Calcium Buffering and Excitation-Contraction Coupling in Developing Avian Myocardium

摘要

This report provides a detailed analysis of developmental changes in cytoplasmic free calcium (Ca²⁺) buffering and excitation-contraction coupling in embryonic chick ventricular myocytes. The peak magnitude of field-stimulated Ca²⁺ transients declined by 41% between embryonic day (ED) 5 and 15, with most of the decline occurring between ED5 and 11. This was due primarily to a decrease in Ca²⁺ currents. Sarcoplasmic reticulum (SR) Ca²⁺ content increased 14-fold from ED5 to 15. Ca²⁺ transients in voltage-clamped myocytes after blockade of SR function permitted computation of the fast Ca buffer power of the cytosol as expressed as generalized values of Bmax and KD. Bmax rose with development whereas KD did not change significantly. The computed SR Ca²⁺ contribution to the Ca²⁺ transient and gain factor for Ca²⁺-induced Ca²⁺ release increased markedly between ED5 and 11 and slightly thereafter. These results paralleled the maturation of SR and peripheral couplings reported by others and demonstrated a strong relationship between structure and function in development of excitation-contraction coupling. Modeling of buffer power from estimates of the major cytosolic Ca binding moieties yielded a Bmax and KD in reasonable agreement with experiment. From ED5 to 15, troponin C was the major Ca²⁺ binding moiety, followed by SR and calmodulin.