Effect of Extracellular [Ca〔2+〕] on Ca〔2+〕 Release from Sarcoplasmic Reticulum in Rat Ventricular Myocytes

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

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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/Fo, 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/Fo/s, respectively, at 0.3 mM [Ca2+]o, they were 0.064±0.005 s (n=9) and 31.1±0.03 F/Fo/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.<br>

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