Serotonin transport in isolated platelet granules. Coupling to the electrochemical proton gradient

The effect of the transmembrane proton gradient (delta pH) and potential gradient (delta psi) upon the rate and extent of amine accumulation was investigated in intact 5-hydroxytryptamine (serotonin) containing dense granules. The granules were isolated and purified from other subcellular organelles under isotonic conditions utilizing a newly developed continuous density gradient of Percoll. As measured by [14C]methylamine distribution, isolated granules suspended in a highly buffered medium at pH 7.0 had an intragranular pH of 5.40, independent of the pH of the external medium. This pH gradient could be collapsed by the addition of 60 mM ammonia. In the presence of Mg-ATP, a transmembrane potential (delta psi) of 30-40 mV, inside positive, was generated and sustained for over 30 min, as measured by [14C]thiocyanate distribution. The addition of carbonyl cyanide p-trifluoromethoxyphenylhydrazone, a proton translocator, resulted in the reversal of the potential to negative values. The Mg-ATP-dependent generation of the delta psi was prevented by addition of dicyclohexylcarbodiimide and trimethyltin, inhibitors of proton-translocating ATPases in this and other subcellular organelles. Ammonia (1-50 mM) addition to highly buffered suspensions of serotonin granules caused a dose-dependent decrease in the delta pH, while thiocyanate added at varying concentrations produced a dose-related collapse of the delta psi and had no effect upon the delta pH. Both the delta pH and delta psi were found to independently drive accumulation of [14C]serotonin into the granules; stepwise collapse of each gradient resulted in a corresponding diminution of [14C]serotonin accumulation. The maximum rate and extent of [14C]serotonin uptake, however, were observed in the presence of both the delta pH and delta psi. The conclusions provide support for the existence of a proton-translocating ATPase in the serotonin granule membrane responsible for the generation of the delta pH and delta psi. Moreover, the results demonstrate a primary role for the electrochemical proton gradient (delta mu H+) in the carrier-mediated active transport of 5-hydroxytryptamine into the platelet granule.