DEVELOPMENTAL-CHANGES IN CA2-UPTAKE, NA+,CA2+-EXCHANGE AND CA2+-ATPASE IN FRESHLY ISOLATED EMBRYONIC, NEWBORN AND ADULT CHICKEN HEART()

Developmental changes in cellular Ca2+-transport mechanisms were studi ed in chick heart by determining cellular Ca2+-uptake and Na+,Ca2+-exc hange activity in freshly isolated ventricular tissues of embryonic (5 -18 days old), newborn (1-2 days old) and young adult (90-100 days old ) heart by monitoring Ca-45 influx. Ca2+-ATPase activity was determine d in microsomal fractions at different stages of development. The Ca2-uptake (per g wet tissue weight) increased with the development of em bryonic as well as post-hatch chick heart, reaching a maximum in the y oung adult chicken. The overall increase in Ca2+-uptake, from embryoni c day 5 to young-adult stage, was more than 3 fold. The Na+,Ca2+-excha nge activity, determined as Na+-gradient-induced Ca2+-uptake in presen ce of either ouabain or zero [Na+](0), showed a 6-fold increase during development of heart from the embryonic day 5 to the young adult stag e. Amiloride, an inhibitor of Na+,Ca2+-exchange, caused a dose-depende nt reduction in a ouabain-induced rise in Ca-45 influx at different st ages of development. The inhibitory effect of amiloride was, however, greater during later stages of development. A progressive increase in Ca2+-ATPase activity was also seen during development. Ca2+-ATPase exh ibited about a 4-fold increase in activity from embryonic day 7 to the young adult. The concomitant increase in Ca2+-uptake, Na+,Ca2+-exchan ge and Ca2+-ATPase activities suggests age-dependent changes in Ca2+-t ransport and storage systems of developing heart during embryogenesis and post-embryonic life. During embryogenesis the developmental increa se in Na+,Ca2+-exchange activity was greater than that during post-hat ch development of heart. However, the increase in Ca2+-ATPase activity was greater during post-hatch development than during embryogenesis. It is suggested that Na+,Ca2+-exchange and Ca2+-ATPase play a prominen t role in maintaining cellular Ca2+ homeostasis during embryogenesis a nd after hatching.