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Regulation of contraction in heart muscle. A Ca2+ gradient model for the Ca(2+)-induced Ca(2+)-release of the sarcoplasmic reticulum.

The sarcoplasmic reticulum (SR) in heart muscle releases a large amount of Ca2+ in response to a small amount of trigger Ca2+ by the mechanism of the Ca(2+)-induced Ca(2+)-release. A model was presented for the regulation of the SR Ca2+ release based on the assumptions that a Ca2+ gradient is formed from the level of Z-line toward the center of the cytoplasm as a result of continuous release of Ca2+ from the SR and that the volume of SR is quite small compared with the cytoplasmic volume. Numerical simulations showed that the amount of the SR Ca2+ release was dependent both on the initial SR Ca2+ content and the transsarcolemmal Ca2+ current. This interprets the effects of beat frequency and external [Ca2+] on twitch contractions in intact heart muscle. The present Ca2+ gradient model predicted that the reduction of the transsarcolemmal Ca2+ current alters the relation between the rate of rise and the peak of the cytoplasmic [Ca2+] in a twitch in such a way that lower the Ca2+ current, smaller the rate of rise of the cytoplasmic [Ca2+] at a given level of the peak cytoplasmic [Ca2+]. Measurements of the relations between the rate of rise and the peak of twitch force in rat papillary muscles and between the rate of rise and the peak of cytoplasmic [Ca2+] examined using fluo-3 in rat single heart myocytes were compatible with the prediction of the present model. Thus the model interpreted some important features in the regulation of the heart muscle contraction.