EFFECTS OF LOW-SODIUM, OUABAIN AND AMILORIDE ON CHICK MYOCARDIAL-CONTRACTION

1. Effects of Ca2+, low Na+, ouabain and amiloride on contractile forc e were examined in myocardial preparations from embryonic and hatched chicks. Measurement of contractile force was performed in an organ bat h with whole hearts for the young embryo (5-6 days old) and with isola ted strips from the right ventricles for the old embryos (16-18-day-ol d), hatched chicks (within 24 hr after hatching) and 1-week-old chicks . 2. The maximum increase in contractile force above the basal value a t each age was determined by increasing extracellular Ca2+, which was smaller in young embryonic hearts than in older ones. 3. Decreasing th e Na+ concentration to half of the normal value increased the contract ile force in all ages examined: the increase in contractile force expr essed as a percentage of the maximum increase at each age was signific antly larger in young embryonic hearts than in older ones. 4. Ouabain produced increases in contractile forces at all ages examined; the eff ective concentration was lower in young embryonic hearts than in older hearts. 5. Amiloride prolonged the relaxation phase of contraction at all ages examined; the degree of prolongation was larger in young emb ryonic hearts than in older ones. 6. The present results suggest that, in the chick myocardium, the relative role of Na+-Ca2+ exchange in re laxation decreases during embryonic development, which may reflect the concurrent increase in sarcoplasmic reticulum function [Tanaka, Takag i and Shigenobu (1993) J. Dev. Physiol. 19, 235-240.