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 interna l perfusion technique were used to study the effects of sarcoplasmic r eticulum (SR) calcium release on L-type calcium current (I-Ca,I-L) in ferret enzymatically isolated right ventricular myocytes. I-Ca,I-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 curren ts. When calcium was the charge carrier, inactivation of I-Ca,I-L was nonmonotonic in many myocytes; after an early rapid phase of inactivat ion, a secondary inward 'hump' component was frequently observed betwe en -40 to -10 mV. The hump component was not present when barium repla ced calcium but was observed when calcium carried the current in low i ntracellular (aspartate) and extracellular (methanesulphonate) chlorid e solutions. When BAPTA 10 mM was perfused internally the amplitude of I-Ca,I-L increased, the kinetics of inactivation slowed and the hump component disappeared. Both caffeine 10 mM and ryanodine 10 mu M incre ased the amplitude of I-Ca,I-L in the hyperpolarized range of potentia ls (negative to 0 mV), slowed the kinetics of I-Ca,I-L inactivation an d caused the hump component to disappear. Under current clamp mode, bo th caffeine and ryanodine significantly prolonged the duration of the action potential. Taken in aggregate, preliminary data demonstrate tha t, in ferret right ventricular myocytes, a secondary inward hump compo nent can be frequently observed after the early rapid phase of inactiv ation of I-Ca,I-L, causing the net inward current to display biphasic, nonmonotonic behaviour. This secondary inward hump current is only pr esent when calcium is the charge carrier, is absent when BAPTA is used as an intracellular calcium chelator and SR calcium release is disrup ted by either caffeine or ryanodine, and is not due to activation of e ither the sodium-calcium exchanger or various putative calcium-activat ed cation or anion channels. Rather, preliminary results strongly sugg est that this secondary inward hump current component is due to modula tion of I-Ca,I-L by SR calcium release. Possible physiological and the oretical implications of the results are discussed briefly.