Guanylate cyclase activity in fetal, neonatal and adult rat heart.

Guanosine 3′, 5′-monophosphate is thought to be an important factor in promoting cell growth. We, therefore, measured the activity of guanylate cyclase, the enzyme which in the developing rat heart catalyzes the production of guanosine 3′, 5′-monophosphate. Cardiac guanylate cyclase activity was found to be increased to 405 ± 44 pmol accumulated/10/min/mg protein in utero and maintained this level of activity for the first 3 weeks after birth. By 28 days, cardiac guanylate cyclase activity had decreased to 165 ± 7 pmol accumulated/10 min/mg protein which paralleled the adult mother cardiac guanylate cyclase activity of 173 ± 9 pmol accumulated/10 min/mg protein. This period of maximal activity of cardiac guanylate cyclase corresponds to the period of rapid growth of the heart and the decreased activity approximates the period of time when cardiac DNA synthesis declines.