Effect of BAY K8644 on cytosolic calcium transients and contraction in embryonic cardiac ventricular myocytes

Cytosolic calcium transients were recorded from spontaneously beating chick embryonic myocardial cell aggregates loaded with the fluorescent [Ca²⁺]_i indicator, indo-1. Calcium transients rose rapidly from an end-diastolic [Ca²⁺]_i of 230±18 nM to a peak systolic [Ca²⁺]_i of 619±34 nM (n=21). Relaxation of the transients was slow, and continued throughout diastole. Bay K8644 (0.5 μM) markedly prolonged the action potential and caused similar prolongation of the calcium transients. Calcium transients in the presence of Bay K8644 had an inflection on their rising phase, which was followed by a more gradual increase that continued until the membrane had repolarized to a negative potential of -15 to -30 mV. Bay K8644 caused marked elevation of peak systolic [Ca²⁺]_i to 955±56 nM (P<0.002), with concomitant elevation of end-diastolic [Ca²⁺]_i to 400±36 nM (P<0.002). Optical recordings of contraction showed changes similar to those in the calcium transient: the initial upstroke of the contraction was followed by a more gradual second component, which gave the contraction a "half-dome" appearance. The time to peak [Ca²⁺]_i and the time to peak contraction increased linearly with action potential duration (APD₅₀). The effects of Bay K8644 were simulated, in part, by CsCl (7.5 mM), which produced equivalent prolongation of the action potential and calcium transients. However, CsCl did not elevate diastolic [Ca²⁺]_i. To determine the mechanism of the diastolic [Ca²⁺]_i, increase, Bay K8644 was applied to aggreagates rendered quiescent by tetrodotoxin. Bay K8644 caused a graded increase in [Ca²⁺]_i, which was followed by resumption of spontaneous beating. Bay K8644 can therefore increase [Ca²⁺]_i in the absence of action potentials. We conclude that the duration of the calcium transient and its companion contraction are tightly coupled to the duration of the action potential in chick embryonic myocardial cells. Besides increasing action potential duration, Bay K8644 has the further effect of elevating diastolic [Ca²⁺]_i, which appears to contribute to the positive inotropic effect.