Prejunctional alpha adrenoceptor desensitization in rat heart after chronic epinephrine treatment

This study examines the effects of acute incorporation of epinephrine (EPI) into cardiac catecholamine stores and chronic elevation of plasma EPI concentrations on sympathetic neurotransmission and adrenoceptor modulation in the rat heart. Chronic elevation of plasma EPI was accomplished by infusion of EPI (100 micrograms/kg/hr s.c.) for 6 days via osmotic minipumps. Vehicle-treated animals served as controls. The cardiac catecholamine stores of the vehicle-treated group consisted of norepinephrine only. Chronic EPI treatment resulted in incorporation of EPI (90% of total catecholamines) without a significant change in the total cardiac catecholamine content. At 0.1 to 1 Hz, stimulus-induced catecholamine overflows in the EPI-treated group consisted of both norepinephrine and EPI and were 66 to 70% higher than those of the vehicle-treated group. A preferential beta-2 adrenoceptor antagonist, ICI 118,551 (1 nM-1 microM) did not alter catecholamine overflows in either group. In contrast, a preferential alpha-2 adrenoceptor antagonist, idazoxan (1 microM), significantly increased catecholamine overflow in the vehicle-treated group but not in the EPI-treated group. After idazoxan treatment, the significant difference between overflows in the chronic vehicle- and EPI-treated groups was abolished. Acute EPI treatment in vitro (1 microM, 1 hr in the presence of phentolamine 10 microM; nadolol, 30 microM) resulted in incorporation of EPI (75% of total catecholamine) into cardiac catecholamine stores and increased the stimulus-induced catecholamine overflow at 0.1 Hz. Blockade of prejunctional beta-2 adrenoceptors abolished the difference between the acute EPI- and vehicle-treated groups. Moreover, the increase in catecholamine overflow resulting from alpha-2 adrenoceptor blockade with idazoxan (1 microM) was comparable between the acute EPI- and vehicle-treated groups. These results suggest that 1) both chronic elevation of plasma EPI and acute incorporation of EPI into cardiac neurotransmitter stores result in facilitation of neurotransmitter release and 2) prejunctional beta adrenoceptor activation appears to be the cause of the increase after acute EPI incorporation, whereas desensitization of prejunctional alpha-2 adrenoceptors, not beta-2 adrenoceptor-mediated facilitation, is the cause of the enhanced overflow after chronic EPI treatment.