Regulation of Contraction in Heart Muscle
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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 Ca2+-induced Ca2+-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.
KeywordsSarcoplasmic Reticulum Heart Muscle Release Channel Beat Frequency Gradient Model
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