Voltage, membrane current and contraction were simultaneously measured in voltage clamp experiments (single sucrose gap) on cat ventricular trabeculae. The pulse programs allowed the determination of the potential dependence of the steady state activation and inactivation as well as the restoration of the calcium-carrying system (slow inward current). 1. The steady state activation variable (d infinity) rose in a sigmoid manner from -50 mV (d infinity nearly 0) to 0 mV (d infinity nearly 1). The experimental values can be described by the function 1/1 + exp [(Vh-V)/S] where half activation (Vh) = -22.5mV and S = 7.6 mV. 2. The steady state inactivation variable (f infinity) declined from 1 at -60mV to 0 at 10mV. The best fit curve is nearly a mirror image of the activation curve with Vh = -28 mV and s = -8.3 mV. 3. The voltage dependence of the (normalized) peak tension was well described by the steady state conductance variables except at potentials positive to +20mV. A "steady state" tension (superimposed on "tonic tension") was found in the potential range where a steady state conductance is predicted by the curves describing steady state activation and inactivation. 5. Following inactivation, the time courses of restoration of the calcium-carrying system and tension were identical. Time courses were exponential with tau = 118 msec at -80 mV, 144 msec at -60 mV, and 198 msec at -40 mV. 6. Two possible models of excitation-contraction coupling in line with the present results are discussed.

Although sarcolemmal (SL) Na+/Ca2+ exchanger is known to regulate the intracellular Ca2+ concentration ([Ca2+]i), its involvement in catecholamine-induced increase in [Ca2+]i is not fully understood. To gain some information in this regard, isolated rat cardiomyocytes were treated with different agents, which are known to modify Ca2+ movements, in the absence or presence of a beta-adrenoceptor agonist, isoproterenol, and [Ca2+]i in cardiomyocytes was determined spectrofluorometrically with fura-2 AM. Treatment with isoproterenol did not alter [Ca2+]i in quiescent cardiomyocytes, whereas the ATP (purinergic receptor agonist)-induced increase in [Ca2+]i was significantly potentiated by isoproterenol. Unlike ryanodine and cyclopiazonic acid, which affect the sarcoplasmic reticulum function, SL L-type Ca2+ channel blockers verapamil and diltiazem, as well as a SL Ca2+-pump inhibitor, vanadate, caused a significant depression in the isoproterenol-induced increase in [Ca2+]i. The SL Na+/Ca2+ exchange blockers amiloride, Ni2+, and KB-R7943 also attenuated the isoproterenol-mediated increase in [Ca2+]i. Combination of KB-R7943 and verapamil showed additive inhibitory effects on the isoproterenol-induced increase in [Ca2+]i. The isoproterenol-induced increase in [Ca2+]i in KCl-depolarized cardiomyocytes was augmented by low Na+; this augmentation was significantly depressed by treatment with KB-R7943. The positive inotropic action of isoproterenol in isolated hearts was also reduced by KB-R7943. These data suggest that in addition to SL L-type Ca2+ channels, SL Na+/Ca2+ exchanger seems to play an important role in catecholamine-induced increase in [Ca2+]i in cardiomyocytes.

The slow upstroke velocity of the action potential of primary pacemaker cells of the sinoatrial node suggests the slow membrane channel to be involved in the excitation process of these cells. In order to prove this the effect of promotors and inhibitors of the slow membrane channel upon the excitation process of the isolated sinoatrial node of rabbits was studied. 1. The action potentials of primary pacemaker cells had an upstroke velocity of 1.7 +/- 1.1 V/sec and an overshoot of 8.1 +/- 4.6 mV with a threshold potential of -40.1 +/- 4.5 mV. A decrease of the extracellular Ca concentration from 2 mM to 0.2 mM led to reduction of upstroke velocity and overshoot whereas an increase from 2 mM to 4 mM augmented both parameters. But the inward current is not only carried by Ca ions but by Na ions as well, since Na withdrawal diminished upstroke velocity and overshoot. 2. The promotors of the slow inward current, isoproterenol and caffeine, produced a significant increase of upstroke velocity and overshoot. On the other hand, verapamil (1 mg/1) and D 600 (0.4 MG/1) completely blocked the excitation process within 20-25 min. The same effect could be produced by the bivalent cations Ni (1 mM), Co (1 mM), and Mn (1 mM). These organic and inorganic inhibitors exerted their blocking effect without significant changes of the maximal diastolic potential and the threshold potential. 3. In the continued presence of Ni, Co and Mn ions their inhibitory effect could be neutralized nearly completely by isoproterenol (2.5 mg/1). But excess Ca (6 mM) increased the upstroke velocity only to a small degree. In contrast to the findings in the ventricular myocardium the blocking effect of verapamil and D 600 could be overcome neither by isoproterenol nor by excess Ca. The excitation process in the sinotrial node is mediated solely by the slow membrane channel. It allows the influx of both Ca and Na ions which act as charge carrier of the slow inward current. The fact, that the inhibitory effect of verapamil and D 600 cannot be neutralized by catecholamines or excess Ca seems to indicate that some properties of the slow membrane channel are not exactly identical in sinotrial pacemaker cells and in the ventricular myocardium.

1. Single Nae-activated K+ channels (KNa) were investigated by means of the inside-out patch clamp technique in ventricular myocytes isolated from the guinea-pig heart.2. Nae-activated K+ channels were observed at very low density (<9 % of patches Distributions of burst duration, between burst duration and openings within bursts were best described by the sum of two exponentials. Lowering [Na+]i decreased the burst duration and the duration of openings within burst. 6. These observations show that the Na+-activated K+ channel from guinea-pig ventricular myocytes has complex gating and bursting behaviour.Of the wide variety of potassium channels that are found in cell membranes, several show a high unit conductance of between 100 and 200 pS. A channel of this type activated by Nae (KNa) at the intracellular surface has been identified in both neuronal and myocardial cells at the whole-cell and single-channel level

1 Fendiline, a diphenylalkylamine type of antianginal drug, was examined for its effects on L-type calcium channels in guinea-pig ventricular myocytes by the whole-cell patch-clamp technique.2 Fendiline (0.3-100 iM) applied extracellularly inhibited the calcium channel current (Ica,) in a concentration-and time-dependent manner. The IC50 of fendiline was 17.0 ± 2.43 gAM and the Hill slope was 1.39 ± 0.23. 3 Inhibition of ICa by fendiline appeared with an onset of less than 3 s. 4 Fendiline inhibited 'Ca at all the membrane potentials tested and shifted the current-voltage curve upwards. The overall calcium channel conductance (&ca) of the cell was reduced and conductancevoltage curve was shifted to the left in the presence of fendiline.5 Isoprenaline (0.5-1 JAM), a P-adrenoceptor agonist, partially reversed the inhibitory effect of fendiline on ICa.6 It is suggested that fendiline applied extracellularly blocks L-type calcium channels and reduces calcium channel conductance of the cell. The calcium channels thus inhibited are, nevertheless, still available for P-adrenoceptor stimulation.