Acute epinephrine treatment fails to facilitate stimulus-induced neurotransmitter overflow in the isolated perfused rat kidney

The effects of acute in vitro epinephrine treatment (40 nM, 1 hr) on stimulus-induced neurotransmitter overflow from the isolated perfused kidney were examined. This treatment resulted in the release of both norepinephrine and epinephrine upon nerve stimulation but no increase in fractional stimulus-induced overflow. Experiments utilizing beta and/or alpha adrenoceptor blockade revealed that beta adrenoreceptor blockade alone had no effect on overflow but that a dose-dependent decrease in stimulus-induced overflow was produced by beta adrenoceptor blockade after alpha adrenoceptor blockade. Thus, under normal conditions the facilitatory influence of epinephrine via prejunctional beta adrenoceptors is masked by the prejunctional alpha adrenoceptor negative feedback loop. The effects of extraneuronal presentation of isoproterenol, salbutamol and epinephrine on stimulus-induced neurotransmitter overflow also were examined. In the absence of alpha adrenoceptor blockade, only epinephrine failed to enhance neurotransmitter overflow. After competitive alpha adrenoceptor blockade, all three agents produced significant increases in stimulus-induced neurotransmitter overflow from the rat kidney, although alpha adrenoceptor stimulation appeared to limit this effect at high agonist concentrations. We conclude that the dominance of prejunctional alpha adrenoceptors in the rat kidney results in the inability of epinephrine to produce beta adrenoceptor-mediated facilitation of neurotransmitter overflow even when it is incorporated into renal neurotransmitter stores and coreleased with endogenous norepinephrine upon nerve stimulation.

Enhanced endogenous neurotransmitter overflow in the isolated perfused rat kidney after chronic epinephrine administration: lack of a prejunctional beta adrenoceptor influence

This study examines stimulus-induced overflow of endogenous catecholamines from the isolated perfused kidney of rats after epinephrine (EPI) administration at 100 micrograms/kg/hr for 6 days via s.c. implanted osmotic minipumps. This regimen resulted in the incorporation of EPI into renal catecholamine stores and the co-release of EPI with norepinephrine during periarterial nerve stimulation. Stimulus-induced (1 Hz) absolute and fractional overflows of neurotransmitter were significantly greater in the EPI-treated rats. These differences in overflow were not altered by uptake blockade. Alpha adrenoceptor blockade with phentolamine (10(-9)-10(-5) alone increased overflow in a dose-dependent manner in both groups, although to a greater extent in the EPI-treated group. However, in the presence of 1 microM propranolol the effect of phentolamine in the EPI-treated group was greatly reduced such that, in the presence of propranolol, phentolamine was less effective in the EPI-treated vs. vehicle-treated rats. Beta adrenoceptor blockade alone with propranolol (10(-9)-10(-6) M) did not alter stimulus-induced overflow in either group but propranolol dose dependently reduced fractional overflow in the EPI-treated rats when the experiments were done in the presence of 10 microM phentolamine. It is concluded that the enhanced fractional overflow observed in the kidneys of EPI-treated rats was not due to beta adrenoceptor activation, but rather, a reduced influence of the prejunctional alpha adrenoceptor-mediated negative feedback loop. Beta adrenoceptor activation by neurally released EPI does not appear to modulate stimulus-induced overflow unless the dominant inhibitory alpha adrenoceptor mechanism is inactivated.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiovascular responsiveness to sympathetic activation after chronic epinephrine administration

The effects of chronic elevation of plasma epinephrine concentrations on pressor and cardiac chronotropic responses to spinal stimulation and exogenous norepinephrine were studied in the pithed rat. Animals were treated s.c. with epinephrine (100 micrograms/kg/hr) or vehicle for 6 days. Treatment with epinephrine resulted in hypertension. Epinephrine treatment significantly reduced pressor and cardiac chronotropic responses to norepinephrine. This treatment also reduced cardiac chronotropic responses to spinal stimulation but had no significant effect on pressor responses to spinal stimulation. The preferential beta-2 adrenoceptor antagonist ICI 118,551 (50 micrograms/kg) had no effect on pressor or cardiac chronotropic responses to norepinephrine in either vehicle-treated or epinephrine-treated animals. Similarly, pressor responses to spinal stimulation were unaffected in the vehicle-treated group. However, ICI 118,551 significantly reduced pressor responses to spinal stimulation after epinephrine treatment. Chronic epinephrine treatment also resulted in accumulation of epinephrine in the heart, kidneys and aorta. However, epinephrine accumulation in the heart was much greater than in the kidney or aorta. Acute infusion of epinephrine, which produced plasma epinephrine concentrations similar to those observed in the chronically treated animals, caused a reduction of pressor and cardiac chronotropic responses to both nerve stimulation and norepinephrine. In conclusion, chronic epinephrine treatment led to accumulation of epinephrine in peripheral tissues and to the development of a facilitatory influence of beta-2 adrenoceptors on pressor responses to nerve stimulation in the pithed rat that were not observed in vehicle-treated animals. The authors believe that this influence is prejunctional in nature.(ABSTRACT TRUNCATED AT 250 WORDS)