Chronic epinephrine treatment fails to alter prejunctional adrenoceptor modulation of sympathetic neurotransmission in the rat mesentery.

Rats were treated chronically with epinephrine (EPI-T; 100 micrograms/kg/hr, s.c.) for 6 days. On day 6 of treatment, the rats were anesthetized and the mesenteric vascular bed was isolated and perfused with Krebs' bicarbonate buffer containing cocaine (10 microM) and corticosterone (40 microM). Stimulus-induced (2 Hz, 120 pulses) overflow of neurotransmitter and its modulation by prejunctional adrenoceptors was studied. After chronic exposure to EPI, 50% of the mesenteric catecholamine stores consisted of EPI with no increase in total catecholamine content as compared to the control group (C). Absolute and fractional overflows of catecholamines upon periarterial nerve stimulation (2 Hz, 1 min) were not significantly different in the two groups. Beta adrenoceptor blockade by propranolol (10(-10) to 10(-6) M) did not alter the overflow of catecholamines. Alpha adrenoceptor blockade by phentolamine (10(-5) M) increased neurotransmitter overflow in both EPI-T and C groups. However, there was no significant difference in total catecholamine overflows between the two groups. Moreover, in the presence of phentolamine, propranolol (10(-6) M) remained without effect on overflow in both groups. These data suggest that EPI-T did not significantly increase the stimulus-induced overflow of catecholamines in the rat mesentery, nor did EPI-T result in prejunctional beta adrenoceptor modulation of neurotransmitter release in the mesenteric vascular bed.