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The contribution of the sarcoplasmic reticulum (SR) and Na⁺−Ca²⁺ exchanger to intracellular Ca²⁺ regulation in mouse cardiac myocytes was investigated by measuring contraction after variable rest intervals, rapid cooling contractures (RCCs) and fast application of caffeine. The results obtained showed differences from other species in the roles played by the SR and the Na⁺−Ca²⁺ exchanger. They suggest that in mouse ventricular myocytes there is significant Ca²⁺ entry via the exchanger during rest and during the latter part of the Ca²⁺ transient.

In cardiac myocytes isolated from transgenic mice overexpressing the cardiac Na⁺−Ca²⁺ exchanger the time to peak and relaxation of twitches and RCCs were faster than in control littermates. The decline of Ca²⁺, assessed by indo‐1 fluorescence, was faster in transgenic myocytes even in the absence of Na⁺ and Ca²⁺ in the superfusing solution. This suggests …