Effects of low sodium ouabain and amiloride on chick myocardial contraction

1. Effects of Ca2+, low Na+, ouabain and amiloride on contractile force were examined in myocardial preparations from embryonic and hatched chicks. Measurement of contractile force was performed in an organ bath with whole hearts for the young embryo (5-6 days old) and with isolated strips from the right ventricles for the old embryos (16-18-day-old), hatched chicks (within 24 hr after hatching) and 1-week-old chicks. 2. The maximum increase in contractile force above the basal value at each age was determined by increasing extracellular Ca2+, which was smaller in young embryonic hearts than in older ones. 3. Decreasing the Na+ concentration to half of the normal value increased the contractile force in all ages examined: the increase in contractile force expressed as a percentage of the maximum increase at each age was significantly larger in young embryonic hearts than in older ones. 4. Ouabain produced increases in contractile forces at all ages examined; the effective 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 embryonic hearts than in older ones. 6. The present results suggest that, in the chick myocardium, the relative role of Na(+)-Ca2+ exchange in relaxation decreases during embryonic development, which may reflect the concurrent increase in sarcoplasmic reticulum function [Tanaka, Takagi and Shigenobu (1993) J. Dev. Physiol. 19, 235-240.