Intracellular [Ca²⁺] Transients in Ca²⁺ Wave in Single Rat Ventricular Myocyte

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Ca²⁺ release from the sarcoplasmic reticulum (SR) in heart muscle grades depending on Ca²⁺ influx in the physiological twitch; Ca²⁺ wave results from regenerative Ca²⁺ release from the SR. To examine if the Ca²⁺ release from the SR in the Ca²⁺ wave takes a duration similar to the physiological one, a transient rise of intracellular [Ca²⁺] ([Ca²⁺]_i transient) was recorded during both a propagating Ca²⁺ wave and an electrically evoked twitch with single rat ventricular myocytes, using a laser scanning confocal microscope. Care was taken to record the fluo-3 fluorescence from a segmental region with little lateral movement, especially during a propagating Ca²⁺ wave. During a typical Ca²⁺ wave, the time-to-peak (TP) and the half-width (HD) of the averaged [Ca²⁺]_i transient were 161 and 253 ms respectively, but they were 76 and 145 ms during an electrically evoked twitch. The difference in the duration between the two types of [Ca²⁺]_i transients could not be accounted for by modification of duration of [Ca²⁺]_i transient by possible asynchronous Ca²⁺ release from the SR during a Ca²⁺ wave, suggesting that the regenerative Ca²⁺ release from the SR in the Ca²⁺ wave occurs more slowly than the physiological one in rat ventricular myocytes.