Contraction of developing avian heart muscle.

1. Developmental changes in contraction of chick heart show strong similarities with those of the mammalian myocardium. 2. Normalized twitch force of intact trabeculae from chick left ventricle increases most markedly during the 3-day period around the time of hatching. 3. At any age, elevation of extracellular [Ca2+] to 10-20 mM increases twitch force to a maximum. 4. Studies using membrane-free ("skinned") trabeculae demonstrate that the developmental increase in twitch force is paralleled by an increase in the maximal contractile capability of the muscle, that is probably due to proliferation of contractile proteins. 5. At all ages studied, maximal twitch force of intact trabeculae at 10-20 mM extracellular [Ca2+] is similar to maximal Ca(2+)-activated force of the trabeculae after skinning. 6. Calcium sensitivity of the contractile apparatus in chick heart decreases with development in parallel with isoform switching in troponin T. 7. The depressant effect of acidosis on calcium sensitivity of the contractile apparatus increases with development in parallel with isoform switching in troponin I. 8. As in mammalian heart, both acidosis and inorganic phosphate (Pi) depress force generation by the contractile machinery of chick heart.