On the mechanism of action of calcium antagonists

A short review is given of possible mechanisms of action of the organic "calcium antagonists". Calcium antagonists comprise a chemically heterogenous group of drugs, and the term may be used to denote agents that inhibit Ca2+-dependent processes or regulatory mechanisms without acting at other sites. Such drugs may be subdivided into those that decrease the availability of Ca2+ to the myoplasm, and those that decrease the cellular effects of Ca2+ without lowering the intracellular Ca2+ concentration. Accordingly, calcium channel blockers, such as verapamil, nifedipine, and diltiazem, form a subgroup of calcium availability inhibitors, as they block influx of extracellular calcium through ion selective channels in the membrane both in cardiac and smooth muscle. However, it cannot be excluded that some of these drugs, particularly in smooth muscle, may have additional sites of action, which must be taken into consideration when they are used as investigational tools.

Preclinical pharmacology of diltiazem

The membrane site of action and intracellular effects of diltiazem on heart and blood vessels are briefly discussed and compared to those of other calcium entry blockers, mainly verapamil and nifedipine. Diltiazem seems to have another site of action in the membrane than verapamil and nifedipine. Even if its main action is exerted at the cell membrane level, diltiazem may, at high concentrations, appears to have intracellular effects. Similar to the haemodynamic effects of verapamil and nifedipine, those of diltiazem are determined not only by direct actions on heart and peripheral vessels, but also by sympathetic reflex activity which modulates the direct effects. Two aspects of the myocardial protective action of diltiazem are discussed, the ability of the drug to reduce the frequency of ventricular dysrhythmias associated with ischaemic damage, and the ability to protect the ischaemic myocardium during reperfusion.

Calcium channel blocker overdose

A case of diltiazem overdose with significant hemodynamic compromise is presented. Multiple therapeutic modalities were attempted with limited results. Control was finally achieved with a combination of norepinephrine, dobutamine, and cardiac pacing. Invasive pulmonary monitoring parameters are reported and were important in the management of this patient. The management of calcium channel blocker overdose and the various available therapeutic modalities are discussed.

The effects of calcium antagonists and prostaglandin E1 on isolated canine coronary arterial tension

The effects of calcium antagonists (nifedipine, nicardipine, diltiazem, and verapamil) and prostaglandin E1 (PGE1) on the tension of isolated canine coronary arterial strips were studied. In a solution containing 20 mEq/L of K+, 127 mEq/L of Na+, the tension was increased by 500-1,000 mg with 4 mEq/L of Ca2+. This increase in tension was suppressed by Ca-antagonists and PGE1 dose-dependently. Nifedipine 10(-5) M, nicardipine 3 x 10(-7) M, diltiazem 3 x 10(-6) M, and verapamil 3 x 10(-6) M completely suppressed the increased tension. The maximal suppression of the tension produced by PGE1 was about 40% at 10(-10) M. In 20 mEq/L K+ solution (0 mEq/L Ca2+, 37 degrees C), the reduction of the Na+ concentrations from 127 mEq/L to 12 mEq/L increased the tension by 50 to 100 mg. This increase in tension was not suppressed by Ca-antagonists or PGE1. In conclusion, this study demonstrated that Ca-antagonists and PGE1 suppressed an increase in the tension caused by Ca2+ but did not suppress an increase in the tension caused by Na+ reduction.

Differential actions of AlF4- and vanadate on canine trachealis muscle

Fluoride (F-) a known stimulator of G-protein, has been reported to inhibit "P"-type ATPase activity in smooth muscles. On the other hand, vanadate, a strong "P"-type ATPase inhibitor, has been reported to stimulate G-protein in some cells. This study was designed to compare the contractile actions of fluoroaluminate (AlF4-) and vanadate and to clarify their mechanisms of actions by measuring changes in the amount of cyclic adenosine monophosphate (cAMP) and inositol phosphates. F- and vanadate induced strong contractions in canine trachealis muscle. The F(-)-induced contraction was potentiated by the addition of aluminum (Al3+, 20 microM) and inhibited by deferoxamine (200 microM), a heavy metal chelator. Ca2+ removal and 10 microM verapamil inhibited the contraction induced by AlF4- and vanadate. AlF4- and vanadate increased 45Ca influx in the absence and presence of verapamil. AlF4(-)-induced contractions were partially relaxed by isoproterenol (38.2 +/- 7.4%) in contrast with those induced by vanadate (72.1 +/- 5.3%), which could be explained by a decrease of tissue cAMP content by AlF4- in forskolin-pretreated tissues. Vanadate increased inositol phosphate accumulation as did AlF4-, although the magnitude of the increase was smaller than that produced by AlF4-. The increases of inositol phosphate content by both drugs were not affected after the pretreatment by pertussis toxin.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of nifedipine on kidney and ureteral function after experimental acute unilateral ureteral obstruction

Nifedipine, a calcium channel blocker, was tested for its in vivo effect on the ureter and kidney during acute obstruction. There was no discernible effect on peristalsis but glomerular filtration rate was improved. Thus, nifedipine would not be expected to decrease renal colic from obstruction.

Nicardipine suppresses bronchoconstrictor actions of pharmacologic agents in guinea pigs

We investigated the inhibitory effects of nicardipine on airway smooth muscle constriction in the guinea pig via both in vitro and in vivo preparations. In the in vitro studies, we created dose-response curves of the excised tracheal strip. The increase in respiratory overflow produced by leukotriene (LT) D4 was measured in the in vivo preparation. In the organ bath, nicardipine (1-12 micrograms/ml) significantly inhibited constriction of the isolated tracheal spiral strip induced by 7 different agonists, acetylcholine (Ach), histamine (HA), serotonin (5-HT), prostaglandin F2 alpha (PGF2 alpha), slow-reacting substance of anaphylaxis (SRS-A), and LTC4 and LTD4. Compared to nifedipine and verapamil (data in the literature), the inhibitory potency of nicardipine on the constriction of the isolated guinea pig tracheal strip is 100-1,000 times greater. In the in vivo studies, nicardipine (50 and 100 micrograms/kg) significantly attenuated the LTD4-induced increase in respiratory overflow. These results suggest that nicardipine inhibits bronchoconstrictor-induced constriction of airway smooth muscle both in vitro and in vivo, and that it is a potent bronchodilator. The inhibitory characteristics of nicardipine may prove to be applicable clinically.

Hypoxic contraction in canine basilar arteries: role of endothelium

The effect was studied of severe hypoxia (partial pressure of oxygen below 20 mm Hg) on the tone of isolated canine basilar arterial rings with and without endothelium. Short-lasting exposure (8 min) to hypoxia induced a pronounced contraction that did not differ in vessels with and without endothelium. The hypoxia-induced changes of vessel tone were reproducible in 3 consecutive episodes of hypoxia interspaced by 45 min of control oxygenation. Reintroduction of oxygen induced a posthypnoxic relaxation in preparations without endothelium and a posthypoxic contraction in part (5 of 15) of the arteries with endothelium. The hypoxic contraction was not influenced by the cyclooxygenase inhibitor indomethacin and the thromboxane A2 receptor antagonist BM 13.505 while the posthypoxic response was diminished by both inhibitors. The Ca(++)-entry blocker D600 abolished both hypoxic and posthypoxic contractions completely.

Cardioprotective effects of amlodipine on ischemia and reperfusion in two experimental models

The cardioprotective effect of amlodipine, a long-acting dihydropyridine derivative, was studied in 2 experimental models of ischemia and reperfusion. Isolated and blood-perfused feline hearts were made globally ischemic for 60 minutes and then reperfused for 60 minutes. Alterations of left ventricular developed pressure and compliance were monitored in both amlodipine-treated hearts and saline-treated control animals. Changes in perfusion pressure indicated that amlodipine significantly reduced myocardial oxygen consumption and coronary vascular resistance. Furthermore, a progressive increase in resting left ventricular diastolic pressure indicated that amlodipine, administered before the onset of global ischemia, attenuated the development of ischemic contracture. Return of contractile function 60 minutes after reperfusion and maintenance of tissue concentrations of electrolytes were significantly better in the amlodipine-treated group than in the control animals. In intact canine hearts, regional myocardial ischemia was induced for 90 minutes, followed by 6 hours of reperfusion. Although the hemodynamic variables and the size of the region of risk did not differ significantly between treated animals and control animals, the infarct size was significantly smaller in the amlodipine-treated group than in the control animals, and a gradual reduction in coronary blood flow was observed in the control group that was prevented in the amlodipine group. A comparison of these findings with those observed with oxygen radical scavengers also is discussed. A detailed report of these studies was published in The American Journal of Cardiology (1989;64:101I-116I). This review is included here to maintain continuity of the symposium for the convenience of the reader.

Clinical neuropharmacology of calcium antagonists

Experimental and clinical data clearly demonstrate that calcium antagonists (CA) may have an action on the central nervous system (CNS). The cerebrovascular action of CA justifies their use in cerebral ischaemia, vasospasm and hypoxia. Several clinical trials have demonstrated such beneficial effects. On the other hand a number of reports indicate that CA may have a direct neuronal effect, although most of such trials have not been verified or are mere case reports. In addition, the large number of conditions susceptible to being corrected by CA is impressive: epilepsy, pain, dystonia, dyskinesia, psychiatric conditions, etc. Other papers are disconcerting that report extrapyramidal disorders induced by flunarizine and cinnarizine in the elderly, whereas nicardipine does not produce such side effects and may even alleviate some parkinsonian symptoms. In various experimental models (e.g. stroke, oedema), pharmacological effects have been shown to vary from one compound to the other. Two main questions are yet to be answered: 1) has the direct neuronal effect of CA been clearly established? 2) are the multiple clinical effects on the CNS really linked to calcium antagonism?

Cardioprotective effects of amlodipine in the ischemic-reperfused heart

Amlodipine is a dihydropyridine derivative belonging to the group of pharmacologic calcium entry blocking agents and is characterized as having a slow onset and relatively long duration of action with minimal effects on cardiac electrophysiology and myocardial contractility. The protective effect of amlodipine was studied in isolated blood-perfused feline hearts made globally ischemic for 60 minutes followed by reperfusion for 60 minutes. Ischemic-induced alterations of left ventricular developed pressure and complicance were monitored. In 11 control and 7 drug-treated hearts, amlodipine produced significant decreases in myocardial oxygen consumption (6.2 +/- 0.4 to 4.4 +/- 0.4 ml oxygen/min/100 g) and coronary vascular resistance, as assessed by changes in perfusion pressure (120 +/- 1 to 100 +/- 4 mm Hg). Amlodipine administered before the onset of global ischemia decreased the development of ischemic contracture as reflected by a progressive increase in resting left ventricular diastolic pressure. The return of contractile function, 60 minutes afer reperfusion, improved significantly in the amlodipine-treated group compared with controls, and there was better maintenance of the tissue concentration of Na+, Ca2+ and K+. A canine model of regional myocardial ischemia (90 minutes) followed by 6 hours of reperfusion was used to assess the cardioprotective effects of amlodipine, 150 micrograms/kg, administered 15 minutes before reperfusion. Infarct size, expressed as a percentage of the area at risk, was smaller in the amlodipine-treated group (n = 10) than in the control group (n = 10) (34.5 +/- 3.8% vs 45.9 +/- 2.8%, p = 0.027). Risk region size did not differ between groups and both groups were comparable with respect to the hemodynamic parameters of heart rate, blood pressure and rate-pressure product. Amlodipine prevented the gradual reduction in coronary blood flow observed in the control group. It is concluded that amlodipine reduces myocardial ischemic injury by mechanism(s) that may involve a reduction in myocardial oxygen demand as well as by positively influencing transmembrane Ca2+ fluxes during ischemia and reperfusion.

Molecular cardiology: new avenues for the diagnosis and treatment of cardiovascular disease

This review summarizes some of the major advances in the investigation of molecular mechanisms underlying both normal and abnormal cardiovascular function. Four major areas are highlighted including cardiac muscle, the blood vessel, atherosclerosis and thrombosis/thrombolysis. The remarkable strides in understanding multifactorial diseases such as atherosclerosis, and the development of innovative new therapies such as the use of thrombolytic agents produced by recombinant deoxyribonucleic acid (DNA) technology, are noted. Moreover, it is concluded that the past decade of basic research has provided a solid framework for improvements in the diagnosis and therapy of other forms of cardiovascular disease as well. An evaluation of current trends in basic cardiovascular research suggests that diagnostic and therapeutic approaches to disease will increasingly target specific molecular processes underlying the pathophysiologic state.

Calcium channel blockers

The calcium channel blockers initially were approved for the treatment of classical and variant angina pectoris. Recent studies indicate that these agents also are useful in such diverse conditions as pulmonary and systemic hypertension, hypertrophic cardiomyopathy, arrhythmias, asthma, Raynaud's syndrome, esophageal spasm, myometrial hyperactivity, cerebral arterial spasm, and migraine.

The role of calcium antagonists in bronchial reactivity

Calcium (Ca) ions play an important pathophysiologic role in allergic reactions. Thus, mediator release from mast cells, synthesis of some newly formed chemical mediators, airway smooth muscle contraction, and nerve-impulse conduction are all dependent on the availability and flux of Ca ions. It is likely, therefore, that Ca antagonists would modify allergic bronchoconstriction. In vitro, Ca antagonists have been demonstrated to inhibit mediator release (histamine, slow-reacting substance of anaphylaxis, and platelet-activating factor) from mast cells, passively sensitized human lung fragments, and leukocytes. Ca antagonists have also been found to inhibit synthesis of leukotrienes in rat lungs and cyclooxygenase products in sheep, possibly by inactivating phospholipase A2 and/or 5-lipoxygenase. In addition, nifedipine, verapamil, and gallopamil have demonstrated inhibition of airway smooth muscle contractions to histamine, carbachol, and antigen in various species. In vivo effects of Ca antagonists are variable, depending on the species, experimental design, the stimulus or the agonist, and the Ca antagonist used. Animal studies have demonstrated the inhibition of histamine, methacholine, citric acid, and prostaglandin F2 alpha-induced bronchoconstriction in guinea pigs and dogs by intravenous nifedipine. In contrast, verapamil inhibited antigen-induced bronchoconstriction in allergic sheep without any effect on histamine- and carbachol-induced responses. Ca antagonists (nifedipine and verapamil) have been of limited value in human subjects and generally have no significant bronchodilating activity. Both nifedipine and verapamil prevent the exercise-induced asthma and partly attenuate the histamine and methacholine-induced bronchoconstriction. Oral nifedipine is generally more effective than oral verapamil against acute antigen-induced bronchoconstriction; however, this efficacy may be limited by systemic side effects. Inhaled Ca antagonists may be more effective and free of systemic side effects, as demonstrated by greater efficacy of inhaled verapamil. A new Ca antagonist, gallopamil (a methoxy derivative of verapamil), is being investigated as an aerosol, and preliminary studies in animals and humans have found it fourfold to seventeenfold more potent than verapamil. In sheep, gallopamil has been found to attenuate histamine, carbachol, and platelet-activating factor-induced bronchoconstriction, as well as to inhibit early and late-phase allergic airway responses. Studies in human subjects have also demonstrated the inhibition of antigen-induced bronchoconstriction by inhaled gallopamil, with efficacy comparable or better than cromolyn sodium.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of oral diltiazem on airway reactivity to methacholine and exercise in subjects with mild intermittent asthma

The effect of increasing doses of oral diltiazem on airway reactivity to methacholine was evaluated in 10 volunteers with mild asthma. Then the highest tolerated dose was compared with placebo in preventing exercise-induced bronchoconstriction. Methacholine challenges were performed 1 h before and 100 min after placebo or after 30, 60, 90, 120, or 180 mg of oral diltiazem, given in a single-blind, crossover manner on different days within 2 wk. Diltiazem, at doses above 60 mg prolonged the P-R interval of the electrocardiograph but had no significant effect on FVC, FEV1, or FEF25-75. The mean +/- SEM ratio of the dose of methacholine required to produce a 20% decrease in FEV1 (PD20) after diltiazem to the PD20 before diltiazem, i.e., the fold increase in PD20, was not significantly different from placebo at any dose: 0.93 +/- 0.11 after placebo, 1.2 +/- 0.1 after 30 mg, 1.3 +/- 0.3 after 60 mg, 1.2 +/- 0.2 after 90 mg, 1.1 +/- 0.1 after 120 mg, and 1.0 +/- 0.1 after 180 mg. One hundred minutes before a standardized exercise challenge, 120 to 180 mg of oral diltiazem and identically appearing placebo tablets were administered in a randomized, double-blind, crossover design on separate days at least 48 h apart. The mean +/- SEM maximal postexercise decrease in FEV1 was 25.5 +/- 3.3% after placebo and 17.0 +/- 4.8% after diltiazem (p less than 0.01). There was no correlation between change in FEV1 and serum concentrations of diltiazem or its active metabolite desacetyldiltiazem.(ABSTRACT TRUNCATED AT 250 WORDS)

The involvement of fast calcium channel activity in the selective activation of phasic contractions by partial depolarization in rat vas deferens smooth muscle

In the prostatic portion of the rat vas deferens, 65% of the preparations studied developed pronounced rapid twitch activity in response to slight depolarization by 15 mM K+ salines. The mechanism underlying this response was studied using treatments designed to inhibit the influence of endogenous transmitters and using recognized calcium antagonist drugs. Although the action of phentolamine was inconclusive, experiments employing guanethidine, reserpine, 6-hydroxydopamine, atropine and alpha,beta-methylene ATP suggest that endogenous transmitter release was not responsible for the observed twitch activity. Twitch activity was strongly dependent upon [Ca]0. The 15 mM K+ twitch activity was inhibited by verapamil (5 X 10(-5) M) but was resistant to 10(-3) M lanthanum. Twitch activity was, however, abolished by 10(-3) M Mn2+ ions and was markedly potentiated by 2 X 10(-3) M TEA. The rapid twitch activity exhibited a strong voltage-dependency, being abolished by [K]0 elevations of 25 mM and above. It is concluded that this phasic activity of the vas deferens smooth muscle may depend upon fast calcium channel activity which, in contrast to voltage-dependent slow calcium channel activity, shows ready voltage-inactivation on substantial depolarization.

Altered calcium-handling properties of jejunal smooth muscle from the nematode-infected rat

We examined changes in contractility of jejunal longitudinal muscle from rats infected 8 days previously with the enteric parasite Nippostrongylus brasiliensis. In uninfected control rats, carbachol-induced contraction was maximal at 1 microM carbachol. In muscle from infected rats, there was no change in ED50 for carbachol, but contraction induced by 1 microM carbachol was increased greater than 75% over that in control rats. No significant differences were observed in muscarinic receptor binding characteristics in smooth muscle cells from control and infected rats when [3H]QNB was used as radioligand. Contraction induced by 5-hydroxytryptamine was also substantially greater in muscle from infected rats. In control rats, carbachol-induced contraction was largely independent of extracellular Ca2+, whereas in muscle from infected rats, withdrawing extracellular Ca2+ reduced contraction to below control levels. Furthermore, whereas adding nitrendipine in the presence of extracellular Ca2+ had no effect on carbachol-induced contraction in control rats, it significantly inhibited contraction in muscle from infected rats.

Novel interactions of cations with dihydropyridine calcium antagonist binding sites in brain

The effects of monovalent (Na+, Li+, K+, Rb+) and divalent (Ca2+, Mg2+, Mn2+) cations on dihydropyridine calcium antagonist binding sites in brain and cardiac membranes were investigated using a low ionic strength buffer (5 mM Tris-HCl, pH 7.4), and the dihydropyridine, [3H]-nitrendipine. At 25 degrees C, the monovalent cations Na+, Li+, and K+ (100 mM) but not Rb+ significantly decreased the apparent dissociation constant (KD) but had no effect on the maximum binding site capacity (Bmax) of [3H]-nitrendipine in brain. The divalent cations Ca2+, Mg2+, and Mn2+ (2 mM) significantly increased the Bmax, but did not affect the KD of [3H]-nitrendipine. The effects of cations were concentration-dependent (EC50 monovalent cations 10-25 mM; EC50 divalent cations 50-200 microM) and demonstrated brain region selectivity. The effect of Ca2+, but not Mg2+ or Mn2+ on [3H]-nitrendipine binding was described by a two-site model. At 25 degrees C, neither mono- nor divalent cations altered the characteristics of [3H]-nitrendipine binding to rat cardiac membranes. At 37 degrees C, Na+ (100 mM) but not K+ (100 mM) significantly increased the Bmax of [3H]-nitrendipine in rat brain membranes. Ca2+ (2 mM) significantly increased the Bmax of [3H]-nitrendipine binding to rat brain membranes to a greater extent than at 25 degrees C. Both Na+ and K+ had no effect on [3H]-nitrendipine binding to cardiac membranes, while Ca2+ (2 mM) significantly decreased the KD of [3H]-nitrendipine. It is suggested that the selective effects of mono- and divalent cations on [3H]-nitrendipine binding to rat brain and cardiac membranes may be associated with differences in the calcium current blocking activity of dihydropyridine calcium antagonists in brain and cardiac tissues.

Acylating phencyclidines irreversibly enhance brain calcium antagonist binding

Phencyclidine was previously shown to allosterically increase the apparent affinity of the dihydropyridine ( [3H]nitrendipine) calcium antagonist binding site in a lysed synaptosomal membrane preparation of rat forebrain. Treatment of a similar preparation of mouse forebrain with 4-isothiocyanato-1-(1-phenylcyclohexyl) piperidine (FOURPHIT), an acylating phencyclidine derivative, resulted in a concentration dependent (0.1-10 microM), irreversible, increase in the apparent affinity of [3H]nitrendipine in contrast to the effects of phencyclidine which were reversible. The FOURPHIT isomer, 1-[1-(3-isothiocyanatophenyl) cyclohexyl] piperidine (METAPHIT), (10 microM) also irreversibly increased the apparent affinity of [3H]nitrendipine, but was much less efficacious than FOURPHIT. Phencyclidine blocked the irreversible increase in the apparent affinity of [3H]nitrendipine produced by FOURPHIT. The interactions of multivalent cations and the calcium antagonist diltiazem with the [3H]nitrendipine binding site were altered following treatment of membranes with FOURPHIT. These studies suggest that FOURPHIT irreversibly interacts with the same sites as PCP, and thus may be a useful tool with which to further probe both the behavioral and biochemical interactions between phencyclidine and the dihydropyridine calcium antagonist binding site.

Effect of nifedipine on arterial hypoxaemia occurring after methacholine challenge in asthma

To investigate whether the effects of nifedipine on methacholine induced broncho-constriction could impair pulmonary gas exchange in bronchial asthma a randomised, double blind, crossover study in 13 symptom free asthmatic subjects was designed. Each patient underwent a methacholine bronchial challenge test on two separate days one week apart, after having either oral nifedipine (20 mg thrice daily) or placebo for three days. Arterial blood gases were measured before and after methacholine challenge in nine subjects. Prechallenge values of forced expiratory volume in one second (FEV1) and arterial oxygen tension (Pao2) were similar after nifedipine and after placebo. After challenge, the cumulative doses of methacholine required to produce a 20% fall in FEV1 (PD20 FEV1) were significantly larger after nifedipine (280 (SD 347)) cumulative breath units (CBU) than after placebo (120 (183) CBU; p less than 0.01). After challenge the fall in Pao2 values (17.1 (1.6) mm Hg; (2.28 (0.21) kPa)) was significantly greater than after placebo (11.7 (2.4) mm Hg; (1.56 (0.32) kPa) p less than 0.03). Our data show that although oral nifedipine significantly reduces airway reactivity in patients with mild bronchial asthma, it also adversely affects pulmonary gas exchange, resulting in a lowered postchallenge Pao2, probably because of worsening ventilation-perfusion relationships.

Myocardial protective effect of amiodarone in hypertrophied hearts during global ischemia

The effect of amiodarone on the ischemic-reperfusion injury was tested in an isolated working preparation, using hypertrophied rat heart at 37 degrees C. Constant filling and afterload pressures and similar heart rates were used. Hearts from spontaneously hypertensive rats (N = 78) had thirty minutes of ischemia. Each received a 12-ml injection, by aortic root infusion, of amiodarone in normal saline or of normal saline alone at 37 degrees C at the onset of ischemia. Heart rate, aortic output, coronary sinus output, atrial pressure, and aortic pressure were recorded before and after global ischemia under steady-state conditions. Dose-response studies were performed at concentrations of 0.01 to 1.0 mg/ml. At every dose administered, amiodarone was found to significantly ameliorate the deleterious effects of global ischemia. The maximal benefit of amiodarone (70 +/- 4.6% recovery of function [mean +/- standard error of the mean], p less than 0.01) was found to be 0.25 mg (0.021 mg/ml), or 0.11 mg/g wet heart weight. Improvement in survival (return of aortic output and heart rate following ischemia) with all doses of amiodarone was statistically significant (p less than 0.002). Decreased recovery of function following global ischemia when doses were greater than 0.25 mg may have been secondary to the known negative inotropic effects of the drug. The mechanisms for the protective effects of amiodarone may be coronary vasodilatation, antiarrhythmic stabilization, or inhibition of calcium flux at the slow channel.

Pharmacological characterization of the effects of verapamil and nifedipine on isolated guinea-pig lung parenchymal strip

Verapamil and nifedipine elicited dose-related relaxations and produced a right downward shift of the concentration-response curves to CaCl2, KCl, acetylcholine and histamine in the isolated guinea-pig lung parenchymal strip. These Ca-entry blockers had greater inhibitory effects on contractions evoked by CaCl2 and KCl than against those elicited by acetylcholine and histamine.

Effect of zinc on neuronal activity in the rat forebrain

Zinc ions, which are unevenly distributed in the CNS and can be released from nerve terminals, have been implicated as causative agents in epileptogenesis. The present study has shown that intraventricular administration to anesthetized rats causes seizure activity of the ECOG and convulsions. Since the manner in which zinc influences neuronal activity and triggers convulsions is unclear, studies were also made of its effect on spontaneous and evoked activity in the rat forebrain. It was found that iontophoretic application of zinc to cortical neurons causes slow and often prolonged increases in firing rate, usually accompanied by bursts of high frequency discharge in just under half the studies. Another cation, barium, evoked excitatory responses of a similar type and a reduction in potassium permeability may underlie the effects of both cations. In contrast, calcium, magnesium, manganese and cerium caused short duration depressant effects. The depression induced by calcium, but not by the other cations, could be blocked by zinc. Similarly, in the hippocampus zinc depressed calcium-dependent potentiation in subfield CA3 evoked by paired-pulse stimulation of mossy fibers; excitatory effects (namely an increase in spike amplitude and appearance of multiple population spikes) were seen at higher zinc concentrations. The depressant effects of an enkephalin analog on cortical firing rate were also blocked by zinc, consistent with studies from another laboratory suggesting enkephalin/zinc interactions. In contrast, the depressant effect of GABA could not be blocked by zinc, although an antagonism has been reported in the lobster muscle. Firm conclusions regarding the mechanism(s) underlying the triggering of seizure activity by zinc cannot yet be drawn, but the results of these studies would be consistent with an interference with calcium and/or potassium ion activity rather than with GABA binding sites.

Calcium antagonists and asthma

Calcium ions participate in the pathogenesis of asthma. Increased cytosolic concentrations of free Ca2+ must develop to trigger smooth muscle contraction, mast cell mediator release, mucous gland secretion, vagal nerve activity, and the movement of inflammatory cells into the walls of the airways. Recent interest has centered on the possibility that Ca2+ antagonists might be useful in the treatment of asthma. Evidence now exists that airway smooth muscle contraction and mast cell and basophil mediator release may be inhibited by the calcium channel blockers nifedipine and verapamil. Other experiments indicate that these drugs may interfere with EIA and bronchoconstriction provoked by cold air and methacholine, for example. CaM antagonists may also interfere with smooth muscle contraction and mediator release. It is possible that more specific calcium antagonists, both Ca2+ channel blockers and CaM-active compounds, will be developed and find use as effective antiasthmatic agents.

Differences in contractile activation between human myometrium and intramyometrial arteries

Small intramyometrial arteries and pieces of adjacent myometrial tissue were obtained from 25 non-pregnant women undergoing hysterectomy. Vascular and myometrial preparations were dissected, mounted in organ baths and isometric tension was recorded. Myometrial strips, but no vascular preparation, developed spontaneous contractile activity. Noradrenaline (NA) and vasopressin (VP) contracted both vessels and myometrium. Prostaglandin F2 alpha (PGF 2 alpha) contracted the myometrial tissue, but had only a minor effect on the vessels. Removal of extracellular calcium almost abolished the myometrial responses to high K+ (124 mM)-solution, PGF 2 alpha, NA and VP. The vascular responses remaining after this treatment were 18% (K+), 34% (NA) and 25% (VP) of control contractions induced by high K+ (124 mM). Nifedipine potently depressed myometrial contractions induced by NA and VP, but was less active against the vascular responses to these agents. In preparations exposed to calcium-free medium, nifedipine (10(-7) M) almost abolished myometrial contractions induced by calcium in the presence of K+ (124 mM), NA or VP. It also effectively depressed vascular responses to calcium in the presence of K+, but was less active if NA and VP were present. It is suggested that PGF2 alpha has almost no contractant effect on intramyometrial arteries, and that the activation process in these vessels is much less dependent on extracellular calcium than that of the myometrium.

Comparison of negative inotropic potency, reversibility, and effects on calcium influx of six calcium channel antagonists in cultured myocardial cells

The negative inotropic effects of calcium channel antagonists on the myocardium were used as a standard for the definition and determination of potency of this group of drugs. The effects of six calcium channel antagonists (verapamil, methoxyverapamil (D600), nifedipine, lidoflazine, perhexiline and diltiazem) were compared on cultured chick embryo ventricular cells. Drug concentrations producing 50% inhibition of contractile amplitude, derived from linearized concentration-response curves, varied from 2.8 X 10(-8)M for nifedipine to 8.3 X 10(-7)M for perhexiline. Equipotent negative inotropic concentrations of verapamil, D600, perhexiline, diltiazem and lidoflazine produced a similar inhibitory effect on 45Ca uptake into cultured cells. Nifedipine produced no significant inhibition of 45Ca uptake. The time required for recovery of contractility after cessation of drug superfusion varied in the order lidoflazine greater than perhexiline greater than D600 greater than verapamil greater than nifedipine greater than diltiazem. This relative order accords closely with the reported in vivo half-lives of these drugs. It is concluded that while some inhibition of 45Ca2+ uptake into cardiac cells can be demonstrated with five of the six calcium channel blockers studied, the relationship between the degree of inhibition of calcium influx and negative inotropic effects may not be uniform for all calcium channel antagonists.

The isolation and characterization of Ca++-accumulating subcellular membrane fractions from cerebral arteries

A study was undertaken using differential centrifugation methods to isolate from rabbit cerebral arteries the subcellular microsomal protein fractions capable of actively sequestering Ca++. One isolated protein fraction displayed a relatively large adenosine triphosphate (ATP)-dependent Ca++-accumulating capacity that was completely inhibited by NaN3, and was therefore designated the "mitochondrial fraction." Electron microscopy confirmed that this fraction consisted of numerous mitochondrial elements. Another isolated membrane fraction possessed a Ca++-accumulating capacity dependent on ATP and oxalate and only partially sensitive to NaN3. In the presence of mersalyl acid or the Ca++ ionophore, A23187, Ca++ uptake by this fraction was inhibited 98.0% and 87.4%, respectively. Electron microscopy revealed that this fraction consisted of numerous membrane vesicles, and measurements of Na+-K+-ATPase (adenosine triphosphatase) activity indicated minimal plasma membrane contamination. It was concluded that this microsomal fraction consisted primarily of sarcoplasmic reticulum. At physiological free [Ca++] levels, Ca++ uptake by this fraction was inhibited by norepinephrine through a process sensitive to tolazoline but not propranolol. The effects on Ca++ uptake of added cyclic adenosine monophosphate (cAMP) alone or with rabbit or bovine protein kinase were inconclusive. The organic Ca++ channel blockers, nifedipine and verapamil, significantly inhibited Ca++ uptake by sarcoplasmic reticulum.

Mechanism of action of natriuretic fraction of urine

The nature of calcium dependence of natriuretic fraction (NF)-induced contractions in the rat smooth muscle anococcygeus and a possible correlation between the effect of NF on sodium transport and its natriuretic potency was investigated. NF and noradrenaline-induced contractions were partially dependent on external calcium concentration. Ouabain and potassium (K) free solution (KoPSS)-induced contractions were totally dependent on external calcium and were more effectively inhibited by calcium antagonists than those of NF and noradrenaline suggesting pharmacomechanical coupling as the mode of calcium dependence of NF. Like other agonists which contract by pharmacomechanical coupling, NF stimulated sodium transport (86rubidium uptake) in dog saphenous vein. Such stimulation correlated positively (r = 0.495, n = 17, P less than 0.05) with the natriuretic potency of NF. NF, like noradrenaline, contracts smooth muscle by pharmacomechanical coupling and NF-induced natriuresis is associated with stimulation of the sodium pump.

Effect of diltiazem on histamine- and carbachol-induced bronchospasm in normal and asthmatic subjects

Recently, several transmembrane calcium-channel blockers have been used in experimental models to investigate the mechanisms through which Ca++ ions contribute to the regulation of the contractile response of airway smooth muscle and to determine the therapeutic use of these drugs in bronchial asthma. Since the data from these studies are inconsistent and inconclusive, we studied the effect of diltiazem, a calcium-channel blocker previously not examined to our knowledge, on histamine- and carbachol-induced bronchoconstriction in healthy and in asymptomatic allergic bronchial asthma. The study was performed in a double-blind, randomized, placebo-controlled fashion, using a single oral dose of 60 mg of diltiazem. Airway reactivity to histamine and carbachol expressed by PD35SGaw was significantly but weakly attenuated by diltiazem in the asthmatic, but not in the normal subjects. Baseline lung function was not significantly influenced by diltiazem. We concluded that the effect of diltiazem on unspecific airway hyperresponsiveness in asthmatic subjects is too weak to justify a recommendation as therapy.

The role of proctolin and glutamate in the excitation-contraction coupling of insect visceral muscle

Proctolin (1 X 10(-10) to 1 X 10(-9) M) had a minimal effect on the spontaneous and evoked electrical events of the hindgut of the cockroach Leucophea maderae. Spontaneous action potentials and contractile activity stopped when the hindgut was exposed to 2 mM Mn2+. Eighty per cent of the response of the hindgut to glutamate was blocked by manganese, but only 35% of the response to proctolin. Hindguts were responsive to proctolin in a calcium-free medium, but not to glutamate. Moreover, proctolin appeared to facilitate the reentry of calcium after depleted preparations were returned to normal levels of external calcium. The results offer evidence for two calcium transmembrane channels in insect visceral muscle.

Renal interactions between norepinephrine and calcium antagonists

The combined action of norepinephrine and calcium antagonists on the glomerular filtration rate (GFR) of isolated rat kidneys perfused at a constant pressure of 105 mm Hg was studied. Norepinephrine, 74 +/- 9 ng/ml (mean +/- SEM), increased the renal vascular resistance (RVR) by 50% and GFR declined by 88% (P less than 0.001). The subsequent addition of either diltiazem or verapamil (5 microM) returned RVR nearly to control and increased the GFR by an average of 86% over control (P = 0.007). Diltiazem or verapamil alone had little effect on RVR or GFR. Norepinephrine (20 ng/ml) did not affect RVR or GFR. In the presence of diltiazem, norepinephrine (20 ng/ml) again did not affect RVR but increased GFR by 58% (P = 0.003). With a low-calcium perfusate containing 0.4 mM total calcium, norepinephrine (90 ng/ml) did not affect RVR or GFR. Increasing perfusate norepinephrine to 236 +/- 8 ng/ml elicited a 50% increase in RVR, and GFR decreased by 95% (P less than 0.001). The subsequent addition of verapamil increased GFR to 2.8 times the original baseline value (P = 0.011) and restored RVR to control levels. These data suggest that diltiazem and verapamil block the vasoconstrictor action of norepinephrine on afferent arterioles selectively. Renal vascular sensitivity to norepinephrine decreases markedly either when the extracellular calcium concentration is reduced or in the presence of a calcium antagonist. However, a low-calcium perfusate does not mimic the property of calcium antagonists to facilitate an increase in GFR in the presence of norepinephrine.

Differential effects of D600, nifedipine and dantrolene sodium on excitation-secretion coupling and presynaptic beta-adrenoceptor responses in rat atria

The stimulation-evoked release of tritium was measured from rat atria labelled with [3H]-noradrenaline. The calcium dependence of evoked release and the facilitation of this release via activation of presynaptic beta-adrenoceptors were examined using D600 (methoxyverapamil), nifedipine and dantrolene sodium. Both D600 and nifedipine at dose levels of 20 and 100 microM inhibited evoked release. Dantrolene (20, 100 microM) reduced release by 25%, the effect being maximal at 20 microM. In the presence of 20 nM isoprenaline, a facilitation of evoked release occurred, which was blocked by 0.1 microM (-)-propranolol. The facilitatory action of isoprenaline was abolished by omission of calcium from the buffer, or by D600 or nifedipine, (100 microM). In contrast, the response to isoprenaline was not modified by dantrolene (20, 100 microM). It is concluded that the evoked release of noradrenaline (NA) utilizes Ca from both intra- and extracellular sources and that isoprenaline increases NA secretion by promoting the depolarization-induced influx of Ca.

Zinc: effect and interaction with other cations in the cortex of the rat

Zinc and other cations were iontophoretically applied to neurones in the frontoparietal cortex of urethane-anaesthetised rats and their effect on the activity of these cells was examined. Slow and often prolonged increases in firing rate occurred in 45.2% of neurones after zinc application and 72.7% after barium. Calcium caused short duration decreases in firing rate in 89.3% of neurones as did manganese (78.6%) cerium (21.9%) and magnesium (33.3%). When applied simultaneously zinc blocked the depressant effect of calcium (in 79.3% of neurones) but not that of manganese or GABA; other ions were ineffective. These results suggest that zinc, which is known to be present in nerve terminals, may modulate neuronal activity.

Fasciola hepatica: the effects of neuropharmacological agents upon in vitro motility

The effects of a wide range of neuropharmacological agents on the motility in vitro of Fasciola hepatica have been determined using an isometric transducer system. The neuromuscular blocking agents tubocurarine and decamethonium cause a long-term stimulation of the basal activity of the fluke. Acetylcholine causes an inhibition of activity. This effect is mimicked by the cholinergic agonists carbachol and nicotine, antagonised by the cholinergic blocking agents atropine and mecamylamine, and potentiated by eserine, a cholinesterase inhibitor. With nicotine and atropine the effects are accompanied by an increase in muscle tone at a concentration of 1 X 10(-2) M. Noradrenaline and adrenaline also cause some inhibition of activity, an effect antagonised by guanethidine, which blocks the release of noradrenaline. In contrast, dopamine stimulates fluke motility, whilst its antagonist dihydroergotamine causes an inhibition of activity. The monoamine oxidase inhibitors iproniazid and p-chloromercuribenzoic acid induce a stimulation of activity; with the latter there is an increase in muscle tone at a concentration of 1 X 10(-3) M. The amine depleting agents chloroamphetamine and reserpine, and the monoamine uptake inhibitors desipramine and nortriptyline produce an inhibition of fluke activity, as does the serotonin uptake inhibitor fluoxetine. High concentrations of chloroamphetamine (1 X 10(-2) M) and the uptake inhibitors (1 X 10(-3) M and above) also induce an increase in muscle tone. Serotonin causes a marked stimulation of motility. The pharmacological evidence is consistent with a neurotransmitter role of acetylcholine (inhibitory), dopamine (excitatory), and noradrenaline (inhibitory). The status of serotonin is discussed.

In vivo effects of three calcium blockers on chickens with inherited muscular dystrophy

Genetically homozygous Line 413 dystrophic chickens were given in separate trials daily i.p. injections of aqueous solutions of the calcium blocker drugs, diltiazem, verapamil, or nifedipine. At a dosage of 20 mg/kg/day, drug therapy in each case significantly prolonged the functional ability of the dystrophic chickens as quantitated regularly by a standardized test for righting ability. Enhanced functional ability, however, was not generally accompanied by a decrease in the usually high plasma creatine kinase activity. In addition, there was no change in the pectoralis muscle mass or protein with any of the drug treatments. Moreover, no significant reduction in the abnormally high total muscle calcium was found with calcium blocker treatment. Also, there was no marked change in the histopathology of muscle from the drug-treated dystrophic chickens. We concluded that drugs with calcium entry blocker activity offer only limited benefit in retarding dystrophic symptoms expressed in the chicken (viz., short-term enhancement in righting ability).

High affinity binding sites for [3H]-nitrendipine in rabbit uterine smooth muscle

The binding properties of the high affinity binding site for [3H]-nitrendipine on rabbit uteri membranes were investigated. The specific component had a dissociation constant of 0.46 +/- .07 nM and a maximal binding of 175 +/- 11 pmol/mg. A variety of other calcium channel blockers inhibited the binding of [3H]nitrendipine with varying potencies. Flunarizine demonstrated a high potency (IC50 = 0.30 microM) in inhibiting binding while verapamil and bepridil were less potent with IC50's of approximately 0.6-1.5 microM. Diltiazem did not displace nitrendipine even at very high concentrations. Verapamil displayed negative cooperative inhibition suggesting that a second site exists on uterine membranes for the binding of other calcium channel blockers.