Ontogeny of cardiac ornithine decarboxylase and its beta adrenergic responsiveness in the rat.

Ornithine decarboxylase (ODC) is a marker of tissue growth and development and, because sympathetic stimulation of beta adrenergic receptors acutely increases ODC in the adult rat heart, measurement of this enzyme can be used to indicate the functional intactness of the beta adrenergic receptor system in the heart. Changes in the postnatal ontogenetic pattern of this enzymatic activity may also indicate abnormal development and ODC appears to be particularly useful in evaluating the effects of prenatal insult on cardiac development. The present study examines the pattern of basal ODC activity and its developing sensitivity to beta adrenergic stimulation during the perinatal period in order to establish a data base for studies on the effect of various environmental agents on the developing cardiovascular system. ODC activity was measured in rat hearts on gestation day (GD) 20 through postnatal day (PND) 28 under saturating conditions of L-ornithine and pyridoxal 5-phosphate. Basal ODC activity fell from 3 nmol of CO2/hr/mg of protein at GD 20 to less than 0.5 nmol of CO2/hr/mg of protein at PND 18, rising again to nearly 1 nmol of CO2/hr/mg of protein at PND 22. The beta adrenergic agonist isoproterenol (10 mg/kg s.c.) resulted in peak ODC stimulation at 4 hr postinjection on PNDs 6, 14 and 21; however, no response was seen at PND 1 at this dose or at GD 20 (300 micrograms/kg s.c.). In dose-response studies, isoproterenol produced a maximal response at 10 mg/kg s.c., resulting in increases from control of 67, 230 and 1700% at PNDs 6, 14 and 21, respectively, indicating that the sensitivity of the heart to beta adrenergic stimulation increases with age, during the perinatal period.