Differentiation of the transmembrane Na and Ca channels in mammalian cardiac fibres by the use of specific inhibitors

The effect of (±)‐, (+)‐ and (‐)‐verapamil on the Ca2+‐binding, Ca2+‐transporting activity, and Ca2+‐dependent adenosine triphosphatase (ATPase) activity of isolated cardiac sarcolemmal preparations was studied. Enzymatic treatment was used to establish the nature of the sites facilitating [14C]‐(±)‐verapamil binding. (±)‐Verapamil 1 μm inhibited the passive binding of 45Ca2+. The (+)‐ and (‐)‐isomers were equiactive. (±)‐Verapamil 1 μm inhibited the ATP‐dependent transport of 45Ca2+ and the associated activation of the Ca2+‐sensitive ATPase. The activity resided in the (‐)‐isomer. Lineweaver‐Burk plots for the initial rates of ATP‐dependent transport showed that the inhibition induced by the (‐)‐isomer was accompanied by a reduced Km and Vmax. Enzymatic removal of N‐acetyl neuraminic acid and galactose residues increased [14C]‐(±)‐verapamil binding; removal of N‐acetylglucosamine and treatment with phospholipase C and trypsin decreased the binding. These results have been interpreted to mean that (‐)‐verapamil interferes with the ATP‐dependent Ca2+‐transporting properties of the sarcolemma, and that this effect is accompanied by an altered activity of the intrinsic Ca2+‐sensitive ATPase. N‐acetylneuraminic acid and galactose residues do not provide binding sites for verapamil at the cell surface.

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

THE EFFECT OF VERAPAMIL ON THE Ca²⁺‐TRANSPORTING AND Ca²⁺‐ATPase ACTIVITY OF ISOLATED CARDIAC SARCOLEMMAL PREPARATIONS

Mas‐Oliva

Nayler

1980

“…These same ions and D600 also block neurotransmitter and hormone release in several tissues (Douglas, 1974;Russell & Thorn, 1974;Pinto & Trifar6, 1976;Trifaro, 1977). D600 can inhibit the release of vasopressin and oxytocin from the neurohypophysis (Russell & Thorn, 1974;Thorn, 1974), the 45Ca uptake by the neurohypophysis (Dreifuss et al, 1973), the slow Ca2+ current in cardiac fibres (Kohlhardt, Bauer, Krause & Fleckenstein, 1972), the influx of Ca2+ through the 'late Ca2+ channel' of the squid axon (Baker et al, 1973;Baker & Reuter, 1975), and the release of catecholamines in response to acetylcholine stimulation or depolarizing concentrations of K+ (Pinto & Trifaro, 1976).…”

Section: Discussionmentioning

confidence: 99%

EFFECTS OF METHOXYVERAPAMIL ON THE STIMULATION BY Ca²⁺, Sr²⁺ AND B^a2+ AND ON THE INHIBITION BY Mg²⁺ OF CATECHOLAMINE RELEASE FROM THE ADRENAL MEDULLA

Aguirre

Falutz

Pinto

et al. 1979

Bovine adrenal glands were perfused with Ca2+‐free Locke solution and catecholamine release was induced either by the introduction of Ca2+, Sr2+ or Ba2+ into the perfusion fluid or by the substitution of Na+ by an osmotically equivalent amount of sucrose. Methoxyverapamil (D600) at a concentration of 3 × 10−4 m blocked the release of catecholamines in response to Ca2+, Sr2+ or Ba2+ stimulation but failed to block the release evoked by the omission of Na+. Mg2+ (10 to 20 mm) blocked the release induced by Na+‐deprivation; however, this inhibitory effect of Mg2+ was not modified by D600. D600 blocked the increase in the efflux of 45Ca from the perfused gland induced by the introduction of Ca2+ into the perfusion fluid and blocked the uptake of 45Ca into adrenal medullary slices induced by K+ depolarization. The results suggest that Ca2+, Sr2+ and Ba2+ may enter the chromaffin cell through the same channel and that this channel is blocked by D600. Mg2+ may enter the cell through the same Ca2+ channel but with a high rate of permeation or it may enter through a channel which is resistant to D600. Alternatively, Mg2+ may exert its inhibitory effect at an extracellular site.

“…The early reports on the effects of local anaesthetics on slow action potentials were not in agreement. Some investigators found that local anaesthetics did not affect slow channels (Kohlhardt et al, 1972;Carmeliet & Verdonck, 1974;Brennan et al, 1978;Hashimoto et al, 1979), whereas others found depressant effects (Reiser et al, 1974;Josephson & Sperelakis, 1976). It has been shown recently that lidocaine, at therapeutic concentrations, impairs the conduction of slow responses through canine Purkinje fibres depolarized by high K+ (Lamanna et al, 1982).…”

Section: Introductionmentioning

confidence: 99%

Effects of lidocaine, procaine, procainamide and quinidine on electrophysiological properties of cultured embryonic chick hearts

Neto

Sperelakis

1985

The effects of lidocaine, procaine, procainamide and quinidine were studied on organ‐cultured embryonic chick (2–3 day‐old) ventricular cells. Lidocaine (10−5 − 10−4 M), in a dose‐dependent manner, reduced the rate of pacemaker discharge, the action potential amplitude (APA), the maximum rate of rise (Vmax) of the upstroke of the action potential and the action potential duration at 50% repolarization (APD50). These changes occurred without alterations in the maximum diastolic potential (MDP). Extracellular electrical field stimulation could still evoke action potentials in cells arrested by 10−4 M lidocaine, but 10−3 M lidocaine completely abolished electrical activity. Procaine, procainamide and quinidine, at 5 × 10−5 M to 10−3 M, depolarized the cells to around − 30 mV and reduced APA and Vmax. Procaine and procainamide increased APD50, but quinidine shortened it. All the effects described disappeared completely in about 40 min of superfusion with drug‐free Tyrode solution. Isoprenaline (5 × 10−7 M) and adrenaline (10−6 M) restored spontaneous firing of preparations arrested by any of the antiarrhythmic agents and repolarized ventricular cells depolarized by procaine, procainamide or quinidine. Propranolol (5 × 10−7 M) did not affect the depolarization produced by procaine (5 × 10−4 M), but antagonized its reversal by isoprenaline. In contrast, isoprenaline (10−6 M) did not produce recovery of automaticity of preparations arrested by verapamil (10−5 M). Histamine (10−5 M) or strontium (10 mM) were not able to restore rhythmic activity in cells arrested procaine. Application of long (10–15 s duration) hyperpolarizing currents did not reverse the blocking effect of procaine, procainamide and quinidine. The input resistance increased during the procaine‐induced depolarization. It is suggested that the four agents studied block the slow Na+ channels responsible for the upstroke of the action potential in young chick heart cells. A drug‐induced decrease in PK may occur in those cells arrested at low levels of membrane potential.