New selective inhibitors of the transmembrane Ca conductivity in mammalian myocardial fibres. Studies with the voltage clamp technique

Verapamil therapy: a new approach to the pharmacologic treatment of hypertrophic cardiomyopathy. I. Hemodynamic effects

The hemodynamic effects of intravenous verapamil administration were examined in 27 patients with hypertrophic cardiomyopathy. Increasing doses of verapamil produced small increases in heart rate and cardiac output and a significant decrease in systolic blood pressure, but had no significance effect on mean pulmonary artery wedge pressure or left ventricular end-diastolic pressure. The highest dose of verapamil increased heart rate from 72 +/- 3 to 81 +/- 6 beats/min and reduced systolic blood pressure from 118 +/- 8 to 99 +/- 5 mm Hg (p less than 0.005). This dose decreased the basal left ventricular outflow tract gradient from 94 +/- 14 to 49 +/- 14 mm Hg and the average left ventricular outflow tract gradient during the Valsalva maneuver from 76 +/- 5 to 63 +/- 13 mm Hg, during amyl nitrite inhalation from 69 +/- 15 to 39 +/- 13 mm Hg, and during isoproterenol infusion from 108 +/- 29 to 70 +/- 21 mm Hg (p less than 0.01). These results indicate that verapamil can significantly decrease left ventricular outflow obstruction in patients with hypertrophic cardiomyopathy and thus may provide an important new therapeutic agent in the treatment of this disorder.

Rapid high-pressure liquid chromatographic analysis of verapamil in blood and plasma

A high-pressure liquid chromatographic assay procedure has been developed for verapamil in blood or plasma. A paired-ion solvent system with a reversed-phase column is employed. The procedure is specific for verapamil and the retention times of the major metabolites are identified. This procedure is sensitive to a lower blood concentration of 1 ng/ml and standard curves were found to be linear up to the highest concentration tested, 500 ng/ml. Several drugs were tested for interference with the assay, but none were found to cause any problems. The procedure is simple, rapid and permits the analysis of up to 25 samples per day.

Effect of replacement of extracellular sodium ions and of D-600 on the activation by adrenalin of adenylate cyclase in intact pigeon erythrocytes

The effect of replacement of extracellular Na+ by Li+, choline K+ or sucrose on cyclic AMP formation in pigeon erythrocytes has been investigated. Replacement of extracellular Na+ by Li+, choline or sucrose but not by K+ inhibited the stimulation by adrenalin of cyclic AMP formation, but had no detectable effect on cyclic AMP content in the absence of adrenalin. This inhibition was observed in the presence or absence of extracellular Ca2+. The relative inhibition caused by Na+ removal decreased with increasing adrenalin concentration. It was concluded that extracellular Na+ or K+ ions were required for maximal activation of adenylate cyclase by low concentrations of adrenalin, and that this effect of monovalent cations may have been due to an effect on the affinity of the receptor for adrenalin. The verapamil derivative D-600 also inhibited the stimulation by adrenalin of cyclic AMP formation. This effect occurred in the absence of extracellular Ca2+ and hence seemed to be unrelated to the inhibition by C-600 of the slow Ca2+ channel in electrically excitable tissues.

New antiarrhythmic agents: amiodarone, aprindine, disopyramide, ethmozin, mexiletine, tocainide, verapamil

The present status, clinical experience, side effects, clinical pharmacology and electrophysiologic actions of seven new antiarrhythmic agents are reviewed. The drugs selected for comment are amiodarone, aprindine, disopyramide, ethmozin, mexiletine, tocainide and verapamil. Each drug has been shown to have clinical efficacy in suppressing cardiac arrhythmias.

Effects of intravenous verapamil on hemodynamics in patients with heart disease

The hemodynamic effects of intravenous verapamil (10 mg.) were evaluated in 13 patients with coronary artery disease and in seven patients with rheumatic valvular disease during cardiac catheterization. The peak effects were apparent at 3 to 5 minutes after injection and lasted about 10 minutes. The mean arterial pressure fell from 97.8 +/- 3.4 to 85.9 +/- 2.7 mm. Hg (-12%; p less than 0.01) accompanied by a significant decrease (-21%, p less than 0.001) in systemic vascular resistance (from 1435 +/- 80 to 1131 +/- 82 dynes-sex.-cm.-5) with an increase in left ventricular end-diastolic pressure (from 11.0 +/- 0.9 to 15.0 +/- 1.0 mm. Hg; +36%, p less than 0.01) and a reduction in LV dp/dt max (from 1343 +/- 152 to 1007 +/- 102 mm. Hg/sex.; -25%, p less than 0.05). The changes in heart rate (from 75.7 +/- 3.0 to 80.2 +/- 2.8 beats/min.), cardiac index (from 3.17 +/- 0.15 to 3.61 +/- 0.17 L./min./M.2), left ventricular minute work (from 3.63 +/- 0.28 to 3.31 +/- 0.23 Kg.-m./min./M.2) and mean pulmonary artery pressures (from 15.7 +/- 1.0 to 18.1 +/- 0.8 mm. Hg) were not statistically significant. The intrinsic negative inotropic action of verapamil is, therefore, minimized by its effect on afterload so that cardiac index is not reduced by the drug in patients with cardiac disease.

Influence of cations on the electrical activity of neuroblastoma X glioma hybrid cells

Electrical excitability is one of the various neuronal properties of neuroblastoma X glioma hybrid cells. At a Ca2+ concentration of 1.8 mM the action potential is inhibited by tetrodotoxin, suggesting that the inward current is carried by Na+ ions. In contrast, at a Ca2+ concentration of 20-36 mM and even in the absence of Na+, spikes (sometimes repetitive) with strong hyperpolarizing afterpotential occur, which are no longer affected by tetrodotoxin. They are, however, blocked by antagonists of Ca2+ like La3+, Co2+, Mn2+, and the synthetic compounds D-600 and BAY a-1040. This seems to indicate that at high concentrations of Ca2+, the inward current of the action potential is essentially carried by Ca2+. Sr2+, but not Mg2+ can effectively substitute for Ca2+. It slows down the time course of the action potential. Ba2+ depolarizes the membrane gradually. If Ca2+ is also present, Ba2+ causes a reduced depolarization and spontaneous action potentials with no hyperpolarizing after-potential are observed.

Effect of drugs on conduction delay and incidence of ventricular arrhythmias induced by acute coronary occlusion in dogs

The effects of various drugs on delayed activation of the ischemic myocardium and the incidence of ventricular arrhythmias were studied in 34 open-chest anesthetized dogs. The left anterior descending coronary artery was occluded for 6 minutes before and 42 minutes after administration of aprindine (2.85 mg/kg body weight), quinidine (8 mg/kg) and verapamil (0.2 mg/kg) and during infusion of isoproterenol (0.2 microng/min). The time intervals from the onset of the QRS complex to the major deflection of the bipolar electrograms recorded within the normal and ischemic zones were measured at cycle lengths of 500, 400 and 300 msec and were correlated with the development of ventricular arrhythmias. At a cycle length of 500 msec, aprindine increased by 19.5 msec the delay in activation time produced by coronary ligation alone (P less than 0.05), whereas verapamil reduced by 10 msec the extent of ischemia-induced conduction delay (P less than 0.05). The delay in activation time in the ischemic zone was not significantly altered by quinidine or isoproterenol. The incidence of ventricular arrhythmias was increased by aprindine (from 1 in 11 to 8 in 11 dogs), decresed by verapamil (from 3 in 7 to 0 in 7 dogs) and was not changes by quinidine or isoproterenol. Thus, delayed activation of the ischemic myocardium appears to play an important role in the genesis of early arrhythmias due to myocardial ischemia, and drugs that significantly depress conduction in the ischemic myocardium may predispose to the development of ventricular arrhythmia whereas those that improve conduction may be protective. Contrary to their effects on slow channel-dependent conduction, verapamil improved and isoproterenol worsened conduction during ischaemia.

Relationship between internal calcium and outward current in mammalian ventricular muscle; a mechanism for the control of the action potential duration?

In sheep and calf ventricular bundles, increasing the internal calcium by increasing the frequency of voltage-clamping to plateau range potentials increased the time-independent outward current. This effect was more marked with higher [Ca]o, and was reduced if the Ca current blockers Verapamil or D 600 were used. 2. If the internal Ca was increased by the addition of cyanide and reduction of external sodium the outward current was also increased. The frequency-dependent increase in outward current also occurred in this Na-poor (12 mM) solution. 3. Tension measurement on the ventricular bundles showed that a Na-free solution with cyanide did not cause a contracture. On changing from Tyrode to a Na-free solution containing cyanide, and on changing back to Tyrode there was a potentiation of the twitch. 4. In Na-poor solution with cyanide, although no contracture was found, ECa was less positive, suggesting that under these circumstances Ca accumulates at the inner side of the membrane, but not around the myofibrils. 5. The prolongation of the action potential in Cl-free solution is frequency-dependent. A greater prolongation is seen at lower frequencies suggesting that Cl current is relatively more important for repolarization at lower frequencies of stimulation. 6. It is suggested that calcium at the inner side of the membrane sets the level of the background outward current. A feed-back mechanism on this basis is proposed for the control of the action potential duration. Various factors that could influence this basic mechanism are discussed.

Calcium dependent electrical activity in twitch muscle fibres of the frog

When twitch muscle fibres of the frog were equilibrated in chloride free saline with 2.5–20 mM tetraethylammonium sulphate [(TEA) 2 SO 4 ], the action potential was followed by a long depolarizing response. This response was greatly reduced by adding chloride ions (10 mM). In 20 mM (TEA) 2 SO 4 the response consisted of an initial depolarization of – 23 mV lasting several seconds, followed by a slow delayed spike reaching + 23 mV. After the delayed spike the cell remained continuously depolarized creeping to a steady depolarization of + 25 mV (mean values). The response can be attributed to an increase in membrane conductance to calcium since the phenomenon was abolished by removing external calcium or by adding cobalt or D-600, and was not greatly affected by reducing or removing external sodium or by adding tetrodotoxin.

Axoplasmic free magnesium levels and magnesium extrusion from squid giant axons

The free magnesium concentration in the axoplasm of the giant axon of the squid, Loligo pealei, was estimated by exploting the known sensitivity of the sodium pump to intracellular Mg2+ levels. The Mg-citrate buffer which, when injected into the axon, resulted in no change in sodium efflux was in equilibrium with a Mg2+ level of about 3--4 mM. Optimal [Mg2+] for the sodium pump is somewhat higher. Total magnesium content of axoplasm was 6.7 mmol/kg, and that of hemolymph was 44 mM. The rate coefficient for 28Mg efflux was about 2 X 10(-3) min-u for a 500-mum axon at 22-25degreesC, with a very high temperature coefficient (Q10=4-5). This efflux is inhibited 95% by injection of apyrase and 75% by removal of external sodium, and seems unaffected by membrane potential or potassium ions. Increased intracellular ADP levels do not affect Mg efflux nor its requirement for Na+/o, but extracellularl magnesium ions do. Activation of 28Mg efflux by Na+/o follows hyperbolic kinetics, with Mg2+/o reducing the affinity of the system for Na+/o. Lanthanum and D600 reversibly inhibit Mg efflux. In the absence of both Na+ and Mg2+, but not in their presence, removal of Ca2+ from the seawater vastly increased 28Mg efflux; this efflux was also strongly inhibited by lanthanum. A small (10(-14) mol cm-2) extra Mg efflux accompanies the conduction of an action potential.

Effects of intravenous verapamil on cardiac arrhythmias and on the electrocardiogram

The effects of intravenous verapamil on the electrocardiogram in 15 patients with heart disease in sinus rhythm and in 44 patients with supraventricular and ventricular tachyarrhythmias were evaluated. Verapamil prolonged the P-R interval without effect on the QRS duration or the Q-Tc interval. In patients with atrial flutter and fibrillation, A-V block was increased, with slowing of the ventricular rate, in almost all cases but sinus rhythm was restored in only 1 of 12 patients in atrial fibrillation and in 2 of the 11 patients with flutter. Verapamil had no effect in 3 patients with atrial fibrillation complicating WPW syndrome; in 1 of 5 patients with ventricular tachycardia it caused reversion to sinus rhythm. Sinus rhythm was restored promptly by verapamil in 13 of 17 patients with paroxysmal supraventricular tachycardias; in 2 others, sinus rhythm became established 1 to 2 hours after administration of the drug. Transient hypotension, not requiring treatment, was the only side effect noted but not in the patients with supraventricular tachycardias, in whom blood pressure generally increased after reversion to sinus rhythm by verapamil.

Electrophysiology and pharmacology of cardiac arrhythmias. VI. Cardiac effects of verapamil

It is clear from clinical and experimental data that have been reported thus far that verapamil is highly effective in the therapy of cardiac arrhythmias, and that it acts by a different mechanism than most of the commonly used antiarrhythmic drugs. The available clinical data indicate that on intravenous administration verapamil is as good as and perhaps superior to quinidine, procainamide and propranolol for the therapy of many atrial arrhythmias. Unfortunately the extent to which it is useful as longterm prophylaxis has not yet been reported, nor has its toxicity during protracted oral administration. The effects of verapamil on cardiac action potentials clearly indicate that it modifies the slow response to a much greater extent than the fast response. Studies of cardiac tissues from diseased human atria have indicated that slow response action potentials occur frequently. It is possible that such action potentials are responsible for the reentrant and automatic arrhythmias which occur in association with clinical cardiac disease. Whether the efficacy of verapamil in the therapy of atrial arrhythmias is primarily due to abolishing slow response activity in diseased atrial tissues or to suppression of propagation through the atrioventricular node is uncertain. However, it is likely that the therapeutic action of the drug may result from altered propagation of an arrhythmia through the atrioventricular junction as well as from the effects of the drug on diseased atrial and ventricular tissues.

Studies on the positive inotropic effect of phenylephrine: a comparison with isoprenaline

1. The effects of phenylephrine and isoprenaline on the isometric contraction of guinea-pig ventricle were compared over the whole range of their respective dose-response curves. 2. In preparations driven at 2.5 Hz the increase in contractile force induced by either isoprenaline of phenylephrine was linearly correlated to an increase in maximum velocity of force development. The relaxation time was shortened by isoprenaline but not by phenylephrine. 3. The negative inotropic effect induced by delta [N-(3,4-dimethoxyphenethyl)-N-methyl-amino]-alpha-(3,4,5-trimethoxyphenyl)alpha-isopropylvaleronitrile hydrochloride (D(600)) was reversed by isoprenaline, but little influenced by phenylephrine. 4. The study of the interval-force relationship shows that the increase in contractile force induced by phenylephrine (3 X 10(-5) M) was relatively greater at low frequencies of stimulation, and that the maximum effect was reached at the frequency of 1 Hz. 5. The positive inotropic effect of phenylephrine (10-4 M) was significantly higher at a frequency of 1 Hz than at 2.5 Hz; the effect of isoprenaline (3 x 10-8 M) was not significantly different at the two driving frequencies. 6. In preparations driven at 1 Hz the inotropic effect of the lower concentrations of phenylephrine was due to an increase in the time to peak tension without any change of the maximum velocity of force development; however an increase of this parameter became evident only after higher concentrations of the amine (10-5 M or more), associated with a progressive shortening of the time to peak. 7. A correlation between mechanical and electrophysiological effects of phenylephrine is attempted; the suggestion is advanced that the prolongation of the action potential and of the active state duration may be an important factor in the inotropic effect of phenylephrine.

Electrical characteristics of pancreatic islet cells

1. The electrical properties of mouse pancreatic islet cells have been explored in vitro using a single intracellular micro-electrode for both voltage recording and current injection. 2. The frequency of spontaneous electrical activity induced in islet cells by concentrations of D-glucose greater than 2-8 mM was enhanced by depolarizing, and reduced by hyperpolarizing, current injection. Post-stimulus inhibition and facilitation were also observed. 3. Intracellular current injection evoked a spike potential in Krebs solution containing a low D-glucose concentration (2-8mM), and in glucose-free, but not Ca-free solution. Evoked spikes were observed in approximately 10% of the cell population impaled. 4. The relationship between the rate of rise of an evoked spike and membrane potential displacement by intracellular current injection a sigmoid curve suggesting the presence of an inactivation process in spike potential genesis. 5. High [K]o, 30-50mM, induced electrical activity rarely, and then only transiently, thereafter blocking it; conditioning hyperpolarizing current tended to restore spike activity. 6. D-600, 5 times 10-minus 5M, blocked the electrical activity induced by D-glucose, tolbutamide or current injection; these inhibitory effects were reversed by a threefold increase in [Ca]o to 7-68 mM. 7. From these results it is concluded that the electrical activity induced in islet cells by Dglucose, tolbutamide and current injection is due mainlu to Ca-2+ influx and is dependent on the level of the membrane potential

Electrical activity in embryonic heart cell aggregates. Developmental aspects

Action potential parameters were measured in beating heart cell aggregates which were formed from trypsin-dissociated cells of embryonic chick heats aged 2 1/2, 4 or 7 days. 1. In aggregates composed of cells from the whole heart there was an increase in the maximum diastolic potential, overshoot, maximum rate of rise of the action potential (V max), and action potential duration between days 2 1/2 and 7. 2. Action potential parameters from 4- or 7-day aggregates composed exclusively of atrial or ventricular cells were similar to those in whole heart aggregates of the same age with the exception of the action potential duration in which atrial less than whole heart less than ventricular. Between days 4 and 7 the increases in duration were approximately 14% in atrial, 35% in whole heart, and 50% in ventricular aggregates. Differences in action potential duration, within or between ages, were not due solely to differences in the rate of beating. 3. Action potentials in whole heart aggregates aged 2 1/2 days were insensivitive to TTX (10-5 g/ml) but abolished by D600 (1 MUG/ML). Conversely, at 7 days activity was suppressed by TTX (2 X 10-8 G/ML) WHILE D600 (1 mug/ml) shortened the action potential duration and reduced the overshoot without influencing V max. 4. Adrenaline (1 mug/ml) restored the action potential overshoot and duration in 7-day aggregates treated with D600. 5. Action potential development in embryonic heart cells appears to be characterized by the functional appearance of fast inward channels. The slow channel mechanism, previously utilized in action potential generation, may gradually assume its adult role of carrying inward current during the plateau phase. 6. In contrast to monolayer cultures, embryonic heart cells cultured in aggregate form seem to have membrane properties similar to those of intact tissue.

Mode of action of sodium nitroprusside on vascular smooth muscle

1. Sodium nitroprusside is a potent relaxant of smooth muscles with a predominantly tonic response, e.g. rat aorta contracted by noradrenaline, angiotensin II, Phe2-Lys8-vasopressin, BaC1(2), or KC1, and guinea-pig tracheal smooth muscle contracted by carbachol. 2. Smooth muscle preparations from the splanchnic region and with varying degrees of phasic contractility are less sensitive and develop tachyphylaxis (portal vein, duodenum of the rat) or are unresponsive to sodium nitroprusside (vas deferens, uterus of the rat). 3. Cardiac auricles of the guinea pig are not affected by sodium nitroprusside in either frequency or amplitude or spontaneous contractions. 4. Sdium nitroprusside causes a parallel shift of the dose-response curve of rat aorta to noradrenaline to the right and reduces the maximum response. 5. The drug has no blocking or stimulant effect on alpha- or beta-adrenoceptors, respectively. 6. Sodium nitroprusside inhibits the contractile response of calcium-depleted depolarized rat aorta to extra-cellular calcium. Like verapamil, it inhibits the increment in 45calcium uptake of rabbit aorta elicited by K+. Sodium nitroprusside significantly reduced 45calcium binding by microsomes prepared from rabbit aorta. 7. Rabbit aorta was incubated with lanthanum chloride to prevent calcium influx; sodium nitroprusside reduced the maintained rapid contraction phase in response to noradrenaline which is believed to be based on the intracellular activation of calcium. 8. In rat aorta, cellular cAMP and ATP levels were not found to be affected by the drug. 9. Rabbit aorta, "skinned" by glycerination is unresponsive to sdoium nitroprusside. 10. It is concluded that sodium nitropruside acts on exictation-contraction coupling predominantly in tonic smooth muscle by interfering with both the influx and the intracellular activation of calcium.

Tetrodotoxin desensitization in aggregates of embryonic chick heart cells

Spontaneous beating of heart-cell aggregates from 4-day chick embryos was initially blocked by 10(-5) g/ml tetrodotoxin (TTX). With continued exposure to the drug, the fraction of blocked aggregates decreased from about 80% at 15 min to about 25% at 2-3 h, at which time, beating aggregates had become desensitized to the toxin, showing no response to a fresh dose. Aggregates from 5-day hearts were more sensitive to TTX, but fewer became desensitized in its presence. Desensitization to TTX was not seen in 6- and 7-day aggregates. Inhibition of protein synthesis by cycloheximide did not affect beating or initial sensitivity to TTX of 4-day aggregates, but desensitization failed to occur. Before TTX, the mean value of maximal upstroke velocity (V(max)) of the action potentials in 4-day aggregates was 33 V/s. After desensitization V(max) was 12 V/s. Activity of desensitized aggregates in the presence of TTX was augmented by elevated calcium levels, and suppressed by presumed inhibitors of slow inward current (manganese, D600). Desensitization was reversible; upon removal of TTX 10(-5) g/ml, aggregates regained their responsiveness to a fresh dose of the drug with a 2-3 h time-course similar to that of desensitization. This was prevented by continued exposure to TTX at concentrations as low as 10(-8) g/ml. These data suggest that (a) desensitization involves a change in the mode of action-potential generating from one involving Na-specific, TTX-sensitive channels to one utilizing slower Mn-sensitive channels; (b) the process of desensitization occurs over a period of 2-3 h and is dependent upon the products of protein synthesis; and (c) desensitization is reversible after removal of TTX over a 2-3 h time-course similar to its onset.

Effects of manganese and other agents on the calcium uptake that follows depolarization of squid axons

1. The Ca-sensitive photoprotein aequorin was injected into squid axons and the light response to stimulation or depolarizing voltage clamp pulses recorded.2. The effects of Mn(2+), Co(2+), Ni(2+), La(3+) and of the organic Ca antagonists D-600 and iproveratril on the early tetrodotoxin-sensitive and late tetrodotoxin-insensitive components of the light response were studied.3. The late tetrodotoxin-insensitive component can be blocked, reversibly, by concentrations of Mn, Co and Ni that reduce but do not block the tetrodotoxin-sensitive component. The late component can also be blocked by La(3+) and the organic Ca antagonists D-600 and iproveratril.4. Mn(2+), Co(2+), Ni(2+) and the drug D-600 all reduce the Na currents, but have little effect on either outward or inward K currents. Tetraethylammonium blocks the outward K current but has no appreciable effect on the tetrodotoxin-insensitive entry of Ca.5. Concentrations of Mn between 5 and 50 mM substantially reduce the light output during a train of action potentials; they also slightly reduce the rate of rise of the action potential.6. On pharmacological grounds it is concluded that the tetrodotoxin-insensitive component of Ca entry does not represent Ca ions passing through the K permeability channels. There must exist a potential-dependent late Ca channel that is distinct from the well known Na and K channels of the action potential. A possible function for this late Ca channel in the coupling of excitation to secretion is discussed.

Pancreatic acinar cells: ionic dependence of the membrane potential and acetycholine-induced depolarization

1. Intracellular recordings of membrane potentials have been made in vitro from the exocrine acinar cells of the mouse pancreas using glass micro-electrodes.2. The mean membrane potential of the acinar cells during superfusion with Krebs-Henseleit solution was -39.2 mV. Increasing [K](o) tenfold decreased the membrane potential by 28 mV when [K](o) was above 10 mM. This depolarization was not affected by atropine (1.4 x 10(-6)M). Strophanthin-G (10(-3)M) slowly depolarized the cells at about 10 mV hr(-1).3. Brief exposure to acetylcholine (ACh), 5.5 x 10(-5)M, or pancreozymin resulted in a short lasting depolarization of the acinar cells. Atropine (1.4 x 10(-6)M) blocked the depolarizing action of ACh but not that of pancreozymin. Adrenaline (5.5 x 10(-5)M) or cyclic AMP (10(-3)-10(-4)M) did not influence the membrane potential.4. The amplitude of the ACh-induced depolarization was not dependent on the presence of CO(2)/HCO(3) in the bathing fluid, but it was closely dependent on the extracellular Na concentration. However, ACh was still able to evoke a small depolarization even after prolonged exposure of the tissue to a Na-free solution.5. During exposure of the tissue to a Ca-free solution the resting membrane potential was decreased and the ACh-induced depolarization was significantly reduced. Some substances which are known in other tissues to inhibit membrane Ca(2+) currents, i.e. La(3+), D-600 and tetracaine, were able to reduce, but never abolish, the ACh-induced depolarization.6. These results suggest that the effect of ACh on the pancreatic acinar cell is to increase the permeability of the membrane to commonly occurring ions with a consequent Na-influx and a small Ca-influx.