Maturation of end-systolic stress-strain relations in chick embryonic myocardium.

The embryonic myocardium increases functional performance geometrically during cardiac morphogenesis. We investigated developmental changes in the in vivo end-systolic stress-strain relations of embryonic chick myocardium in stage 17, 21, and 24 white Leghorn chick embryos (n = 10 for each stage). End-systolic stress-strain relations were linear in all developmental stages. End-systolic strain decreased from 0.50 +/- 0.02 to 0.31 +/- 0.01 (mean +/- SE, P < 0.05), while average end-systolic wall stress was similar at 3.29 +/- 0.34 to 4.19 +/- 0.43 mmHg (P = 0.14) from stage 17 to 24. Normalized end-systolic myocardial stiffness, a load-independent index of ventricular contractility, increased from 2.98 +/- 0.19 to 6.03 +/- 0.39 mmHg from stage 17 to 24 (P < 0.05). Zero-stress midwall volume increased from 0.024 +/- 0.002 to 0.124 +/- 0.004 microl from stage 17 to 24 (P < 0.05). These results suggest that the embryonic ventricle increases normalized ventricular "contractility" while maintaining average end-systolic wall stress over a relatively narrow range during cardiovascular morphogenesis.