Does the calcium current modulate the contraction of the accompanying beat? A study of E-C coupling in mammalian ventricular muscle using cobalt ions.

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Abstract
This study was prompted by a conclusion that emerged from experiments by others on mammalian ventricular muscle i.e., that the magnitude of the calcium current (Isi) does not influence contraction on the accompanying beat. In the key experiments, muscles were potentiated with paired pulses, rested for 2 minutes, and then re-stimulated. When Co2+ was present during the rest, the first post-rest action potential (and presumably Isi) was depressed but contraction was unchanged. In the present study, we have measured the effect of Co2+ on the action potential, Isi, and tension of bovine, cat, and rabbit ventricular muscle. All three parameters were depressed by 1-2 mM Co2+ when muscle was stimulated at 20/min. The key experiments of the earlier study were repeated. Co2+ (1-2 mM depressed both the action potential plateau and contraction on the first post-rest response. We conclude that the use of Co2+ in this type of experiment does not provide evidence against a role for Lsi in the modulation of contraction.