Ursula Ravens

The role of sodium channels in the effects of the cardiotonic compound DPI 201-106 on contractility and membrane potentials in isolated mammalian heart preparations

The novel compound DPI 201-106 (4-3-(4-diphenyl-methyl-1-piperazinyl)-2-hydroxypropoxy-1H-indole-carbon itrile) prolonged the action potential duration (APD) and enhanced force of contraction in isolated papillary muscles of the guinea-pig. The effective concentration range was 0.1-3 mumol/l. These effects persisted upon removal of the compound, even after extensive washings. Both prolongation of APD and the positive inotropic effect were readily reversed or prevented after exposure to tetrodotoxin, 3 mumol/l. Slow action potentials of partially depolarized preparations in high potassium solution were hardly influenced by DPI 201-106 (1 mumol/l) or were depressed (3 mumol/l). In isolated myocytes DPI 201-106 induced a slowly decaying net inward current, that disappeared again after exposure to tetrodotoxin. With the exception of the lack of reversibility by washing, these effects were similar to the ones reported previously for the Anemonia sulcata polypeptide ATX II. ATX II and DPI 201-106 did not affect the post-rest contraction. The biphasic response in APD after a transient interruption of stimulation was accentuated by ATX II and became monophasic with DPI 201-106. It is concluded that the effects of DPI 201-106 are also mediated by an interaction with the Na channels, but DPI 201-106 and ATX II probably affect the channels in a different manner.

Electromechanical studies of an Anemonia sulcata toxin in mammalian cardiac muscle

Summary1.In guinea-pig papillary muscles toxin II isolated from the sea anemone Anemonia sulcata (ATX) enhanced the force of contraction without changing the time-to-peak tension. ATX increased the relaxation time. The positive inotropic effect was accompanied by a prolongation in action potential duration. The action potential amplitude and the resting membrane potential remained essentially unchanged. The velocity of depolarization (dV/dtmax) decreased slightly. The effects were not modeified by pretreatment of the animals with reserpine.2.In preparations partially depolarized by elevated extracellular potassium (14.7 mM KCl) a concentration of 1×10−8 M ATX was ineffective with respect to increase in force of contraction or delay of repolarization. The effects of 1×10−8 M ATX could be reversed by 5×10−7 M tetrodotoxin.3.Our results suggest that ATX affects the action potential duration by delaying the inactivation of the fast sodium permeability change which initiates the action potential. It is not clear, however, in which manner this effect is related to the increase in force of contraction.

The time courses of the changes in contractile force and in transmembrane potentials induced by cardiac glycosides in guinea-pig papillary muscle

1 The effects of ouabain, digoxin, digoxigenin monodigitoxoside, digoxigenin bisdigitoxoside and digitoxigenin bisdigitoxoside on the force of contraction and on the transmembrane action potential were compared in isolated papillary muscles of guinea‐pigs. 2 All cardiac glycosides studied had a dose‐dependent positive inotropic effect and simultaneously shortened the duration of the action potential at all levels of repolarization from the start of drug action. 3 In every instance, the reduction of the action potential duration developed more slowly than the increment in contractile force. However, the ratios between the two rates were independent of the concentrations used and seemed to be characteristic for the individual cardiac glycosides. 4 All cardiac glycosides had a biphasic effect on the time‐to‐peak tension. An initial increase was followed by a dose‐dependent decrease. 5 The results are discussed with respect to the possible sites of action. Taking into account the different rates as well as the different ratios, it is proposed that more than one site of action is involved in producing the different effects of cardiac glycosides on heart muscle.

The influence of the rate of electrical stimulation on the effects of the Anemonia sulcata toxin ATX II in guinea pig papillary muscle

In guinea pig papillary muscle, the rate of electrical stimulation (0.1-2 Hz) strongly influenced the effects of the Anemonia sulcata toxin ATX II on action potential duration (APD) and contractile force. In the concentration range studied (10-8-10-7 M), ATX II always produced a larger prolongation in APD at low rates of stimulation. At 0.1 Hz there was a temporal dissociation between the onset of the APD-prolonging and the positive inotropic effect. However, under equilibrium conditions there was a positive relationship between the APD expressed as a fraction of the time during which the membrane was depolarized, and the contractile force irrespective of the change in experimental conditions being variation of stimulation frequency or the addition of ATX II. The results suggest that the positive inotropic effects of both ATX II and increased stimulation frequency could be induced by a similar mechanism, e.g. an increase in sodium of the heart muscle.

The effects of lanthanum on electrical and mechanical events in mammalian cardiac muscle

Summary1.The effects of various concentrations of lanthanum (La3+) on the force of contraction and on action potentials was investigated in isolated papillary muscles of the guinea-pig.2.The force of contraction was initially reduced by La3+. During long exposure to high concentrations (i. e. 50, 100 and 500 μM) the contraction amplitude was increased again and a rise in resting tension was observed. These late changes in force of contraction also occurred after short exposure to 500 μM La3+ (2 min) when La3+ had been removed from the medium.3.The action potentials were shortened at 20% of repolarization (plateau level) during La3+ exposure, whereas at 90% of repolarization a transient prolongation was observed. The resting potentials were reduced only at high concentrations and long duration of exposure.4.500 μM La3+ reduced the force of contraction to the control value obtained before addition of ouabain (5×10−7 M). The ouabain-induced shortening in action potential duration at the plateau level was not reversed by the same concentration of La3+.5.It is concluded that the action of La3+ on cardiac muscle cannot be explained by a simple displacement of superficially bound calcium. The characteristic effects observed at high concentrations reduce the value of La3+ as a pharmacological tool used in the study of calcium turnover.

STUDIES ON THE KINETICS OF [3 H]-OUABAIN UPTAKE AND EXCHANGE IN THE ISOLATED PAPILLARY MUSCLE OF THE GUINEA-PIG

1 The uptake, wash‐out and exchange of [3 H]‐ouabain was studied in isolated, resting, and electrically stimulated papillary muscles of the guinea‐pig. 2 At the equilibrium level of uptake, a different tissue/medium ratio was obtained for each of the concentrations used, i.e. 3.4, 1.8 and 0.82 for 1 × 10−7 M, 7.7 × 10−7 M, and 5 × 10−6 M ouabain, respectively. Equilibrium was reached more rapidly at high concentrations of ouabain. 3 The maximum number of binding sites for ouabain was estimated to be 1 × 1015 binding sites/g wet weight. 4 No difference in [3 H]‐ouabain uptake could be detected between resting and electrically stimulated papillary muscles. 5 The kinetics of the ouabain uptake, wash‐out and exchange are discussed. The results suggest that there is a saturable compartment in papillary muscle which can best be demonstrated if low concentrations of ouabain are used. Because of its small size, the saturable compartment submerges in the process of the filling of the extracellular space at high concentrations (5 × 10−6 M).

Responses to hypertonic solutions in guinea-pig atria: changes in action potentials, force of contraction and calcium content

In left atria of guinea-pig hearts we studied the effects of Tyrode solution made hypertonic by the addition of mannitol (50 to 300 mosm/l). Electrical and mechanical performance were investigated at 0.1, 1 and 3 Hz. Up to 150 mosm/l a positive inotropic effect was observed, addition of 300 mosm/l caused a positive inotropic response followed by a long lasting negative inotropic effect. The positive inotropic effect depended on the frequency of stimulation: at the low frequency of 0.1 Hz the increase in force of contraction was so large, that in hypertonic solution the absolute value of developed tension at 0.1 Hz was actually larger than at 3 Hz. With 150 mosm/l mannitol the time to peak tension and the twitch duration were prolonged. These effects were also most prominent at 0.1 Hz. The action potential duration was shortened; the membrane hyperpolarized. The observed changes in action potential duration and the membrane hyperpolarization may reflect changes in cation concentration due to cellular dehydration. The propagation velocity of excitation was reduced in the absence of a detectable decrease in maximum rate of depolarization during the upstroke of the action potential. The cells did not behave as perfect osmometers because loss of tissue water as estimated from wet weight and dry weight measurements was smaller than expected for a 1.5-fold increase in tonicity. After having established complete exchange of tissue calcium with 45Ca, hypertonicity-induced changes in the cellular calcium content could be determined by changes in 45Ca content. At both frequencies of stimulation, the cellular calcium content increased to the same extent.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellularly applied sodium mimics the effects of ouabain in single cardiac myocytes.

Sodium was applied intracellularly, in single myocytes from guinea-pig hearts via the potential-recording electrode. With electrodes containing 75 mM NaCl (but not with 15 mM) contractility increased and the action potential duration shortened. Aftercontractions, delayed afterpotentials and spontaneous activity as observed with toxic concentrations of ouabain (10 microM) appeared within 1 min when 150 mM NaCl was applied. The results demonstrate that sodium load without pump inhibition can mimic the effects of cardiac glycosides.

The effects of the cardiotonic dihydropyridine derivatives Bay k 8644 and H160/51 on post-rest adaptation of guinea-pig papillary muscles

SummaryIn isolated guinea-pig papillary muscle, the effect of the two dihydropyridine derivatives Bay k 8644 (0.03–10 μmol/l) and H160/51 (0.1–3 μmol/l) on transmembrane action potentials and force of contraction were investigated at regular stimulation (1 Hz) and after a period of rest (10 min). The following results were obtained:1.At regular stimulation of the preparations, Bay k 8644 and H160/51 enhanced force of contraction without affecting time-to-peak tension. The time required for relaxation and the action potential duration were prolonged. These effects were transient with exposure to high concentrations of Bay k 8644 (>3 μmol/l).2.The amplitude of the post-rest contraction thought to depend entirely on transmembrane calcium influx was small under control conditions and increased because of prolongation in time-to-peak tension in the presence of either dihydropyridine derivative. Isoprenaline (30 nmol/l) — as opposed to Bay k 8644 and H160/51 — increased the rate of force development of post-rest contractions.3.Bay k 8644 and H160/51 prolonged the duration of the first action potential after 10 min of rest. In the course of adaptation to steady state stimulation this prolongation transiently increased further resulting in a biphasic pattern which was attenuated by addition of nifedipine. With isoprenaline the biphasic pattern changed into a monotonous adaptation to pre-rest control.4.Our results show that the small enhancement of the post-rest contraction in the presence of Bay k 8644 or H160/51 is due to prolonged action potential duration after rest, whereas isoprenaline enhances the intensity of post-rest activation. Although the agents used are known to increaseiCa, they affect post-rest contractions and adaptation patterns in a different manner.

Electrophysiological studies of some semisynthetic cardiac glycoside derivatives in isolated papillary muscle of the guinea-pig

1 The effects of digitoxin, 3α‐methyl‐digitoxigenin‐3β‐monoglucoside (3α‐MDM), 3α‐methyl‐digitoxigenin (3α‐MD), proscillaridin, 4, 5‐methylene‐procillaridin (4, 5‐MP), and 3β‐hydroxy‐4, 5‐methylene‐A, B‐trans‐scillarenin (3β‐HMTS) on force of contraction and on the transmembrane action potentials were examined in isolated papillary muscles of guinea‐pigs. 2 All derivatives exhibited the typical cardiac glycoside effects: i.e. they increased the force of contraction and shortened the action potential duration at 20% (plateau phase) and 90% of repolarization. With digitoxin, 3β‐HMTS and 4, 5‐MP a transient prolongation in action potential duration was observed at the lower concentrations. The action potential amplitude and the resting membrane potential were reduced consistently only with the higher concentrations used. 3 The onset of the positive inotropic effects of 3α‐MDM, 3α‐MD and 3β‐HMTS was more rapid than that of digitoxin and proscillaridin. The increment in contractile force reached a maximum well before the full shortening effect on the action potential duration had developed. The shortening of the action potential is thought to be responsible for the biphasic nature of the positive inotropic effect. 4 With 3α‐MD and 3α‐MDM even toxic effects, e.g. increase in baseline tension, were completely reversible after washing in drug‐free solution. 5 The dose‐response curves for the positive inotropism can only be compared reliably once the equilibrium of drug action has been established. This steady state is probably reflected by the development of the full shortening in action potential duration.