achieved by NA. The log dose ratio for all agonists (except amidephrine) were similar following the addition of 5 nM prazosin. Amidephrine had no effect in the presence of this concentration of prazosin. It is concluded that all agonists tested were acting on alpha 1-adrenoreceptors. Prazosin (5 nM) but not corynanthine (2.5 microM) slowed the rate of rise of contraction of all agonists tested. It is proposed that the rate of rise of agonist/receptor combinations is an important determinant of alpha-adrenoreceptor agonist induced EGTA-resistant responses in rat aorta.

Characterization of alpha-adrenoreceptor mediated reduction of flow in a saline-perfused constant pressure model of rat perfused hindquarters

An analysis of vasopressor responses elicited by alpha-adrenoreceptor agonists and KCl in a constant pressure saline perfused model of rat perfused hindquarters is presented. Using methoxamine (alpha 1), cirazoline (alpha 1), B-HT 920 (alpha 2) and St 587 (alpha 1) as alpha-adrenoreceptor agonists, and rauwolscine and prazosin as selective alpha 2- and alpha 1-adrenoreceptor antagonists, respectively, it is concluded that alpha-adrenoreceptor mediated vasoconstriction in this model is mediated by the alpha 1-subtype. Pretreatment of the animals with reserpine (i.p., at various doses and dose-regimens) or addition of angiotensin II (10(-9)-10(-6) M), PGF2 alpha(10(-8) & 5 X 10(-7) M) or KCl (2 & 4 X 10(-3) M), or changing [Mg2+] did not significantly affect the potency or intrinsic activity of the alpha-adrenoreceptor agonists studied. Experiments with the calcium entry blocking drugs gallopamil and nifedipine, or deletion of CaCl2 from the perfusion solution, did not reveal a contribution from the influx of extracellular calcium towards alpha 1-adrenoreceptor-mediated reduction of flow in RPH. KCl-induced vasoconstriction in RPH was found to be sensitive to the blockade of calcium entry by calcium entry blocking drugs, but not to alpha-adrenoreceptor-blocking drugs and calmodulin antagonist. It is concluded that the model described here is of potential value in the characterization of alpha-adrenoreceptor antagonists and calcium entry blocking drugs under in vitro conditions in resistance vessels of the rat.

Classification of calcium antagonists

Drugs of several chemical families have been identified as calcium antagonists. This article examines some pharmacologic properties of these drugs to clarify their terminology and their classification and to provide a rationale for their clinical use. Studies with nifedipine show quantitatively that the therapeutic effect in angina is related to the interaction of this drug with membrane calcium channels in human coronary arteries. This gives support to a classification based on studies at the molecular, tissue and organ levels. Among calcium antagonists, calcium entry blockers are defined as agents able to block calcium inward fluxes evoked by various stimuli. They may be subdivided in 2 groups. Group I is the group of selective calcium entry blockers. Group IA consists of those agents selective for slow calcium channels in myocardium (slow channel blockers); the leading agents are verapamil, nifedipine and diltiazem. Group IB contains agents without action on slow calcium channels in myocardium but with selective action on arteries; the leading agents are cinnarizine and flunarizine. Group II is the group of nonselective calcium entry blockers. Group IIA contains agents acting at similar concentration on calcium and on fast sodium channels. Group IIB consists of agents interacting with calcium channels while having another primary site of action. Other agents modulate calcium movements by an action on sodium-calcium exchange and by an action within the cell. Their identification requires the use of cell biology. The actual clinical uses of these drugs are consistent with this pharmacologic classification.

Inhibitory effect of calcium antagonist drugs on vasoconstriction induced by vascular alpha 2-adrenoceptor stimulation

A survey is given of the mechanisms of the antihypertensive effect of calcium entry blockers. The main background of the antihypertensive/hypotensive action is dilatation of precapillary arterioles (resistance vessels that cause a reduction in total peripheral resistance and, hence, a decrease in blood pressure). The vascular relaxation is caused by an inhibition of the transmembranous calcium influx and, probably less so, by interference with calmoduline. Calcium entry blockers significantly reduce the vasoconstriction induced by the excitation of vascular postsynaptic alpha 2 adrenoceptors. The inhibitory effect of calcium entry blockers is reversed by the calcium entry promoter Bay k 8644. The vasoconstriction induced by alpha 1-adrenoceptor stimulation is less generally influenced by calcium entry blockers than the alpha 2 effects. The interference with alpha 2-adrenoceptor-induced vasoconstriction may contribute to the vasodilator action of the calcium entry blockers, especially in hypertensive patients who show a hyperreactivity to pressor responses toward catecholamines.

Diphenylalkylamine calcium antagonists interact with alpha-adrenoceptor binding sites in aortic membranes

Some interactions of calcium antagonists with [3H]prazosin and [3H]yohimbine binding sites were investigated in bovine aorta membranes. Diphenylalkylamines (flunarizine, cinnarizine and bepridil) acted as competitors of the two ligands with Ki values in the microM range. With the exception of verapamil, reference compounds (nifedipine, Bay-K 8644, diltiazem) and the peripheral benzodiazepine receptor antagonist PK 11195 did not displace the ligands. The apparent affinity of the diphenylalkylamines for alpha-adrenoceptor was consistent with the concentrations producing vasodilatation.

Influence of extracellular calcium and nifedipine on alpha 1- and alpha 2-adrenoceptor-mediated contractile responses in isolated rat and cat cerebral and mesenteric arteries

The influence of extracellular Ca2+ and nifedipine on contractile responses to 10 microM noradrenaline (NA) was investigated in isolated rat and cat middle cerebral (RCA, CCA) and mesenteric (RMA, CMA) arteries. In the CCA (containing predominantly alpha 2-adrenoceptors), the NA-induced contractions developed considerably more slowly than in the RCA, RMA (containing mainly alpha 1-adrenoceptors) and CMA (sensitive to both alpha 1- and alpha 2-adrenoceptor selective antagonists). The tonic component of the NA-induced contraction in the four types of artery was substantially suppressed after only short periods in Ca2+-free solution. In each type of artery, excluding the CCA, the contractile response to 124 mM K+ was more sensitive to Ca2+ deprivation than that to NA. This suggests that NA, besides mobilizing extracellular Ca2+, can also release Ca2+ from an intracellular pool in the RCA, RMA and CMA, but not in the CCA. Thus, alpha 1-adrenoceptor-mediated contractions in the RCA and RMA seem to depend on both Ca2+ influx and intracellular Ca2+ release, whereas alpha 2-adrenoceptor-mediated contractile responses in the CCA appear to rely almost entirely on Ca2+ influx. Both the maximum response and the tonic component of the NA-induced contraction were significantly more sensitive to nifedipine in the CCA than in the RCA. In comparison with the NA-induced contractions in these arteries, those in the RMA and CMA were relatively resistant to nifedipine. In the CCA exposed to NA in Ca2+-free medium, nifedipine almost abolished the contraction induced by re-addition of Ca2+, whereas in the other types of artery, Ca2+ re-application evoked a significant contraction also in the presence of the drug. The differential effects of nifedipine presumably reflect differences between the arteries, not only in the relative contribution of Ca2+ influx and intracellular Ca2+ release to the contractile activation, but also in the nifedipine sensitivity of the Ca2+ entry pathways utilized by NA. It is concluded that the mechanisms through which NA induces contraction seem to be related both to the subtype of alpha-adrenoceptor stimulated by NA and to the type of vessel studied.

Preferential antagonism by diltiazem of alpha 2-adrenoceptor mediated vasoconstrictor responses in perfused tail arteries of spontaneous hypertensive rats

Vasoconstrictor responses mediated by the alpha 2-adrenoceptor agonist TL99, were particularly sensitive to blockade by the calcium antagonist drug diltiazem in isolated perfused tail arteries of spontaneously hypertensive rats (SHR). In contrast, the vasoconstrictor responses induced by the alpha 1-adrenoceptor agonist methoxamine were significantly more resistant to antagonism by diltiazem. At higher concentrations (greater than 300 nmol/l) diltiazem became an effective antagonist of all alpha-adrenoceptor mediated responses. In normotensive Wistar Kyoto (WKY) or Sprague-Dawley (SD) rats diltiazem was significantly less potent against vasoconstrictor responses to TL99 than in SHR. The blockade of alpha 1-adrenoceptor mediated vasoconstriction by diltiazem was not significantly different when normotensive rats and SHR were compared. The vasoconstrictor responses evoked by 5HT in the perfused tail arteries were particularly resistant to blockade by diltiazem in SHR arteries. The responses to endogenously released noradrenaline, evoked by electrical field stimulation, were significantly antagonised by diltiazem (30 nmol/1-3 mumol/l) in SHR-tail arteries, while they were not modified in WKY-tail arteries. At the concentrations of diltiazem which blocked end organ responses to field stimulation, there was no modification of total tritium overflow from SHR-tail arteries after labelling the tissue with 3H-noradrenaline, indicating that diltiazem does not inhibit transmitter release at these concentrations. The tail artery preparation of SHR contains a population of postsynaptic alpha 2-adrenoceptors which mediate contraction in this blood vessel and the calcium entry blocker diltiazem is a potent antagonist of vasoconstrictor responses mediated by vascular alpha 2-adrenoceptors in hypertensive rats. These findings may be relevant to the antihypertensive action of diltiazem.

Measurements of 45Ca2+ uptake and contractile responses after activation of postsynaptic alpha 1-adrenoceptors in the isolated canine saphenous vein: effects of calcium entry blockade

The increase in 45Ca2+ content produced by the EC100 concentrations of a series of alpha 1-agonists in the canine saphenous vein (CSV) was determined in the presence of 10(-7) M rauwolscine and found to be correlated (r = 0.92) with the intrinsic activities of the alpha 1-agonists in CSV. The degree of inhibition of 45Ca2+ uptake by nifedipine (1 microM) was inversely correlated (r = -0.97) with the intrinsic activities of the alpha 1-agonists and also inversely correlated (r = -0.95) with the 45Ca2+ uptake induced by the agonists. In the presence of 5 mM LaCl3, there was no significant 45Ca2+ uptake elicited by KCl (80 mM) or activation of postsynaptic alpha 1-adrenoceptors in this tissue, and the alpha 1-agonists were found to have a varied ability to release internal Ca2+ for contractions. It is concluded that (1) activation of alpha 1-adrenoceptors in CSV utilizes both intracellular and extracellular Ca2+ for contractions; (2) the increase in 45Ca2+ content after activation of post-synaptic alpha 1-adrenoceptors in CSV is directly correlated with the intrinsic ability of the alpha 1-agonists to induce contractions; and (3) the sensitivity of the 45Ca2+ uptake to nifedipine is inversely related to the intrinsic ability of the alpha 1-agonists to translocate extracellular Ca2+.

Invariable susceptibility to blockade by nifedipine of vasoconstriction to various alpha 2-adrenoceptor agonists in pithed rats

The sensitivity of the increase in diastolic pressure brought about by the selective agonists of alpha 2-adrenoceptors, B-HT 920, B-HT 933, xylazine, UK-14,304, M-7, TL-99 and DP-6, 7-ADTN in pithed normotensive rats to blockade by the calcium entry inhibitor nifedipine has been investigated. To exclude any participation of vascular alpha 1- and beta 2-adrenoceptors, as well as cardiac beta 1-adrenoceptors, in the pressor responses, the study was made after treatment of the pithed rats with prazosin (0.1 mg kg-1) and (-)-propranol (1 mg kg-1). Without exception, the preferential agonists of alpha 2-adrenoceptors elicited vasoconstrictor responses which were susceptible to inhibition by nifedipine (0.03-1 mg kg-1) in a dose-dependent manner regardless of the differences in intrinsic activity of the compounds. The pressor activity was almost completely abolished after 1 mg kg-1 of nifedipine. The results show that vasoconstriction induced in pithed rats by various selective stimulating agents of postjunctional vascular alpha 2-adrenoceptors is invariably and equally sensitive to attenuation by nifedipine. This susceptibility of alpha 2-adrenoceptor-mediated vasoconstriction to impairment by blockade of calcium entry is not dependent on the nature, the potency or the efficacy of the agonist.

Interactions between the putative calcium entry promotor Bay k 8644 and pressor responses produced by alpha 1- and alpha 2-adrenoceptor agonists in the pithed normotensive rat

Interactions between the putative calcium entry promotor Bay k 8644 and both alpha 1- and alpha 2-adrenoceptor mediated increases in diastolic pressure were studied in the pithed normotensive rat. The alpha 2-adrenoceptor mediated pressor responses elicited by B-HT920, TL-99, DP-6,7-ADTN and B-HT958 were potentiated by Bay k 8644, reflected by a leftward shift and an increase in the maximum of the log dose-pressor response curves. The alpha 1-adrenoceptor-mediated effects elicited by cirazoline, methoxamine, (-)-amidephrine, St 587, (-)-phenylephrine and Sgd 101/75 were less enhanced by Bay k 8644. Only a leftward shift of the dose-response curves was observed, which was most pronounced for (-)-phenylephrine and Sgd 101/75. The alpha 1- and alpha 2-adrenoceptor-mediated pressor components of (-)-noradrenaline were similarly distinguished by Bay k 8644 as observed for the selective alpha 1- or alpha 2-adrenoceptor agonists. Effects of Bay k 8644 on the increase in diastolic pressure mediated by B-HT 920, St 587 and cirazoline were also studied after pretreatment with the calcium entry blocker nifedipine. After additional pretreatment with nifedipine the potentiation by Bay k 8644 observed for B-HT 920 and St 587 was more pronounced. The presence of nifedipine had no effect on the interaction between Bay k 8644 and cirazoline. It is concluded that Bay k 8644 behaves as a mirror image of nifedipine.+

Lack of relationship between intrinsic activity and susceptibility of pressor responses to blockade by nifedipine among the alpha 2-adrenoceptor agonists B-HT 920 and B-HT 958

Following i.v. bolus injections into pithed normotensive rats, the maximal diastolic pressor responses to B-HT 920 and B-HT 958 amounted to 115 and 35 mm Hg, respectively. Prazosin (0.1 mg/kg, i.v., -15 min) was without effect on the log dose-pressor effect curve of B-HT 958, whereas yohimbine (1 mg/kg, i.v., -15 min) shifted this curve about 30-fold to the right, showing the exclusive participation of alpha 2-adrenoceptors in the vasoconstrictor response to B-HT 958. In doses of 10 and 30 mg/kg, B-HT 958 displaced the log dose-vasoconstrictor effect curve of B-HT 920 approximately 6- and 30-fold, respectively, to the right, illustrating the partial agonism of B-HT 958 at postjunctional vascular alpha 2-adrenoceptors. Despite the marked difference in intrinsic activity of B-HT 920 and B-HT 958, the calcium entry blocker nifedipine exhibited a comparable inhibitory action on the vasopressor responses to both agonists. This finding indicates that partial and full agonism at vascular alpha 2-adrenoceptors are not related to the susceptibility of the initiated pressor response to inhibition by calcium entry blockade.

Interference of calcium entry blockade in vivo with pressor responses to alpha-adrenergic stimulation: effects of two unrelated blockers on responses to both exogenous and endogenously released norepinephrine

To the extent that calcium availability is the final common mediator of vasoconstrictor responses, calcium entry blockade might interfere with physiologic responses to adrenergic stimulation. To test this hypothesis, we studied the effects of calcium entry blockade on pressor responses to norepinephrine in pithed, normal Sprague-Dawley rats in two different ways: (1) by evaluating the effects on pressor responsiveness to exogenous norepinephrine during differential blockade of alpha 1-(prazosin, 0.3 mg/kg) and of alpha 2-receptors (yohimbine, 0.3 mg/kg) and (2) by comparing the effects of calcium entry blockade with those of prazosin and those of rauwolscine (a specific alpha 2-antagonist) on pressor responses to infusions of both endogenously released norepinephrine (electrical stimulation of the pithing rod) and exogenous norepinephrine. In the presence of alpha 1-blockade, both nitrendipine (0.01 mg/kg) and verapamil (0.6 mg/kg) shifted the norepinephrine pressor dose-response curve to the right but were ineffective in alpha 2-blocked animals. Furthermore, nitrendipine (range 0.01 to 0.3 mg/kg) proved to be more effective (p less than .001) against exogenous norepinephrine than against electrical stimulation of the spinal cord, a behavior opposite that of selective alpha 1-blockade (prazosin) and directionally comparable to that of selective alpha 2-antagonism (rauwolscine). These data indicate that calcium entry blockade in vivo preferentially antagonizes the alpha 2-pressor component of exogenous norepinephrine. In addition, both calcium entry blockers were consistently more active (p less than .01) than rauwolscine (0.01 to 1 mg/kg) in antagonizing the pressor response to neural stimulation, suggesting that mechanisms different from "classical" alpha 2-antagonism may also contribute to the overall effect of calcium entry blockade on the adrenergic control of peripheral vascular tone.

Differential effect of calcium entry blockers on alpha 1-adrenoceptor-mediated vasoconstriction in vivo

The effects of the calcium entry blockers nifedipine, (-)-verapamil and the dihydropyridine derivative PY 108-068 were evaluated on the increase in diastolic pressure of pithed normotensive rats caused by the selective alpha 1-adrenoceptor agonists cirazoline, (-)-phenylephrine, (+/-)-erythro-methoxamine, (-)-amidephrine and St 587 [(2-chloro-5-trifluoromethylphenylimino)-2-imidazolidine] as well as by the mixed alpha 1/alpha 2-adrenoceptor agonists clonidine and DPI [(3,4-dihydroxyphenylimino)-2-imidazolidine]. The calcium entry inhibitors (up to 3 mg/kg) caused 3- to 5-fold, parallel rightward shifts of the log dose-pressor effect curves to cirazoline, (-)-phenylephrine, (+/-)-erythro-methoxamine and (-)-amidephrine accompanied by only a slight depression of the maximal pressor response. In contrast, the calcium entry inhibitors produced a dose-dependent profound depression of both maximum and slope of the log dose-pressor response curves to St 587 and clonidine. For DPI about 10- and 100-fold parallel displacements to the right without reduction of the maximum were found following treatment with 1 and 3 mg/kg of nifedipine, respectively. Infusion of vasopressin to counteract the vasodilatory action produced by the calcium entry inhibitors did not significantly change the pattern of interference observed under the conditions of decreased baseline diastolic pressure. The results indicate that alpha 1-adrenoceptor-mediated vasoconstriction in the pithed normotensive rat, which is characterized by its sensitivity to blockade by prazosin and its relative insensitivity to antagonism by yohimbine or rauwolscine, can be subdivided into two distinct processes which are differentially influenced by blockade of calcium entry.(ABSTRACT TRUNCATED AT 250 WORDS)

Attenuation of sympathetic vasoconstriction by nifedipine: the role of vascular alpha 2-adrenoceptors

The effects of nifedipine on hindlimb vasoconstriction caused by norepinephrine infusion and sympathetic stimulation (0.1-1.0 Hz) were compared in dogs given 0.12 mg/kg prazosin or 0.3 mg/kg rauwolscine. Constrictions due to stimulation or norephinephrine with prazosin, presumed to be mediated by vascular alpha 2-adrenoceptors, were significantly attenuated by 30 micrograms/kg nifedipine, while constrictions with rauwolscine, presumably alpha 1-mediated, remained unaffected. These data support the hypothesis that the antihypertensive effect of calcium antagonists is based upon interference with alpha 2-mediated sympathetic vasoconstriction.

Calcium entry blocking drugs, 'calcium antagonists' and vascular smooth muscle function

The group of drugs known as "calcium antagonists' is under extensive investigation in experimental animals and man and a re-evaluation of their pharmacological properties is overdue. Recent proposals to adopt the more specific nomenclature of calcium entry blockers for some of these compounds (Vanhoutte & Bohr, 1981) should be supported since there is much confusion in the literature with this class of compound. In this review, which concentrates on vascular smooth muscle, only nifedipine, verapamil, their close chemical analogues and diltiazem are recognised as being relatively selective calcium entry blocking drugs. Whilst definitive evidence for calcium entry blockade must include the demonstration of a selective inhibition of Ca2+-influx into a tissue over a range of concentrations also inhibiting contraction, it is nevertheless possible to define several simple pharmacological criteria which may aid in the identification of such activity. These criteria include the selective antagonism of K+ and Ca2+-induced contractions, relative to those of noradrenaline in suitable vascular smooth muscle preparations and a selective inhibition of alpha 2- as opposed to alpha 1-adrenoreceptor mediated pressor responses in, for example, pithed rat preparations. Recent pharmacological and biochemical studies have identified 3 major subgroups of "calcium antagonist' drugs but the compounds within each subgroup varies with the technique adopted. It is therefore suggested that a combination of both pharmacological and ligand-binding studies be used for purposes of classification. Which mechanism, if any, of inhibiting calcium entry is therapeutically most desirable remains an important question for future research.