Effect of extracellular [Ca2+] on Ca2+ release from sarcoplasmic reticulum in rat ventricular myocytes

The effect of external [Ca2+] ([Ca2+](o)) on Ca2+ release from the sarcoplasmic reticulum (SR) was examined with rested-state twitches in rat ventricular myocytes. The magnitude of transient rise of intracellular [Ca2+] ([Ca2+](i)) relative to the resting one, F/F-o, as measured with fluo-3, was 1.75 +/-0.07 (mean +/- SEM, n=9) and 1.86 +/-0.13 (n=9) at 0.3 and 1.8 mM [Ca2+](o), respectively; the difference was insignificant. The time from onset to peak and the rate of rise of the [Ca2+](i) transient were 0.107 +/-0.017 s (n=9) and 18.8 +/-3.38 F/F-o/s, respectively, at 0.3 mM [Ca2+](o), they were 0.064 +/-0.005 s (n=9) and 31.1 +/-0.03 F/F-o/s, respectively, at 1.8 mM [Ca2+](o). The difference in the corresponding values at the two [Ca2+](o) was significant (t-test, p <0.05). The half decay time of the [Ca2+](i) transient was 0.217 +/-0.016 s (n=8) at 0.3 mM [Ca2+](o) and was similar to the value of 0.230 +/-0.022 s (n=8) at 1.8 mM [Ca2+](o), indicating that the rate of decrease of [Ca2+](i) is independent of the [Ca2+](o). The duration of action potential was similar at 0.3 and 1.8 mM [Ca2+](o) as examined with papillary muscle. The results suggest that a lowering of [Ca2+](o), i.e., reducing the Ca2+ influx, slows the rate of Ca2+ release from the SR fully loaded with Ca2+ with little effect on the total amount of the Ca2+ release. An instantaneous relationship between the [Ca2+](i) and the myocyte shortening at 0.3 and 1.8 mM [Ca2+](o) suggested that the time course of unloaded contraction is related not only to the magnitude, but also to the rate of rise of [Ca2+](i).