Modulation of L-type calcium current kinetics by sarcoplasmic reticulum calcium release in ferret isolated right ventricular myocytes

The gigaohm seal patch clamp (whole cell configuration) and an internal perfusion technique were used to study the effects of sarcoplasmic reticulum (SR) calcium release on L-type calcium current (ICa,L) in ferret enzymatically isolated right ventricular myocytes. ICa,L (22 to 24 degrees C) was isolated by using various sodium- and potassium-free salines, which eliminated or greatly minimized activation of the sodium-calcium exchanger and calcium-activated cation and anion currents. When calcium was the charge carrier, inactivation of ICa,L was nonmonotonic in many myocytes; after an early rapid phase of inactivation, a secondary inward 'hump' component was frequently observed between -40 to -10 mV. The hump component was not present when barium replaced calcium but was observed when calcium carried the current in low intracellular (aspartate) and extracellular (methanesulphonate) chloride solutions. When BAPTA 10 mM was perfused internally the amplitude of ICa,L increased, the kinetics of inactivation slowed and the hump component disappeared. Both caffeine 10 mM and ryanodine 10 microM increased the amplitude of ICa,L in the hyperpolarized range of potentials (negative to 0 mV), slowed the kinetics of ICa,L inactivation and caused the hump component to disappear. Under current clamp mode, both caffeine and ryanodine significantly prolonged the duration of the action potential. Taken in aggregate, preliminary data demonstrate that, in ferret right ventricular myocytes, a secondary inward hump component can be frequently observed after the early rapid phase of inactivation of ICa,L, causing the net inward current to display biphasic, nonmonotonic behaviour. This secondary inward hump current is only present when calcium is the charge carrier, is absent when BAPTA is used as an intracellular calcium chelator and SR calcium release is disrupted by either caffeine or ryanodine, and is not due to activation of either the sodium-calcium exchanger or various putative calcium-activated cation or anion channels. Rather, preliminary results strongly suggest that this secondary inward hump current component is due to modulation of ICa,L by SR calcium release. Possible physiological and theoretical implications of the results are discussed briefly.