A comparison of excitation-contraction coupling in heart and skeletal muscle: An examination of “calcium-induced calcium release”

Abstract

Vascularly perfused guinea pig semimembranosus accessorius (SMA) and frog gastrocnemius (FG) muscles and perfused interventricular septa of guinea pig heart were used to examine the hypothesis which proposes the physiological regenerative release of Ca. Membrane resting potentials and excitability were maintained by including 100 μ m La in Ca-free perfusion media (0-Ca, 100-La). Tension decline in 0-Ca, 100-La was rapid in the heart with halftime (t 1 2 ) = 11 s and correlated with the most rapid component of 45Ca efflux. In contrast, decline of the major component of tension in SMA and FG muscles was much slower with t 1 2 = 22 to 33 min . The rate of tension decline was virtually identical to the t 1 2 of the slow component of 45Ca efflux from these muscles. Neither perfusion with 0-Ca, 2 m m EGTA solutions after 30 min 0-Ca exposure (at which time interstitial [Ca 2+] was estimated < 3 × 10 −9 m) nor addition of 100 μ m La to 0-Ca, 100-La perfusates after 60 min (SMA) or 120 min (FG) significantly altered the rate of tension decline. The data emphasize the difference in source of contractile Ca in heart and skeletal muscle. It is concluded that in these physiologically perfused muscles with sarcolemma intact there is no evidence for a role of “trigger Ca” in the process of E-C coupling.