Reflex changes in sympatho-adrenal medullary functions in response to baroreceptor stimulation in anesthetized rats

Effects of increased and decreased blood pressure on adrenal sympathetic nerve activity and catecholamine secretion were examined in anesthetized Wistar rats. Efferent nerve impulses were counted and adrenal venous plasma was collected just before and 30 s after the onset of hypotensive stimulation induced by bleeding (average decrease in systolic blood pressure over the stimulation period was 38 mm Hg) or a hypertensive stimulation induced by phenylephrine administration (average increase in systolic blood pressure over the stimulation period was 41 mm Hg). While nerve activity and epinephrine and norepinephrine secretion were not significantly altered by the change in blood pressure in baroreceptor denervated animals, there were increases in these variables (131.1% of control levels for epinephrine, 117.5% for norepinephrine, and 127.1% for nerve activity) in response to bleeding and a marked decrease (to 63.0% of control levels for epinephrine, 53.9% for norepinephrine, and 54.0% for nerve activity) in response to phenylephrine administration with baroreceptors intact. These changes were significant both with respect to control blood pressure levels and with regard to the effect of stimulation in the baroreceptor-denervated animals. The reflex responses in adrenal nerve and catecholamine secretion were generally proportional in magnitude. The present results are discussed with regard to the effects of somatic stimulation on nerve activity and catecholamine secretion.