Slowly inactivating component of sodium current in ventricular myocytes is decreased by diabetes and partially inhibited by known Na(+)-H(+)Exchange blockers.

Recent evidence has suggested a major role for a slowly inactivating component of Na(+)current (I(NaL)) as a contributor to ischemic Na(+)loading. The purposes of this study were to investigate veratrine and lysophosphatidylcholine (LPC)-induced I(NaL)in single ventricular myocytes of normal and diabetic rats and to analyse the effects on this current of three pharmacological agents, known as Na(+)/H(+)exchange inhibitors, whose selectivity has been questioned in several studies. A decrease in Na(+)/H(+)exchange activity has been previously shown to be associated with diabetes, and this has been found to confer some protection to the diabetic heart after an episode of ischemia/reperfusion. Recordings were made using the whole-cell patch-clamp technique. I(NaL)was stimulated either by veratrine (100 mg/ml) or by LPC (10 micromol/l) applied extracellularly. Veratrine as well as LPC-induced I(NaL)was found to be significantly decreased in ventricular myocytes isolated from diabetic rat hearts. Veratrine- and LPC-induced I(NaL)in ventricular myocytes of normal rats was significantly (in the range 10(-7)to 10(-4)mol/l) inhibited by the Na(+)/H(+)exchange blockers HOE 694, EIPA and HOE 642. HOE 694 was the most potent inhibitor, followed by the amiloride derivative EIPA and HOE 642. The sensitivity of veratrine-induced I(NaL)to inhibition by HOE 694 and EIPA was markedly reduced in diabetic ventricular myocytes, with no observed inhibition by HOE 642. These data may have important implications as to the protection that may be afforded against ischemic and reperfusion injury, especially during ischemia and when ischemia occurs in a diabetic situation.

Decrease in sodium-calcium exchange and calcium currents in diabetic rat ventricular myocytes.

This study was designed in order to gain insight into possible changes in the inward sodium-calcium exchange current (INa-Ca) and the L-type calcium current (ICa), in ventricular myocytes isolated from streptozotocin-induced diabetic rats. Recordings were made using the nystatin-perforated patch technique which minimizes interference with the normal intracellular Ca2+ buffering mechanisms. The averaged INa-Ca current density elicited by Ca2+ current was smaller in diabetic than in normal myocytes at all potentials tested. INa-Ca activated by rapid application of caffeine was significantly reduced and the decay phase was prolonged. The density of ICa was also significantly reduced by diabetes in the range of test potentials between -10 and +50 mV. In addition, the fast time constant of ICa inactivation, which represents mainly the sarcoplasmic reticulum (SR) Ca2+ release-induced inactivation, was significantly higher in diabetic than in normal myocytes. The decrease in ICa, which is the main source of trigger Ca2+ for SR Ca2+ release, may explain the significantly lowered peak systolic [Ca2+]i previously shown in diabetic myocytes. As activation of ICa is essential for subsequent stimulation of INa-Ca, reduced ICa may contribute to decreasing activation of the Na+-Ca2+ exchanger.

Influence of Bistramide A on the twitch tension in rat atrial heart muscle.

Bistramide A, a new toxin isolated from the Urochordate Lissoclinum bistratum Sluiter, was applied to rat auricular heart muscle bundles. At a stimulation frequency of 0.2 Hz, the toxin induces a dose-dependent reduction of the stimulated twitch tension force; it decreases Vmax and shortens the duration of the plateau and the slow repolarizing phase of the action potential. In the control solution, switching from a stimulation frequency of 0.2 Hz to 1 Hz decreases the force with which a positive potentiation develops either at a maintained high frequency or after switching from 1 Hz to 0.2 Hz. Bistramide A reduces both the force evoked at 1 Hz and the potentiation. The data suggest that Bistramide A blocks Na+ conductance; inhibits Ca++ channels in a time- and frequency-dependent manner; reduces Na(+)-Ca++ exchange activity; but does not modify the ability of the sarcoplasmic reticulum to be refilled although the rate of Ca++ accumulation is decreased.

Comparison of effects of aprikalim and of hypoxic and ischaemic preconditioning on extracellular potassium accumulation, metabolism, and functional recovery of the globally ischaemic rat heart.

OBJECTIVE: The aim was to compare the effects of a potassium channel opener, aprikalim, and of hypoxic and ischaemic preconditioning on extracellular K+ concentration change, metabolism, and ventricular function in isolated globally ischaemic rat hearts. METHODS: Isovolumetric rat hearts (37 degrees C) were treated with 1 microM (apri 1) or 30 microM (apri 30) aprikalim, or preconditioned with either 10 min of hypoxia (N2PC) or 5 min of ischaemia followed by 5 min of perfusion (IPC5) or 10 min of ischaemia followed by 3 min of perfusion (IPC10). Control hearts received neither treatment nor preconditioning. All hearts received 30 min of sustained ischaemia followed by 25 min of reperfusion. Extracellular K+ concentration was measured with a potassium sensitive electrode inserted into the extracellular space of the left ventricular wall. RESULTS: Recovery of left ventricular developed pressure after 25 min of reperfusion was only 19.20(SEM 5.09)% of the preischaemic level in the control group. No recovery was obtained for the apri 1 group. In contrast, a very good recovery was obtained for the apri 30 group [96.69(10.92)%], the N2PC group [104.92(17.40)%], and the IPC10 group [84.96(9.86)%]. The IPC5 group, however, did not have improved recovery of left ventricular pressure [14.15(5.61)%]; this is likely to be related to differences in the stimulation of anaerobic glycolysis. The protection was also markedly attenuated by pretreatment with 50 microM glibenclamide in the apri 30, N2PC, and IPC10 groups [22.76(9.00), 66.06(6.09), and 46.18(7.06)%, respectively]. Hearts treated with aprikalim before inducing ischaemia showed a concentration dependent increase in [K+]e. Hypoxic (N2PC) and ischaemic preconditioning (IPC5 and IPC10) were also associated with an increase in [K+]e over the 5-10 min period preceding the 30 min of sustained ischaemia. During sustained ischaemia all groups showed a nearly triphasic pattern of extracellular K+ changes with an early rising phase, with the exception of the N2PC group for which the early [K+]e rise was barely detectable. CONCLUSIONS: An increase in [K+]e before sustained ischaemia is one of the mechanisms involved in the conditions affording protection. Although important, this is not sufficient, and further protection may be accomplished by decreased stimulation of anaerobic glycolysis during the sustained ischaemia.

The polyether Bistramide A affects the calcium sensitivity of the contractile proteins in frog atrial heart muscle.

The effects of Bistramide A, a new toxin isolated from the Urochordate Lissoclinum bistratum Sluiter have been studied on the mechanical activity of frog heart atrial muscle preparations. The peak tension of isolated trabeculae was sensitive to nanomolar concentrations of Bistramide A. Lineweaver-Burk relationships suggest that Bistramide A competes with Ca for a common site. In voltage-clamped trabeculae, the toxin inhibited both the cadmium-sensitive Ca current and the phasic component of the tension with a dissociation constant of 3.3 microM and a stoichiometry of 2. Bistramide A decreased the isometric tension of skinned fibres in a dose-dependent manner with a dissociation constant of 400 nM and a stoichiometry of 2. The toxin reduced the maximum Ca activated force and decreased the sensitivity of the contractile proteins to Ca. The data suggest that Bistramide A decreases the Ca-sensitivity of contractile proteins prior to blocking the Ca current.

[Extemporaneous analysis of myocardial performance by a programmable automatic dynamometer]. / Analyse extemporanée de la performance myocardique par dynamomètre automatique programmable.

This article describes a biophysical investigation to determine the physiological integrity of isolated cardiac fibers obtained during surgery. Immediate biopsy of myocardium can be analysed dynamically in various physiological conditions using an intelligent dynamometer controlled by an "APPLE II" microcomputer generally in less than fifteen minutes. Multiple tests including pharmacological and electrical properties of fibers can now be done with this cardiac analyser. Primary investigations on heart diseases show the importance of the method for evaluation of heart necrosis.

Opposite effects of adrenaline and ouabain on the resting potential of rat atrial cells.

In the left rat atrium changes in diastolic potential (E max) evoked by sudden stimulation train were modified by adrenaline and ouabain. The early stimulation depolarization phase (SDP) of E max occurring on stimulation was shortened by adrenaline, but lengthened and strongly enhanced by ouabain. The stimulation repolarization phase (SRP) following SDP was markedly inhibited by ouabain, while accelerated and increased by adrenaline. In continuously stimulated atria E max was decreased by ouabain and augmented by adrenaline. The adrenaline-induced hyperpolarization was reduced or suppressed in the presence of 10-4 M or 10-3 M ouabain, respectively. The present data suggest that adrenaline could stimulate the electrogenic sodium pump in the rat atrium.

[Antagonistic effects of acetylcholine and caffeine on the contractility of the auricular myocardium of rats]. / Effets antagonistes de l'acétylcholine et de la caféine sur la contractilité du myocarde auriculaire de rat.

In the rat atrium, the contraction rebound occurring after ACh treatment could not be related to an increase in action potential duration and disappeared in the presence of atropine or caffeine. Contractures elicited by caffeine (10 and 20 mM) were inhibited by ACh. This inhibition was suppressed by atropine. The hypothesis that ACh could increase the sarcoplasmic reticulum Ca diastolic reuptake is discussed.

[Correlation between inhibition of stimulatory membrane repolarization and positive inotropic response caused by ouabain in rat heart]. / Corrélation entre l'inhibition de la repolarisation membranaire stimulative et la réponse inotrope positive provoquées par l'ouabaïne sur le myocarde de rat.

During a stimulus train, the diastolic membrane potential of rat atria exhibits a depolarization phase followed by a slower repolarization phase which has been attributed to the activation of an electrogenic sodium pump (ATPase Na+, K+). This pump seems to be all the more active as stimulation frequency is higher. The parallel evolution of the sodium pump inhibition and a positive inotropic effect in response to ouabain perfusion, suggests that the enzymatic inhibition is directly involved in the development of the cardiotonic effect of digitalis.

Effect of ouabain on mechanical and electrical properties of rat and guinea pig hearts at 180 C.

The effects of ouabain 10(-6) M on rat and guinea pig hearts have been studied at 18 degrees C, in order to reduce almost fully both the Na+, K+-dependent ATPase activity and the ouabain induced inhibition of this enzyme. In isolated guinea pig hearts the positive inotropic response to ouabain obtained at 32 degrees C disappeared at 18 degrees C. On the contrary, the contractile strength of rat hearts was slightly reduced by ouabain and in the same manner at both temperatures. Current and voltage clamp experiments carried out at 18 degrees C in ventricular fibres revealed that ouabain 10(-6) M decreased both the action potential overshoot and the fast sodium current in rat and guinea pig, by reduction of the membrane sodium conductance. Ouabain did not change the calcium current in guinea pig preparations, whereas in rat heart muscle this current was reduced. The effects of ouabain on both the action potential plateau and outward repolarizing current indicated some inconsistencies from preparation to preparation and cannot therefore be considered as significant. The persistence of the ouabain induced alterations of g Na (in rat and guinea pig) and calcium current (in rat) at 18 degrees C supports the hypothesis of two ouabain cell receptors in heart muscle.

[The effect of caffeine on action potentials induced by noradrenaline in the depolarized heart ventricle of the rat]. / Modifications par la caféine des potentiels d'action induits par la noradrénaline sur le myocarde ventriculaire dépolarisé de rat

In Rat myocardium depolarized by high external KCl concentration, norepinephrine restores excitability in the form of slow rising electrical responses. Caffein alone does not induce such responses. This result agrees with the hypothesis that caffein reduces the slow inward current in the Rat myocardium. Caffein increases the effects produced by norepinephrine. This suggests an indirect action of the caffein on the calcium permeability mediated by intracellular AMP cyclic level.

Effects of taurine on the transmembrane ionic currents of rat and guinea pig ventricular fibres.

The effects of taurine on electrical properties of isolated myocardial ventricular fibres were studied in rat and guinea pig. The action potential amplitude was decreased in these species while the plateau and the repolarization phase were not altered. This suggests that simple inspection of the plateau phase is not sensitive enough to pick up changes in outward current since voltage clamp experiments showed that taurine decreased both potassium current and fast inward current. However, the slow inward calcium current remained unchanged with taurine in each species. These results suggest that the inotropic effects of taurine on rat and guinea pig heart contraction already described, were not due to modifications of the cell membrane permeability for calcium. It seems more reasonable to assume that the cardiac actions of this compound result from intracellular changes in calcium availability.

[Effect of taurine of the membrane electrical properties in guinea pig hearts]. / Effets de la taurine sur les propriétés électriques membranaires du couer de Cobaye

The effects of taurine on electrical properties of isolated ventricular fibres were studied in guinea pig. The action potential amplitude was decreased while the plateau and the repolarization phase was not altered. Voltage clamp experiments at 18 degrees C revealed that taurine reduced the fast inward sodium current and the potassium current. The slow inward calcium current remained unchanged. These results suggest that the inotropic effects of taurine on guinea pig heart contractions already described were not due to modifications of the membrane permeability for calcium but rather to intracellular changes in calcium availability.