Role of Calcium Current and Sarcoplasmic Reticulum Calcium Released in Control of Myocardial Contraction in Rat and Rabbit Myocytes

The relative importance of calcium current and sarcoplasmic reticulum in supplying contraction-related calcium was investigated in isolated rat and rabbit cardiac ventricular myocytes using patch-clamp and video techniques. Calcium current and contractile response were manipulated via variation of [Ca2+]o between 0.25 and 8.0 mM. Sarcolemmal calcium influx was derived from integration of the calcium current. An increase in [Ca2+]o from 0.25 to 8 mM augmented the amplitude of contractile shortening from 7 to 375% of control (amplitude at 1.0 mM) in rat but only from 36.5 to 177.6% in rabbit. Upon variation in [Ca2+]o from 0.5 to 8.0 mM calcium current increased from 67 to 175% of control in rat and from 65 to 150% in rabbit. Sarcolemmal calcium influx in rat was, however, only about half of that in rabbit for the entire range of [Ca2+]o and saturated at [Ca2+]o over 2.0 mM in both species. Application of 1 μM ryanodine increased calcium current and slowed its inactivation time course in the rat, but in the rabbit a slight decrease in current was observed and the inactivation time course was not obviously affected. The sarcoplasmic reticulum calcium release: transsarcolemmal calcium flux ratio is at least four-fold greater in the rat than in the rabbit. The results from this study further explain the well-known marked differences in contractile control in the two species.