Effect of increased pressure on ventricular growth in stage 21 chick embryos

We studied the effect of increased ventricular pressure on heart growth in the stage 21 (3.5-day) chick embryo. Ventricular pressure was increased by constricting the conotruncus with a loop of 10-0 nylon tied in an overhand knot. The embryos were reincubated, and physiology and cellular morphology were evaluated at successive stages of development, stages 21, 24, 27, and 29. Ventricular pressure was measured with a servo-null pressure system, and cardiac output was measured with a 20-MHz pulsed Doppler velocity meter. Ventricular and embryo dry weights were measured on an electronic microbalance, myocyte organelle composition was measured by a point counting technique, and cell growth response was measured by DNA and protein assay. The conotruncal loop increased ventricular pressure in experimental compared with control embryos, i.e., at stage 24, 2.88 +/- 0.13 vs. 1.96 +/- 0.05 (SE) mmHg (P less than 0.05), respectively, without affecting cardiac output. Ventricular dry weight increased in experimental vs. control embryos, i.e., at stage 24, 114 +/- 7 vs. 85 +/- 4 micrograms (P less than 0.05), respectively, whereas embryo weights were similar between the two groups. The difference in ventricular weights was due to myocyte hyperplasia, since organelle proportion of myofibrils and mitochondria, DNA-to-protein ratio, and myocyte area were similar in experimental voice control embryos. Thus the adjustment of myocardial mass to ventricular work occurs even during the earliest stages of embryonic development. Cardiac growth and morphogenesis are parallel but separable processes.