Effects Of Calcium Entry Blockers Research Articles

The blocking of capacitative calcium entry by 2-aminoethyl diphenylborate (2-APB) and carboxyamidotriazole (CAI) inhibits proliferation in Hep G2 and Huh-7 human hepatoma cells

Calcium entry is a component of the processes regulating the proliferative phenotype of some types of cancer. In non-excitable cells, capacitative calcium entry (CCE) and non-capacitative calcium entry (NCCE) are thought to be the main pathways of Ca 2+ influx into cells. Thus, blocking calcium entry may prevent normal and pathological cell proliferation and there is evidence to suggest that molecules blocking calcium entry also have antiproliferative properties. Carboxyamidotriazole (CAI), a novel inhibitor of the non-voltage-dependent calcium entry has been shown to have such properties in model systems in vitro and in vivo. We used Hep G2 and Huh-7 human hepatoma cells to investigate the effects of calcium entry blockers on cell proliferation. CAI (10 μM) and 2-APB (20 μM) completely blocked CCE in thapsigargin-treated Huh-7, and CAI and 2-APB inhibited cell proliferation with IC 50 of 4.5 and 43 μM, respectively. The plateau phase of the [Ca 2+] i increases triggered by 10% FCS were abolished in the absence of external Ca 2+ and in the presence of CAI or 2-APB. We, therefore, suggest that CCE is the main pathway involved in regulation of the processes leading to cell proliferation.

Influence of depolarization on vasorelaxant potency and efficacy of Ca2+ entry blockers, K+ channel openers, nitrate derivatives, salbutamol and papaverine in rat aortic rings.

We examined the effects of various KCl concentrations on the actions of some vasodilators belonging to different pharmacological classes in rat aortic rings. In some experiments, tissues were precontracted with noradrenaline after blocking voltage-dependent channels to assess the effects of depolarisation unaccompanied by the entry of extracellular Ca2+ into the cytosol. Concentration/response curves for the vasorelaxant effect of calcium entry blockers (e.g. diltiazem), K+ channel openers (e.g. aprikalim), nitrate derivatives (e.g. nitroglycerin), a beta2-adrenergic agonist (salbutamol) and papaverine were obtained by using endothelium-denuded rat aortic rings precontracted with KC1 (20-60 mM) to determine the potencies and efficacies of the drugs. The efficacies and potencies of calcium entry inhibitors were virtually independent of the [KCl]. A reduction in the potency (up to 18-fold) of papaverine occurred without changes in efficacy when the [KCl] was raised from 20 to 60 mM. The decline in potency was even greater for nitrate-like compounds. The potency of K+ channel openers in aortic rings precontracted with 30 mM KCI decreased by three- to sixfold compared with those precontracted with 20 mM KCl. With the exception of pinacidil, the efficacy of these agents already started to decline in preparations precontracted with 25 mM KCI and was virtually zero in preparations precontracted with 60 mM KCI. In contrast to other K+ channel openers, the vasorelaxant action of pinacidil was relatively resistant to glibenclamide. Salbutamol produced only a slight relaxation even in preparations precontracted with 20 mM KCl. In nitrendipine-pretreated, noradrenaline-precontracted aortic rings, the vasorelaxant effects of aprikalim, but not those of linsidomine or papaverine, declined when the [KCl] of the bathing medium was increased. In conclusion, the vasorelaxant potency and efficacy of calcium entry blockers is independent of the [KCI] used to precontract rat aortic rings, and thus, of the degree of membrane depolarisation. In contrast, increasing the [KCl] strongly reduces the potency and the efficacy of K+ channel openers not only in this preparation but also in noradrenaline-precontracted rings in which the entry of extracellular Ca2+ was prevented with nitrendipine. This indicates that, with the exception of pinacidil, the vasorelaxant activity of K+ channel openers depends on the degree of membrane depolarisation. Finally, the vasorelaxant potency and efficacy of nitrate-like compounds and papaverine are independent of depolarisation per se but they are markedly affected by the influx of Ca2+ accompanying elevated [KCI]. Thus, the degree of vessel depolarisation should be taken into consideration when attempting to compare potencies and efficacies among vasorelaxant agents.

Calcium entry blockers increase interleukin-10 production in endotoxemia.

Intracellular calcium is an important mediator of the cellular response in endotoxemia and shock. Here we investigated the effects of verapamil and diltiazem, two calcium entry blockers, on endotoxin (bacterial lipopolysaccharide, LPS)-induced production of pro- and anti-inflammatory cytokines and of nitric oxide in mice. LPS-induced interleukin-10 plasma levels were significantly enhanced, and circulating tumor necrosis factor-alpha concentrations were significantly suppressed in animals pretreated intraperitoneally with verapamil (10 mg/kg) or diltiazem (20 mg/kg). However, LPS-induced interleukin-6 levels were unaffected by the calcium antagonists. Similarly, LPS-induced production of nitrite/nitrate (breakdown products of nitric oxide) was not affected by verapamil and diltiazem. We conclude that calcium entry blockers; selectively modulate the production of some pro- and anti-inflammatory mediators in endotoxemia. These effects may contribute to the cytoprotective and anti-inflammatory effects of calcium entry blockers in shock and trauma.

Rhythmic contractions of isolated small arteries from rat: role of calcium.

In order to investigate the mechanisms behind rhythmic contractions in small arteries of the mesenteric arcade from Wistar rats, the calcium dependency of the oscillations in response to noradrenaline activation was tested on isolated vessels. Application of 1 microM ryanodine or 30 microM TMB-8 (procedures known to inhibit Ca2+ release from intracellular stores) totally abolished the rhythmic activity, even though the antagonists had opposite effects on the amplitude of the contractile response to noradrenaline. Verapamil (1 microM) or felodipine (1 nM) (agents known to inhibit influx of extracellular Ca2+) also abolished the oscillations and reduced the maximal noradrenaline response by about 40%. Reducing the extracellular Ca2+ concentration to 0.1 mM reduced the amplitude of the noradrenaline response to a similar extent as 1 nM felodipine, but did not eliminate the oscillations. This may indicate that the effect of calcium entry blockers was to eliminate the voltage-dependency of Ca2+ inflow rather than just reducing the Ca2+ level. Manoeuvres that would increase the cytosolic Ca2+ concentration (exposure to caffeine or to the calcium agonist BAY-K 8644) increased the frequency of the oscillations. These observations indicate an important role, not only for voltage-operating channels, but also for intracellular calcium stores in the generation of rhythmic contractions in these small arteries. Oscillations appear to be generated by an interplay between membrane activation and intracellular calcium stores.

The role of atrial natriuretic peptide in the diuretic effect of Ca 2+ entry blockers

The effect of three calcium entry blockers — verapamil, nifedipine and felodipine — on diuresis, natriuresis, the renin-aldosterone axis, and atrial natriuretic peptide (ANP) levels was studied in 30 previously untreated patients with mild to moderate essential hypertension. All three blockers produced significant antihypertensive effects after 2 and after 24 h. Heart rate tended to decrease, but did not change significantly after verapamil, but increased significantly 1–2 h after nifedipine and felodipine. Plasma renin activity (PRA) did not change significantly with any treatment, and plasma aldosterone decreased with all three agents. ANP levels did not change significantly after verapamil, but increased significantly after nifedipine and felodipine. There was a significant positive correlation between the maximal change in ANP level and urinary sodium after nifedipine and felodipine. The increase in urinary sodium after verapamil was not significant. The role of the direct renal effect of calcium entry blockers in their persistent natriuretic action is discussed, as well as their short-term enhancement of ANP levels, which may account for the initial diuretic and natriuretic effects seen with this class of dihydropiridines.

Nimodipine decreases resuscitability in a cardiopulmonary arrest model in the rat.

Although calcium has been implicated in ischemia-induced brain death or dysfunction, many animal studies do not show a beneficial effect of calcium-entry blockers given after resuscitation from a cardiopulmonary arrest (CPA). This may be due to the fact that treatment was started too late; we, therefore, evaluated the effect of the calcium-entry blocker nimodipine administered at the earliest feasible postischemic moment, i.e. at the start of the resuscitation attempts. In anesthetized Wistar rats, CPA was induced by an intra-cardiac injection of KCl, and maintained for 7 min by chest restriction. At the start of the resuscitation attempts, 50 rats were blindly and randomly assigned to intravenous treatment with either nimodipine (10 micrograms/kg over 2 min, followed by 1 micrograms/kg per min for 60 min; n = 25) or saline (n = 25). In the nimodipine group, significantly less rats could be resuscitated (11/25 versus 20/25) and the survival rate at the end of the 7 days evaluation period tended to be lower (5/25 versus 11/25). In the rats surviving after 7 days, there was no difference between both groups in incidence of seizures, neurological status and histological lesions in the hippocampus. It is concluded that nimodipine, in the dose tested and given during resuscitation in this rat model, has a detrimental effect on resuscitability and no beneficial effect on the neurological outcome in the surviving animals.

Cardiovascular interactions of local anesthetics and calcium entry blockers in conscious dogs.

To assess the pharmacologic risk of regional anesthesia in patients treated with calcium entry blockers (CEBs), the cardiovascular effects of 30-minute infusions of lidocaine (60 micrograms.kg-1.minute-1 after an intravenous bolus of 1.5 mg.kg-1), bupivacaine (15 micrograms.kg-1) or normal saline (1 ml.minute-1) were studied with concurrent 40-minute infusions of equihypotensive doses of verapamil (25 micrograms.kg-1.minute-1) and diltiazem (20 micrograms.kg-1.minute-1) in seven conscious dogs chronically instrumented for continuous recording of arterial pressure and left ventricular (LV) contractility assessed by the peak of LV dP/dt. The effects of CEBs alone (saline group) were stable throughout the infusion time. Besides a 10% decrease in mean arterial pressure, verapamil increased heart rate, prolonged atrioventricular (AV) conduction and decreased LV dP/dt, whereas a lesser and isolated tachycardia was observed in the case of diltiazem. At the end of a 30-minute concomitant infusion: (1) bupivacaine (1.4 micrograms/ml) further decreased arterial pressure (with verapamil and diltiazem; -8% and -11%, respectively) and LV dP/dt (with verapamil, -13%), whereas PR interval was further lengthened with verapamil and diltiazem (+36% and +17%, respectively) without change in heart rate; (2) lidocaine (approximately 3.5 micrograms/ml) increased heart rate (with verapamil and diltiazem; +19% and +14%, respectively) and arterial pressure (with verapamil, +8%) without change in LV dP/dt, while the PR interval was further lengthened during verapamil infusion (+34%). Second-degree AV blocks appeared with lidocaine (one of seven dogs) and bupivacaine (three of seven dogs) during verapamil infusion. Caution should therefore be exercised when patients under effective verapamil or diltiazem treatment are given regional anesthesia resulting in significant plasma levels of local anesthetics.(ABSTRACT TRUNCATED AT 250 WORDS)

Combined negative inotropic effects of calcium entry blockers and isoflurane on canine isolated heart muscles.

The combined negative inotropic effects of isoflurane and calcium entry blockers (verapamil, diltiazem, nifedipine, nicardipine) were studied utilizing isolated heart preparations of ventricular muscles from dogs. All of these calcium entry blockers exerted dose-dependent decreases in maximal velocity of shortening (Vmax), maximal developed isometric force (Fm), and the maximal first derivative of Fm (maximal dF/dt). Dose-dependent decreases of these variables of muscle mechanics were augmented in isoflurane-depressed myocardium. At equimolar concentrations, direct myocardial depression was demonstrated in the following order of severity: nifedipine > diltiazem = verapamil > nicardipine. Percent depressions of Vmax, Fm and maximal dF/dt were significantly greater in muscles when calcium entry blockers were combined with 1MAC isoflurane than in muscles of calcium entry blockers alone. These data suggest that the negative inotropic effects of verapamil, diltiazem, nifedipine, and nicardipine were potentiated by isoflurane.

Renal effects of calcium entry blockers

Calcium entry blockers exert several characteristic effects on renal function that contribute to their blood-pressure lowering capacity. They are able to dilate renal vasculature and, in certain circumstances, can increase the glomerular filtration rate, both effects being dependent on the preexisting vascular tone. Calcium blockers are also able to increase renal sodium excretion, mainly through a direct tubular effect that remains during the chronic administration of these drugs. These effects clearly differentiate calcium entry blockers from nonspecific vasodilators and contribute to their effectiveness when they are used as first-step drugs in the therapy of arterial hypertension.

Effect of calcium entry blockers on calcium influx in cultured opossum kidney cells (OK)

Effect of calcium entry blockers on calcium influx in cultured opossum kidney cells (OK)

Reactivity and sensitivity of mesenteric vascular beds and aortic rings of spontaneously hypertensive rats to endothelin: effects of calcium entry blockers

1. The vasoconstrictor effects of endothelin-1 were studied in perfused mesenteric vascular beds (MVB) and aortic rings of 14-16 week-old spontaneously hypertensive rats (SHR) and age-matched Wistar Kyoto rats (WKY). 2. Reactivity to endothelin-1 was increased in MVBs of SHR, as indicated by the maximum perfusion pressure obtained (264 +/- 8 and 141 +/- 9 mmHg respectively) (P less than 0.001), whereas sensitivity was not significantly different between the two strains (EC50 171 +/- 21 and 102 +/- 19, respectively). 3. In aortic rings, in contrast, reactivity to endothelin-1 was reduced in SHR as compared to WKY, whereas sensitivity was similar (EC50 0.78 +/- 0.08 and 0.87 +/- 0.09 nM). 4. As with endothelin-1, reactivity to noradrenaline and potassium chloride was increased in MVBs, but not in aortic rings of SHR. Endothelin-1 was 30 times more potent than noradrenaline in MVBs of SHR, and 15 times more potent than noradrenaline in aortic rings. 5. In both strains, nifedipine and nitrendipine almost completely blocked potassium-induced contractions in MVB and aortic rings, respectively, whereas contractions induced by endothelin-1 or noradrenaline were only partially inhibited. 6. It is concluded that calcium influx via the voltage-operated calcium channel is only partially responsible for the vasoconstrictor action of endothelin-1 in MVBs and aortic rings of SHR and WKY rats. The increased reactivity of the MVB of SHR to endothelin-1 at this stage of the hypertensive process is most likely to be the result of a change in vascular structure rather than due to a primary hypertensive mechanism.

Effects of calcium entry blockers on contractions evoked by endothelin‐1, [Ala3,11]endothelin‐1 and [Ala1,15]endothelin‐1 in rat isolated aorta

1. The aim of the present study was to examine the contractile responses evoked by the recently characterized vasoactive peptide endothelin-1 (ET-1) and by two of its structural analogues, [Ala3,11]ET-1 and [Ala1,15]ET-1 in endothelium-denuded rat isolated aorta, and also to assess the extent of dependence of these responses on extracellular calcium entry. 2. ET-1 (0.3 to 10 nM), [Ala3,11]ET-1 (2.25 to 225 nM) and [Ala1,15]ET-1 (0.04 to 1.36 microM) evoked concentration-dependent contractions in normal, calcium-containing medium with the order of potency: ET-1 greater than [Ala3,11]ET-1 greater than [Ala1,15]ET-1. 3. Preincubation of tissues for 60 min with diltiazem (1 microM) induced a significant 3 fold rightward shift of concentration-effect curves to ET-1 without affecting maximal responses elicited by 10 nM of this peptide, whereas the same treatment failed to modify concentration-effect curves to [Ala3,11]ET-1. 4. Preincubation of tissues for 60 min with nifedipine (0.1 or 1 microM) markedly inhibited contractions elicited by either ET-1 (10 nM) or [Ala1,15]ET-1 (0.41 microM). Furthermore, when added cumulatively to tissues maximally contracted by ET-1 (10 nM), nifedipine (3 nM to 1 microM) induced concentration-dependent relaxations with an IC50 value of 21.8 +/- 5.9 nM. Maximal relaxation to nifedipine, 1 microM, amounted to 56.9 +/- 11.5%. 5. Submaximal concentrations of ET-1 (3 nM), [Ala3,11]ET-1 (75 nM) and [Ala1,15]ET-1 (0.41 microM), gave about 85% of maximal contractions elicited by noradrenaline (1 microM) in normal, calcium-containing medium. These contractile responses were all reduced by about 70% in calcium-free medium. These contractile responses were all reduced by about 70% in calcium-free medium. Pretreatment of tissues with diltiazem (1 microM) or with nifedipine (0.1 or 1 microM) did not affect these residual contractions. 6. Upon readdition of calcium (10 microM to 10mM) to tissues in calcium-free medium, preincubated with submaximal concentrations of one or the other of the peptides, concentration-dependent contractions were elicited with EC50 values of 1.21 + 1.1 mm (ET-1-exposed rings), 74.6 + 9.1 microM ([Ala31 ']ET-1-exposed rings) and 102 + 27 microM ([Ala' 15]ET-1-exposed rings). Calcium-induced responses were significantly inhibited by diltiazem (1 microM) in both ET-1- and [Ala3"1 ']ET-1-exposed tissues and by nifedipine (0.1 and 1 microM) in ET-1-exposed tissues. However, nifedipine did not significantly affect calcium induced responses in [Ala"'15]ET-1-exposed rings. 7. Readdition of calcium to a calcium-free medium containing 40mm K+, evoked concentrationdependent responses that were unaffected by the presence of ET-1 (3 nM). 8. Overall, these results indicate that contractions to ET-1 and its analogues in rat aorta are to some extent dependent on extracellular calcium. However, the mode of antagonism by diltiazem and nifedipine of responses to the peptides in normal medium, as well as of contractions induced by readdition of calcium in peptide-exposed tissues in calcium-free medium, argue against a direct activation by endothelin or its analogues of L-type calcium channels. Finally, removal of one or the other of the disulphide bonds of ET-1 reduces the potency of the peptide.

Should calcium chloride still be used for cardiopulmonary resuscitation?

The essential role played by calcium ions in myocardial contractility has been well known since the original work done by Ringer more than 100 years ago. On the basis that calcium is essential to myocardial contraction and that its administration has positive inotropic effects, the American Heart Foundation made the recommendation to administer calcium chloride in electromechanical dissociation (EMD) or asystole. Recent work however has related calcium overload to ischaemic cell damage so that the administration of exogenous calcium in cardiopulmonary resuscitation (CPR) has been questioned. In addition, data have also emerged to support the beneficial effects of calcium entry blockers during global ischaemia. In view of the paucity of clinical data to support or refute the use of calcium chloride in CPR, the discussion must remain highly conjectural.

Effects of calcium entry blockers on human platelet metabolism measured by microcalorimetry.

1. The direct overall metabolic effects of calcium entry blockers on human platelets were evaluated using a sensitive microcalorimetric method. 2. The effect on platelet metabolism of four calcium entry blockers with different profiles of action was examined and compared with the relaxant response on human cerebral vessels in vitro. 3. Diltiazem (10(-8), 10(-4) M) and nifedipine (10(-10), 10(-6) M) were without effect on overall platelet metabolism. On the other hand flunarizine (10(-4) M) and verapamil (10(-4) M) significantly reduced metabolism, while lower concentrations of these agents did not change heat production. 4. The thermogenic response occurred at concentrations of the calcium entry blockers that were much higher than those observed during therapy and the concentrations which relax human cerebral arteries in vitro.

In Vivo and in Vitro Evidence of an Adenosine-Mediated Mechanism of Calcium Entry Blocker Activities

Drugs such as dipyridamole (200 micrograms/kg/min), an adenosine uptake inhibitor, and theophylline (300 micrograms/kg/min), an adenosine receptor antagonist, respectively increased and decreased postischemic hyperemia in normal subjects, as well as in POAD patients. Moreover, dipyridamole pretreatment was able to antagonize the reduction of peak flow induced by nifedipine, and the potentiating effect of flunarizine on postischemic hyperemia was affected significantly by theophylline, thus suggesting a possible interference of calcium entry blocker drugs with the endogenous adenosine system. In a cellular model (polymorphonuclear leukocytes--PMN) the inhibitory effect of calcium entry blockers on stimulated functions (degranulation and free radical production) was highly antagonized by theophylline. Finally, a 1H-NMR spectroscopy study showed a binding interaction between adenosine and flunarizine on the cell membrane. An adenosine-receptor coupling to the calcium entry blocker channels is suggested.

Effects of halothane and calcium entry blockers on atrioventricular conduction-a comparative study of verapamil, diltiazem, and nifedipine.

The effects of halothane on AV nodal function were evaluated in dogs with verapamil, diltiazem, or nifedipine during atrial pacing using the technique of His-bundle electrocardiography. Fifty-one mongrel dogs were divided into six groups. Anesthesia was induced with ketamine 100 mg im. and thiamylal 25 mg/kg iv. The animals were intubated and mechanically ventilated at normocapneic levels. Anesthesia was maintained with 50% nitrous-oxide in oxygen with pancuronium 2 mg im. Dogs in groups I, III, and V were anesthetized with 0.8% halothane and 50% nitrous-oxide in oxygen. We observed interactions between halothane and intravenous administration of either verapamil 0.1 mg/kg, diltiazem 0.15 mg/kg, or nifedipine 0.01 mg/kg respectively. Dogs in groups II, IV, and VI were administered either verapamil, diltiazem, or nifedipine iv without halothane. There were prolongations of sinus cycle length (SCL) (414 +/- 10 to 542 +/- 19 msec.), atrium-His (AH) interval (73 +/- 3 to 97 +/- 5 msec.), and functional refractory period (FRP) of the AV-node (227 +/- 5 to 260 +/- 5 msec.) in halothane anesthesia in groups I, III, and V. There were more prolongations of these variables after iv administration of verapamil (SCL; 617 +/- 35, AH; 118 +/- 7, FRP of the AV node; 311 +/- 4) and diltiazem (SCL; 554 +/- 19, AH; 118 +/- 12, FRP of the AV node; 283 +/- 12) but no prolongations after nifedipine (SCL; 533 +/- 19, AH; 99 +/- 8, FRP of the AV node; 272 +/- 9). Comparing effects of calcium entry blockers with and without halothane in groups I and II, III and IV, or V and VI, there were additive depressing effects of halothane with either verapamil or diltiazem on AV nodal function. And there is a difference between the effects of nifedipine on SCL with and without halothane.

Modulation of anaphylactic histamine release by calcium channel agonists and antagonists

Calcium antagonists have been reported to exert protective effects in hypersensitivity reactions in man and animals. However, their effect on anaphylactic histamine release is highly variable and controversial. In the present paper we evaluate the effect of calcium entry blockers and BAY K 8644 on the response to specific antigen in isolated hearts taken from actively sensitized guinea-pigs and from isolated rat and guinea-pig mast cells, actively or passively sensitized. Verapamil, diltiazem, nifedipine and prenylamine dose-dependently decreased anaphylactic histamine release in isolated actively sensitized guinea-pig mast cells. BAY K 8644 was found to be ineffective. In isolated, passively sensitized rat mast cells, verapamil showed a highly significant inhibitory effect, while prenylamine (10(-4) M) was able to evoke a histamine releasing effect. In cardiac anaphylaxis verapamil, diltiazem, prenylamine, but not nifedipine, were active in reducing the release of histamine without modifying the antigen-induced arrhythmias and positive chronotropic and inotropic effects.

Persistence of the Natriuretic Effect of Calcium Entry Blockers

In order to elucidate whether or not the natriuretic properties of calcium entry blockers persist during the long-term administration of the drug, we have investigated the renal capacity to excrete an intravenous sodium load (2,000 ml of isotonic saline in 4 h) in a group of eight mild to moderate essential hypertensive patients, before and after 1, 8, and 24 weeks of treatment with nitrendipine. The following parameters were measured before and hourly during saline infusion: blood pressure, and plasma and urine sodium and potassium. For 5 days prior to the performance of the test, the patients received a diet containing 120 mEq of sodium daily. Nitrendipine induced a significant fall of both systolic and diastolic blood pressure (p less than 0.05-0.01) that was maintained throughout the study. A significant increase of the cumulative renal sodium output (microEq/min/h) was observed after 1, 8, and 24 weeks (p less than 0.05-0.001) of therapy with the calcium entry blocker. Meanwhile, plasma sodium and potassium and the kaliuresis did not change significantly. These results indicate that the natriuretic effect of calcium entry blockers is still present after long-term treatment.

Effects of calcium entry blockers on calcium-dependent contractions of rat portal vein.

1 The effects of six calcium entry blockers belonging to the dihydropyridine (isradipine or PN 200-110, nifedipine, nicardipine), verapamil (D888 or desmethoxyverapamil, D600 or gallopamil) and diltiazem classes were investigated on isometric spontaneous contractions and contractions induced by high-K+ solutions, noradrenaline, acetylcholine and caffeine. 2 The rank order of potency was PN 200-110 greater than nicardipine = nifedipine = D888 greater than D600 greater than diltiazem from experiments on spontaneous contractions and high-K+ induced contractions. With depolarized preparations, the concentration-response curves for nicardipine, PN 200-110, nifedipine and D600 were significantly shifted to the left indicating that the calcium entry blockers show voltage-dependent inhibitory properties. This effect was not significant with D888 and diltiazem. 3 All the calcium entry blockers strongly reduced the noradrenaline (NA)- and acetylcholine (ACh)-induced contractions at concentrations which produced complete inhibition of spontaneous contractions. They had a slight effect on caffeine-induced contractions. 4 In Ca2+-free, EGTA-containing solutions, both ACh, NA and caffeine produced transient contractions, the amplitude of which could be taken as a measurement of the amount of internal calcium present in a drug-sensitive calcium store. The filling of the calcium store was maximal after 10-12 min of calcium loading in 2.1 mM Ca2+, while the depletion was complete after 4-6 min of perfusion in Ca2+-free solution. 5 At concentrations which abolished spontaneous contractions, PN 200-110, nifedipine, D888 and D600 had no appreciable effect on contractions evoked in Ca2+-free solutions by ACh, NA and caffeine. When added in Ca2+-containing solutions diltiazem and, to a lesser extent, nicardipine strongly reduced the contractions evoked in Ca2 -free solutions, suggesting that they inhibited the filling of the internal calcium store. 6. These results indicate that the six calcium entry blockers are potent inhibitors of calcium influx through voltage-dependent calcium channels. Two of them (diltiazem and nicardipine) may exert an additional effect to depress contractions dependent on intracellular calcium release.

Multiple effects of calcium entry blockers on renal function in hypertension.

Characterization of the renal effects of calcium entry blockers has not been easy because the inhibition of Ca2+ cellular influx alters several regulatory functions. The ability of calcium blockers to dilate renal vasculature and to increase glomerular filtration rate is largely determined by the preexisting vascular tone. However, the increments in sodium excretion could occur without alterations in renal hemodynamics. Calcium blockers could increase sodium excretion by inducing a redistribution of renal blood flow toward juxtamedullary nephrons, by inhibiting tubuloglomerular feedback responses, or by a direct action on the tubular transport of sodium. These effects are poorly understood at present. In vitro studies show that the blockade of calcium entry enhances renin secretion and decreases prostaglandin synthesis. This dissociation has not been found during long-term administration, which has been proved to be effective for the treatment of essential hypertension with normal maintenance of renal function. In this respect, there are reports indicating that calcium blockers are particularly effective in a subgroup of patients with essential hypertension who exhibit subtle but detectable alterations in calcium metabolism. Further studies are needed to determine whether this significant response to calcium blockers is due to correction of an early defect of calcium cellular kinetics that initiated the increase in blood pressure.