Calcium buffering and excitation-contraction coupling in developing avian myocardium

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 B-max and K-D. B-max rose with development whereas K-D 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 B-max and K-D in reasonable agreement with experiment. From ED5 to 15, troponin C was the major Ca2+ binding moiety, followed by SR and calmodulin.