1. Archive
2. Vol.51, No.2

Effect of Extracellular [Ca²⁺] on Ca²⁺ Release from Sarcoplasmic Reticulum in Rat Ventricular Myocytes

Abstract: 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, n9) and 1.86±0.13 (n9) 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 (n9) and 18.8±3.38 F/F_o/s, respectively, at 0.3 mM [Ca²⁺]_o, they were 0.064±0.005 s (n9) 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 (n8) at 0.3 mM [Ca²⁺]_o and was similar to the value of 0.230±0.022 s (n8) 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.

[Full Text (PDF)]