PARASYMPATHETIC CONTROL OF HEART PERIOD DURING EARLY POSTNATAL-DEVELOPMENT IN THE RAT

Basal autonomic control of heart period (HP) changes considerably duri ng the early postnatal period in the rat. Although studies in the deve loping animal have examined the ability of the sympathetic branch to d ecrease HP during physiological challenge, few studies have examined t he emerging capabilities of the parasympathetic branch to alter HP dur ing early development. To determine the extent of parasympathetic cont rol of HP in the young rat, we used a modified dive reflex procedure a nd electrical stimulation of the vagal nerve to examine the range of p arasympathetic effects on HP in postnatal day 3-24 rats. Modified dive reflex manipulations produced maximal parasympathetically-mediated HP s that were longer just after birth and at weaning than at intervening ages. Direct vagal nerve stimulation studies revealed significant dec reases with age in the HP at maximal vagal activation and in the intri nsic HP. The dynamic range, or difference between minimal and maximal parasympathetic effects on HP was similar across ages when assessed fr om the results of vagal stimulation. Nerve stimulations also revealed age-independent and relatively linear transfer functions relating para sympathetic stimulation frequency and HP during early life. Therefore, several parameters characterizing parasympathetic control of HP, incl uding the dynamic range acid transfer function, remain reasonably stab le throughout the early postnatal period in the rat. These data provid e a framework delineating the autonomic limits within which cardiac re sponses operate in the young rat. Knowledge of changes in these limits across time affords a firmer physiological basis for cross-age compar isons of autonomically-mediated cardiac changes.