Mechanisms of accumulation of tyramine, metaraminol, and isoproterenol in isolated chromaffin granules and ghosts

Abstract

The effects of the transmembrane pH gradient (delta pH) and the transmembrane potential gradient (delta psi) on the uptake of several sympathomimetic amines were investigated, using bovine adrenal chromaffin granules isolated in isotonic sucrose. As previously described [R. Johnson and A. Scarpa, J. Biol. Chem. 254 3750 (1979)], freshly isolated chromaffin granules maintain an intragranular pH of 5.5 as measured by [14C] methylamine distribution and, in the presence of ATP, generate a delta psi of 80 mV, positive inside, as measured by [14C] methylamine distribution. When tyramine, metaraminol, and isoproterenol (1-50 mM) were added to well-buffered suspensions of granules at pH 7.0, a dose-related alkalinization of the granule interior was observed. Study of the time-resolved influx of the same amines labeled radiochemically (5-21 microM) revealed that all the amines were accumulated against an apparent concentration gradient. However, while accumulation of [14C] serotonin and [3H] isoproterenol was totally inhibited by reserpine, [14C] tryramine accumulation was inhibited by only 60% and [14C[ metaraminol uptake was unaffected. The ATP-dependent generation of a delta psi produced a stimulation of amine uptake in the order: serotonin greater than isoproterenol greater than tyramine; metaraminol accumulation was not enhanced by ATP addition. The relationship between the electrochemical proton gradient (delta micro H+) and the electrochemical gradient for each of the sympathomimetic amines (delta micro A) was investigated utilizing chromaffin ghosts devoid of endogenous matrix gradients or components. All amines were accumulated in the presence of delta pH alone. In the presence of delta psi alone, [14C] serotonin, (14C] tyramine, and [3H] isoproterenol were accumulated, but no [3H] metaraminol uptake was demonstrable. The results indicate that serotonin and isoproterenol accumulated in isolated chromaffin granules and ghosts via a reserpine-sensitive mechanism, driven by the magnitude of the electrochemical proton gradient. Conversely, metaraminol permeated the membrane of the chromaffin granule through the apolar lipid phase and distributed according to the delta pH alone. Tyramine uptake proceeded by both mechanisms. The implications of the mechanism of accumulation of these potent physiologic and pharmacologic agents for their in vivo action are discussed.