The effects of nifedipine on contraction and monophasic action potential of isolated cat myocardium

Nanocrown electrodes for parallel and robust intracellular recording of cardiomyocytes

Drug-induced cardiotoxicity arises primarily when a compound alters the electrophysiological properties of cardiomyocytes. Features of intracellular action potentials (iAPs) are powerful biomarkers that predict proarrhythmic risks. In the last decade, a number of vertical nanoelectrodes have been demonstrated to achieve parallel and minimally-invasive iAP recordings. However, the large variability in success rate and signal strength have hindered nanoelectrodes from being broadly adopted for proarrhythmia drug assessment. In this work, we develop vertically-aligned nanocrown electrodes that are mechanically robust and achieve > 99% success rates in obtaining intracellular access through electroporation. We validate the accuracy of nanocrown electrode recordings by simultaneous patch clamp recording from the same cell. Finally, we demonstrate that nanocrown electrodes enable prolonged iAP recording for continual monitoring of the same cells upon the sequential addition of four incremental drug doses. Our technology development provides an advancement towards establishing an iAP screening assay for preclinical evaluation of drug-induced arrhythmogenicity.

Accurate nanoelectrode recording of human pluripotent stem cell-derived cardiomyocytes for assaying drugs and modeling disease

The measurement of the electrophysiology of human pluripotent stem cell-derived cardiomyocytes is critical for their biomedical applications, from disease modeling to drug screening. Yet, a method that enables the high-throughput intracellular electrophysiology measurement of single cardiomyocytes in adherent culture is not available. To address this area, we have fabricated vertical nanopillar electrodes that can record intracellular action potentials from up to 60 single beating cardiomyocytes. Intracellular access is achieved by highly localized electroporation, which allows for low impedance electrical access to the intracellular voltage. Herein, we demonstrate that this method provides the accurate measurement of the shape and duration of intracellular action potentials, validated by patch clamp, and can facilitate cellular drug screening and disease modeling using human pluripotent stem cells. This study validates the use of nanopillar electrodes for myriad further applications of human pluripotent stem cell-derived cardiomyocytes such as cardiomyocyte maturation monitoring and electrophysiology-contractile force correlation.

Field and action potential recordings in heart slices: correlation with established in vitro and in vivo models

BACKGROUND AND PURPOSE

Action potential (AP) recordings in ex vivo heart preparations constitute an important component of the preclinical cardiac safety assessment according to the ICH S7B guideline. Most AP measurement models are sensitive, predictive and informative but suffer from a low throughput. Here, effects of selected anti-arrhythmics (flecainide, quinidine, atenolol, sotalol, dofetilide, nifedipine, verapamil) on field/action potentials (FP/AP) of guinea pig and rabbit ventricular slices are presented and compared with data from established in vitro and in vivo models.

EXPERIMENTAL APPROACH

Data from measurements of membrane currents (hERG, I(Na) ), AP/FP (guinea pig and rabbit ventricular slices), AP (rabbit Purkinje fibre), haemodynamic/ECG parameters (conscious, telemetered dog) were collected, compared and correlated to complementary published data (focused literature search).

KEY RESULTS

The selected anti-arrhythmics, flecainide, quinidine, atenolol, sotalol, dofetilide, nifedipine and verapamil, influenced the shape of AP/FP of guinea pig and rabbit ventricular slices in a manner similar to that observed for rabbit PF. The findings obtained from slice preparations are in line with measurements of membrane currents in vitro, papillary muscle AP in vitro and haemodynamic/ECG parameters from conscious dogs in vivo, and were also corroborated by published data.

CONCLUSION AND IMPLICATIONS

FP and AP recordings from heart slices correlated well with established in vitro and in vivo models in terms of pharmacology and predictability. Heart slice preparations yield similar results as papillary muscle but offer enhanced throughput for mechanistic investigations and may substantially reduce the use of laboratory animals.

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.

Effects of inhibition of calcium and potassium currents in guinea-pig cardiac contraction: comparison of beta-caryophyllene oxide, eugenol, and nifedipine

1. To investigate the effects of the clove oil constituents beta-caryophyllene oxide and eugenol on the heart muscle, experiments were performed on isolated papillary muscles and on ventricular myocytes of the guinea-pig. The results were compared with those obtained with the dihydropyridine, nifedipine. 2. All three substances exerted negative inotropic effects in heart muscle although with different potencies and different influences on the time course of the contraction curve. 3. They all reduced rested-state contractions (RSCs) in the presence of isoprenaline which are thought to be due to the Ca(2+) current (I(Ca)). 4. beta-Caryophyllene oxide, eugenol and nifedipine inhibited the I(Ca) in single cells from the guinea-pig ventricle in a concentration-dependent, reversible way, but with different potencies. 5. In addition to the I(Ca)-inhibiting effect, beta-caryophyllene oxide strongly inhibited and eugenol slightly inhibited the potassium current. 6. The action potential of papillary muscles at a 1 Hz contraction frequency was greatly shortened by nifedipine, slightly shortened by eugenol, but not changed by beta-caryophyllene oxide. 7. The inhibition of the potassium current by beta-caryophyllene oxide obviously prevents a shortening of the action potential due to the diminution of the I(Ca). Accordingly, the negative inotropic effect of beta-caryophyllene oxide is closely related to the inhibition of I(Ca). In contrast, eugenol and nifedipine, which shorten the action potential, exert stronger negative inotropic effects than expected from their influence on I(Ca). 8. The results show that the negative inotropic effect of a calcium channel blocker can be attenuated by an additional inhibition of potassium channels.

Cardiotoxicity of emetine dihydrochloride by calcium channel blockade in isolated preparations and ventricular myocytes of guinea-pig hearts

1. The cardiotoxic effects of emetine dihydrochloride on mechanical and electrical activity were studied in isolated preparations (papillary muscles, sinoatrial and atrioventricular nodes, ventricular myocytes) of the guinea-pig heart. 2. Force of contraction was measured isometrically, action potentials and maximum rate of rise of the action potential were recorded by means of the intracellular microelectrode technique. Single channel L-type calcium current (Ba2+ ions as charge carrier) was studied with the patch-clamp technique in the cell-attached mode. 3. Emetine dihydrochloride (8-256 microM) reduced force of contraction in papillary muscles and spontaneous activity of sinoatrial and atrioventricular nodes concentration-dependently; the negative inotropic effect was abolished when the extracellular Ca2+ concentration was increased. 4. Maximum diastolic potential, action potential amplitude, maximum rate of rise of the action potential and the slope of the slow diastolic depolarization were decreased by emetine in sinoatrial as well as atrioventricular noes, while action potential duration was prolonged in both preparations (1-64 microM). 5. The amplitude of the L-type calcium single channel current was not altered by emetine dihydrochloride, while average open state probability was decreased concentration-dependently (10, 30 and 60 microM). 6. The most prominent effect of emetine dihydrochloride on single channel current was an increase of sweeps without activity. 7. At 60 microM, emetine dihydrochloride caused a decrease of the mean open time an increase of the mean closed time. The number of openings per record and number of bursts per record were reduced. 8. It is concluded that emetine dihydrochloride produces an L-type calcium channel block which might contribute to its cardiac side effects.

QT-interval prolonging drugs: mechanisms and clinical relevance of their arrhythmogenic hazards

The antiarrhythmic principle of drug-induced QT-interval prolongation is well known. However, with the widespread use of the presently known and new Class III antiarrhythmic agents under investigation, and the growing number of agents not primarily designed as antiarrhythmic drugs that potentially cause QT prolongation, we have also become aware of the proarrhythmic hazards associated with many of these agents. The proarrhythmic risk differs markedly from one agent to another and interferes with many individual clinical variables (e.g., hypokalemia, sinus bradycardia). This paper summarizes the present data on the proarrhythmic risk of drug-induced QT prolongation, including the value and problems of the rate-corrected QT interval, the mechanisms involved in the genesis of proarrhythmia, and the clinical cofactors that facilitate the occurrence of proarrhythmic events. In addition, an extensive database provides information on the known proarrhythmic risk of all currently used QT-prolonging agents.

Effect of nifedipine on thyrotropin, prolactin, and thyroid hormone release in man: a placebo-controlled study

Nifedipine (10 mg tid) or placebo was administered in a randomized double-blind trial for 1 wk to 2 groups of 10 mildly hypertensive euthyroid patients. Hormonal concentrations (thyrotropin [TSH], prolactin [PRL], thyroxin, triiodothyronine, and reverse triiodothyronine) before and during a TRH test were assessed in the 2 groups before (D0) and after (D7) each treatment. Parameters of the TRH test were determined (peak values, area under the curve [AUC], release [Kr], and elimination [Ke] rate constants) and their D7:D0 ratios were compared in the 2 groups. The TSH (peak values and Kr) and PRL (peak values and AUC) responses to TRH were significantly decreased in the nifedipine group compared to the placebo group. Neither basal nor TRH-stimulated thyroid hormone levels were modified. These results confirm experimental data but seem to be clinically irrelevant.

The mechanism of action of calcium antagonists on arrhythmias in early myocardial ischaemia: studies with nifedipine and DHM9

1. Nifedipine and DHM9 (carboxymethyl methyl 1,4-dihydro-2,6-dimethyl-4-(3-nitrophenyl)-3,5-pyridinedicarboxylate) were studied for their effects on arrhythmias resulting from regional myocardial ischaemia in conscious rats, and for their effects on left ventricular developed pressure in vitro. 2. Nifedipine possessed antiarrhythmic activity at a high dose of 10 mg kg-1 i.v., but not at 0.5 or 2 mg kg-1. Ventricular fibrillation (VF), tachycardia (VT), and ventricular premature beats (VPB) were all attenuated to a similar degree; nifedipine did not have a selectivity of action for high frequency arrhythmias. 3. Before coronary occlusion, the three doses of nifedipine reduced arterial blood pressure by a similar magnitude, indicating a similar (maximal) degree of systemic vasodilatation. The reductions in blood pressure were accompanied by reflex tachycardia. Heart rate and blood pressure did not correlate with the incidence or severity of arrhythmias. 4. DHM9 had no influence on arrhythmias, haemodynamic variables or the ECG, even at 20 mg kg-1 i.v. 5. Nifedipine concentration-dependently reduced contractility in perfused paced (5 Hz) rat ventricles in vitro. Raising the concentration of K+ in the perfusion fluid from 3 to 10 mequiv.l-1 increased the potency (-log10 EC50) of nifedipine up to four fold, and caused a significant depression in excitability. 6. DHM9 at up to 3 x 10(-5) M had no significant influence on ventricular contractility in vitro. 7. The results provided indirect evidence in support of the hypothesis that calcium antagonists inhibit ischaemia-induced arrhythmias by virtue of inhibition of the slow inward current (Isi) in the ischaemic ventricular myocardium.

Intravenous nifedipine for control of hypertension in patients after coronary artery bypass graft surgery

A study was undertaken to assess the use of intravenous nifedipine in controlling hypertension in patients following coronary artery surgery. A combined hemodynamic and metabolic assessment was carried out in 15 patients on data recorded at six sequential time intervals: (1) baseline, (2) control of blood pressure, (3) 30 minutes after control of blood pressure, (4) 1.5 hours after control of blood pressure, (5) 3.5 hours after control of blood pressure, and (6) 30 minutes after discontinuing nifedipine. Coronary sinus and great cardiac vein blood flows were measured by the continuous thermodilution technique using the Baim coronary sinus flow catheter. Intravenous nifedipine was run initially at an average rate of 1.82 microg/kg/min. It took an average time of 12 minutes to lower the blood pressure to less than 130 mmHg systolic. There were highly significant decreases in systolic, mean, and diastolic blood pressures (P < .001), associated with significant decreases in systemic vascular resistance (P < .001) and left ventricular stroke work index (P < .05). There was an increase in cardiac output at 30 and 90 minutes of infusion (P < .05), and the stroke volume was increased 90 minutes after starting nifedipine (P < .05). The increase in heart rate was not significant. There was no significant effect on conduction times as measured by PR and QRS intervals on the ECG. However, the QTc interval was decreased after 3.5 hours (P < .05). There was an increase in right atrial pressure at 90 minutes and again 30 minutes after stopping nifedipine. (P < .05). The pulmonary artery pressure also was increased after stopping the infusion (P < .05). The pulmonary capillary wedge pressure, pulmonary vascular resistance, and right ventricular stroke work index remained unchanged. Coronary sinus and great cardiac vein flows were maintained despite a decrease in perfusion pressure, suggesting that nifedipine is a potent coronary vasodilator. Indeed, coronary vascular resistance was significantly decreased (P < .05). Myocardial oxygen consumption remained unchanged. The lactate extraction indicated that myocardial metabolism remained aerobic regionally and globally. Thus, the results suggest that blood pressure was easy to control and that there were no adverse effects on atrioventricular conduction, cardiac performance, regional and global myocardial oxygen utilization, or lactate extraction.

Calcium channel blockers in systemic hypertension

Alterations in transmembrane flux of calcium ions may be playing a role in the pathophysiology of systemic hypertension. Calcium channel blockers have been shown to be effective antihypertensive drugs with excellent safety profiles. They are efficacious in the long term treatment of systemic hypertension in all population subgroups, and have special applicability for treating patients with hypertensive urgencies and individuals with concomitant diseases such as angina pectoris and arrhythmias.

Basic mechanisms underlying prenylamine-induced 'torsade de pointes': differences between prenylamine and fendiline due to basic actions of the isomers

The calcium antagonists prenylamine and fendiline both bind with rather low affinity to the dihydropyridine (nifedipine) binding site. As calmodulin (CaM) antagonists, they both inhibit CaM-dependent enzymes and relax smooth muscle preparation in nearly the same concentration range. If compared with other calcium antagonists, their action on smooth muscle develops rather slowly and cannot be inhibited by the calcium agonist Bay k 8644. In contrast, basic pharmacology reveals major differences of the actions of prenylamine and fendiline in heart muscle, indicating that, after all, the change in structure close to the asymmetric carbon strongly influences the molecular action of the compounds and their respective isomers. The negative inotropic effect of racemic prenylamine is rather independent of stimulation rate, whereas fendiline preferably depresses contraction at high rate stimulation. The negative inotropic potencies are determined by the (-)-isomers, but only in the case of prenylamine the isomeric ratio of 6 reveals a considerable stereoselectivity of action. In low concentrations and preferably at low rate stimulation, (+)-prenylamine exerts a strong positive inotropic effect. At low rate stimulation, total duration of transmembrane action potential is prolonged by (+/-)- and (+)-prenylamine, but discretely shortened by (+/-)- and (+)-fendiline. At high rate stimulation, it is shortened by (+/-)- and (-)-prenylamine, but prolonged (only) at the very final repolarization level by (+/-)- and (-)-fendiline. The positive inotropic action of prenylamine and the prolongation of action potential at low stimulation rate can be interpreted as a calcium agonistic side-effect due to the action of the (+)-isomer. It seems possible that, under the condition of low heart rate, prenylamine (as reported for the calcium agonist Bay k 8644) increases the potential-dependent transmembrane calcium current. In addition, it is argued that during the long-lasting action potential, a reactivation of the calcium current induces early after-depolarizations. These effects are postulated to represent the main mechanisms triggering torsade de pointes during therapy with prenylamine. Though fendiline, from a chemical point of view, rather resembles prenylamine, its pharmacological profile is different. In particular, in regard to electrophysiology, torsade de pointes are not expected to be induced by fendiline.

The peripheral, high affinity benzodiazepine binding site is not coupled to the cardiac Ca2+ channel

Ro 5-4864, the prototype ligand of the peripheral benzodiazepine binding site caused a decrease of the action potential duration in isolated guinea-pig cardiac myocytes. Voltage-clamp experiments showed that, at concentrations below 3 X 10(-6) M, Ro 5-4864 caused a parallel outward shift of the membrane current elicited by depolarization to + 10 mV from a holding potential of -50 mV. The peak inward Ca2+ current (ICa) and the inwardly rectifying K+ current were not affected. ICa was reduced by Ro 5-4864 at concentrations above 3 X 10(-6) M. At these concentrations, Ro 5-4864 also caused a negative inotropic effect in isolated guinea-pig papillary muscles, reduced K+ depolarization-induced contractures of the isolated rat aorta and inhibited [3H]nitrendipine binding to guinea-pig cardiac membranes. These data provide no evidence that the peripheral high affinity benzodiazepine binding site is coupled to the cardiac Ca2+ channel. The possibility cannot be excluded that a postulated micromolar affinity benzodiazepine receptor is associated with the Ca2+ channel.

Arterial pressure and heart rate responses to calcium channel blockers administered in the brainstem in rats

The study examined the possibility that organic calcium channel blockers alter autonomic nervous system function by acting on brainstem neurons. Intracisternal administration of diltiazem or verapamil produced dose-related decreases in arterial pressure and heart rate. Diltiazem administered by drop on the dorsal surface of the brainstem at the obex or microinjected directly into the nucleus tractus solitarius also decreased arterial pressure and heart rate. The responses were absent or markedly attenuated in rats previously treated with 6-hydroxydopamine or with bilateral electrolytic lesions of the nucleus tractus solitarius but were preserved in rats treated with atropine or with sham nucleus tractus solitarius lesions. Nifedipine or ethylene glycol-bis-(beta-aminoethyl ether)N,N'-tetraacetic acid administered on the dorsal surface of the brainstem at the obex decreased arterial pressure and heart rate. Vehicle, acid saline, or sucrose solution failed to alter arterial pressure or heart rate. These results suggest that organic calcium channel blockers produce excitation of the nucleus tractus solitarius neurons, directly or indirectly, which results in the withdrawal of sympathetic nervous activity and in the decrease in arterial pressure and heart rate. The results suggest that calcium ion plays an important role in maintaining integral function of neurons in the brainstem, particularly in the nucleus tractus solitarius.

Hypotension and sinus arrest with nifedipine in pulmonary hypertension

An adverse reaction to the acute administration of sublingual nifedipine is reported. Subsequent reversal was by intravenous administration of calcium chloride.

Effect of nifedipine on TRH stimulation of TSH and PRL release by the pituitary gland

Studies in vitro and in vivo have shown that thyrotropin-releasing hormone (TRH)-induced calcium ion changes in the adenohypophysial cells play an important role in release of hormones by the anterior pituitary. To determine the effect of the calcium blocker nifedipine on TRH-induced thyroid-stimulating hormone (TSH) and prolactin (PRL) release, TRH stimulation tests were performed before and after 74 hours of nifedipine therapy in ten patients. Although the magnitude of the TSH and PRL mean peak increase above baseline was slightly lower during calcium blocker administration (TSH 14.1 +/- 4.8 SEM v 16.4 +/- 4.5 SEM; PRL 37.7 +/- 4.5 SEM v 41.7 +/- 5.4 SEM), this was not statistically significant. Use of nifedipine in clinically effective doses does not appear to significantly interfere with TRH-stimulated release of TSH or PRL, in vivo.

Basic understanding of the electrophysiologic actions of antiarrhythmic drugs. Sources, sinks, and matrices of information

The author creates an intellectual framework consisting of key electrophysiologic principles, basic mechanisms of arrhythmogenesis, and important drug reactions that will allow the rational use of antiarrhythmic drugs. Basic principles have been emphasized because current understanding requires it.

Depression of automaticity of the rabbit SA node by bepridil and nifedipine

The effects of bepridil and nifedipine, newly developed Ca2+ antagonists, were studied on the spontaneous action potentials of the true pacemaker cell preparations (0.5 X 0.5 mm) obtained from the rabbit sino-atrial (SA) node. At the lower concentrations, bepridil (10(-6) M) and nifedipine (10(-8) M) decreased the spontaneous activity in accordance with the decrease of the maximum rate of rise of the action potential (Vmax), without changing the maximum diastolic potential (MDP) and the 50% action potential duration ( APD50 ). At the high concentrations, bepridil (5 X 10(-6) M) and nifedipine (5 X 10(-8) M) stopped the sinus automaticity with the depolarization of the MDP. The action potential could not be induced by the hyperpolarizing electrical stimulation in the presence of the higher concentration of these drugs. The results suggest that bepridil and nifedipine have a primary effect of the blocking action on the SA node slow channels, similar to the action of verapamil. This can account for their negative chronotropic action.

Calcium antagonistic drugs differ in ability to block the slow Na+ channels of young embryonic chick hearts

Young (3-day-old) embryonic chick hearts have slowly rising action potentials (APs) dependent on slow Na+ channels, and these slow APs are blocked by verapamil, but not by Mn2+ (1 mM). In contrast, old embryonic hearts possess slow Ca2+-Na+ channels which can be blocked by both verapamil and Mn2+. In the present experiments, we compared the effects of a number of Ca antagonistic drugs on the spontaneous APs recorded from intact 3-day-old embryonic chick hearts (ventricles). At 10(-5) M, verapamil, D-600, and nifedipine abolished the spontaneous APs (3 X 10(-6) M produced some depression of Vmax). Electrical stimulation did not elicit APs, and long-duration hyperpolarizing pulses (applied because the maximum diastolic potential was moderately decreased) did not allow APs to be elicited by stimulation. This blockade was not reversed by elevation of [Ca]0 from 0.6 mM (control concentration used to reduce contractions) to 3.6 mM or 5.4 mM. However, lowering of [Na]0 from 137 mM to 68.5 mM facilitated the blockade. Washout of the drugs for 30 min produced nearly complete recovery. In contrast to the effects of verapamil, D-600, and nifedipine, 10(-5) M diltiazem, bepridil, and mesudipine had little or no depressant effect on Vmax; the same was true for 1 mM Mn2+. These results indicate that verapamil, D-600, and nifedipine are blockers of slow Na+ channels (as well as of slow Ca2+-Na+ channels), whereas diltiazem, bepridil and mesudipine do not block slow Na+ channels. Therefore, the Ca antagonistic drugs vary in their ability to block slow Na+ channels.

Dihydropyridine Ca2+ entry blockers selectively inhibit peak I cAMP phosphodiesterase

The vasodilatory action of Ca2+ entry blockers is due primarily to slow Ca2+ channel inhibition; however, these drugs may have additional sites of action that contribute to vasodilation. Eleven Ca2+ entry blockers were evaluated for their inhibition of the two major forms of bovine heart cAMP phosphodiesterase which were separated by DEAE cellulose chromatography. Nifedipine and four other dihydropyridine Ca2+ entry blockers selectively inhibited peak I phosphodiesterase activity with IC50 values between 2 and 3 microM but were weak inhibitors of peak II phosphodiesterase with IC50 values of 100 microM or greater. The selective inhibition of peak I phosphodiesterase activity by these dihydropyridine Ca2+ entry blockers may be an intracellular mechanism for producing vasodilation in addition to slow Ca2+ channel inhibition.

Comparison of the direct effects of nifedipine and verapamil on the electrical activity of the sinoatrial and atrioventricular nodes of the rabbit heart

We compared the effects of nifedipine and verapamil on the rabbit sinus and atrioventricular nodes. Both drugs slowed the rate of impulse initiation by sinus node cells. Verapamil exerted a greater negative chronotropic effect at low concentrations, but at higher concentrations verapamil and nifedipine were equipotent. Nifedipine also reduced the amplitude of sinus node action potentials and the Vmax of phase O, effects which are identical to those previously described for verapamil. Both drugs slowed AV nodal conduction and prolonged refractory periods, but verapamil was more potent than nifedipine. Nifedipine reduced the amplitude of AV nodal action potentials and Vmax of phase O the same as verapamil. Nifedipine and verapamil, therefore, have nearly identical direct effects on the nodes. The failure of nifedipine to depress AV nodal conduction in situ and abolish reentrant AV nodal tachycardia is probably a result of the decreased sensitivity of the AV node to nifedipine compared to verapamil.

Effect of the Ca2+ antagonist nifedipine on the release of platelet-activating factor (PAF-acether), slow-reacting substance and beta-glucuronidase from human neutrophils

Ca2+ antagonists such as nifedipine (Nif) inhibit processes that depend on extracellular Ca2+ in many muscular and secretory cells. The effect of Nif on mediator release and Ca2+ uptake by human polymorphonuclear neutrophils (PMN) has been investigated. Nif caused a concentration-dependent inhibition of the Ca2+ ionophore-induced release of platelet-activating factor (PAF-acether), slow-reacting substance (SRS) and to a lesser degree beta-glucuronidase (beta-glu). Nif inhibited the synthesis of PAF-acether rather than its release. Increasing Ca2+ concentration in the bathing medium from 1.3 to 2.8 mM completely reversed the effect of Nif on PAF-acether secretion. Nif at 1 and 5 microM reduced PMN45Ca2+ uptake induced by the Ca2+ ionophore A 23187. These results indicate that the inhibition by Nif of mediator release depends probably on the Ca2+-antagonistic property of the drug. A preliminary ex vivo study suggests that this inhibitory effect on neutrophil functions exists during therapeutic use.

Characterization of [3H]nifedipine binding sites in rabbit myocardium

A specific, high affinity (KD 1.8 nM) binding site for the calcium entry blocking drug [3H]nifedipine was identified in homogenates of rabbit myocardium. [3H]Nifedipine binding was rapid (t1/2 3 min) and reversible (t1/2 11 min). Calcium entry blockers with different chemical structures competed with [3H]nifedipine binding in the potency order: nifedipine much greater than D600 = verapamil greater than tiapamil greater than cinnarizine = prenylamine. Diltiazem and perhexiline did not significantly inhibit [3H]nifedipine binding. The potencies of these drugs to inhibit binding were similar to their abilities to depress contractions of the isolated rabbit papillary muscle. The stereoselectivity of D600 and verapamil ((-)-much greater than (+)-isomers) as inhibitors of papillary muscle contractions was not apparent in [3H]nifedipine competition experiments. The slopes of the concentration-inhibition curves for D600 and verapamil were significantly less than for nifedipine. It is concluded that [3H]nifedipine may be labelling part of the myocardial Ca2+ channel, and that verapamil-like substances and nifedipine differ in their mode of interaction with this binding site.

Comparative effects of nitroglycerin and nifedipine on myocardial blood flow and contraction during flow-limiting coronary stenosis in the dog

Both nifedipine and nitroglycerin are used to treat angina pectoris. The comparative effects of these agents on myocardial blood flow and contraction in the setting of flow-limiting coronary stenosis are poorly understood. Thus 24 open chest dogs underwent carotid to left anterior descending coronary arterial perfusion with coronary flow probe and perfusion pressure monitoring. Segment length was measured with ultrasonic crystals in the subendocardial ischemic and nonischemic zones. Myocardial blood flow was measured with radioactive microspheres. Partial coronary occlusion was performed to attain a diastolic perfusion pressure of 40 mm Hg. Twelve dogs received intravenous nifedipine, 3 micrograms/kg per min, and 12 received intravenous nitroglycerin to reduce aortic pressure by 20 mm Hg. Partial occlusion resulted in a slight but significant decrease in segment shortening in the ischemic zone. Neither nitroglycerin nor nifedipine affected shortening in the ischemic zone. After occlusion, blood flow decreased in the subendocardial ischemic zone but was unchanged in the subepicardium. Nifedipine increased subendocardial blood flow in the nonischemic zone and decreased it in the ischemic zone but caused no change in subepicardial flow in the ischemic zone. In contrast, nitroglycerin decreased subendocardial and subepicardial blood flow in both the ischemic and nonischemic zones. In the setting of coronary stenosis, different classes of vasodilators may have varying effects on myocardial blood flow, suggesting different sites and mechanisms of action. In addition, segment function may not always reflect changes in myocardial blood flow.

Actions of verapamil, diltiazem and other divalent cations on the calcium-current of Helix neurones

1 Effects of organic Ca(2+)-antagonists, verapamil and diltiazem, and cations, Ni(2+), Mn(2+), Co(2+) and La(3+) on Ca(2+) current (I(Ca)) separated from other ionic currents in a Helix neurone were studied. A suction pipette technique which allows internal perfusion of the cell body and voltage clamp was used.2 Verapamil and diltiazem (10(-6)-10(-4) M) increased the threshold, and decreased both the amplitude and rate of rise of the soma Ca(2+)-spike. Both agents inhibited I(Ca) over the entire range of the current-voltage (I-V) relationship dose-dependently, without shifting the threshold of the I-V relationship. Increases in external Ca(2+) overcame the inhibitory action of the agents.3 Divalent cations, Ni(2+), Mn(2+), Co(2+) and the trivalent cation, La(3+) inhibited I(Ca) dose-dependently, but induced shifts of the I-V relationship to more positive voltages. The order of potency of inhibition of I(Ca) among these cations was as follows; Ni(2+) > La(3+) > Mn(2+) > Co(2+).4 Double reciprocal plots for peak I(Ca) versus external Ca(2+) concentrations in the presence or absence of both organic and inorganic Ca(2+)-antagonists intersect at the ordinate. Results indicate that both organic and inorganic Ca(2+)-antagonists compete for Ca(2+) at the common binding site for Ca(2+).5 Internal application of the organic Ca(2+)-antagonists (10(-4) M) inhibited I(Ca) in a time-dependent manner to about 40-60% of the control. Ni(2+), when applied internally, also depressed I(Ca).6 The results provide evidence that organic Ca(2+)-antagonists occupy the binding site for Ca(2+) in a competitive manner at the surface of the soma membrane of the Helix neurone, while divalent and trivalent cations, in addition to inhibiting I(Ca) in a similar manner to the organic Ca(2+)-antagonists, change the surface charge of the soma membrane.

Nifedipine in the treatment of cardiovascular disease

Nifedipine, a calcium channel blocking agent, has been shown safe and effective in the treatment of various cardiac disorders. In Prinzmetal's variant angina, nifedipine relieves the acute spasm of a large coronary artery, and thereby reverses the sudden decrease in myocardial oxygen supply. In chronic stable angina, the efficacy of nifedipine stems from an interplay of direct and reflex effects. A decrease in systemic vascular resistance, and thus, myocardial oxygen demand, is considered the most important effect. Although this is partially offset by baroreceptor-mediated reflex increases in blood pressure and heart rate, the net effect is a decrease in myocardial oxygen consumption. The importance of nifedipine-induced increases in coronary blood flow in relieving stable angina remains controversial. Numerous clinical trials in patients with chronic angina have demonstrated that nifedipine increases exercise tolerance by lowering the heart rate-blood pressure product (an approximation of myocardial oxygen consumption). The maximal double product during exercise, however, does not increase, suggesting that the drug does not importantly improve myocardial oxygen supply. Nifedipine may also be of value in the therapy of unstable angina, acute myocardial infarction, and hypertension, as well as in the protection of the myocardium at risk during open heart surgery. However, definite recommendations on the use of the drug for these indications must await results of ongoing randomized studies. The effects of nifedipine are dose-related. Larger doses have been shown more effective than smaller doses, as long as an appropriate blood pressure is maintained. The side effects of nifedipine are mild and related to its vasodilating action.

Comparative pharmacology of calcium antagonists: nifedipine, verapamil and diltiazem

Calcium antagonists (slow channel blocking agents) are a very heterogeneous group of agents with dissimilar structural, electrophysiologic and pharmacologic properties. Nifedipine is a potent, long-acting vasodilator that has proved highly efficacious in relieving anginal symptoms caused by coronary vasospasm. In vivo, it exerts no myocardial depressant effects and has no antiarrhythmic properties. Treatment with nifedipine can safely be combined with administration of a beta receptor blocking agent. VErapamil prolongs atrioventricular (A-V) conduction (A-H interval) in a dose-dependent manner. It is the drug of choice for the treatment of reentrant supraventricular arrhythmias, irrespective of whether reentry occurs within the A-V node or through an accessory pathway (the Wolff-Parkinson-White syndrome). Verapamil is only moderately effective as an antianginal agent. Diltiazem is efficacious for the treatment of angiospastic angina, but its value as an antiarrhythmic agent remains to be delineated.

Some effects of sodium nitroprusside, methoxyverapamil (D600) and nifedipine on rat portal vein

1 The effects of methoxyverapamil (D600), nifedipine and sodium nitroprusside on noradrenaline-induced electrical and mechanical activity in rat portal vein have been examined. 2 D600 and nifedipine produced a concentration-dependent reduction in mechanical responses to noradrenaline whilst sodium nitroprusside had no effect. The effects of D600 and nifedipine were reversed by increasing the extracellular calcium concentration. 3 The mechano-inhibitory effects of D600 were accompanied by a marked reduction in electrical activity with some evidence of electromechanical uncoupling. 4 The mechano-inhibitory effects of nifedipine were associated with considerable electromechanical uncoupling. 5 It is concluded that in the concentrations used, D600 acts primarily by inhibiting calcium influx with some effect on electromechanical coupling whilst nifedipine interferes mainly with the coupling process. The inactivity of sodium nitroprusside suggests that the excitation-contraction coupling process in rat portal vein is relatively simple and further studies with this tissue seem indicated.

The effect of nifedipine on the monophasic action potential and refractoriness of the right ventricle of the dog heart in situ after beta-adrenergic receptor blockade

The effect of nifedipine, a calcium-antagonistic drug, was studied on the electrophysiology of the right ventricle in the dog heart in situ. Monophasic action potential recordings were obtained by the suction electrode technique and refractoriness was measured by means of programmed electrical stimulation. Pentobarbital anaesthesia was used. As the basic cardiac effects of nifedipine can be altered by the release of catecholamines from sympathetic nerves of the heart and vessels, the dogs were pretreated with the beta-adrenergic receptor blocking agent acebutolol which increased the action potential duration and the refractoriness. Intravenous injection of nifedipine 30 microgram/kg body weight decreased the times for 50 and 90 per cent repolarization of the monophasic action potential and to a smaller extent the effective and functional refractory period. It is suggested that nifedipine decreases the action potential duration and the refractoriness of the right ventricle of the dog heart in situ due to a direct effect of the drug on the myocardium.