Sodium channel, sodium pump, and sodium-calcium exchange activities in synaptosomal plasma membrane vesicles

Two mechanisms of Na+ influx have been observed using synaptosomal plasma membrane vesicles purified by density gradient centrifugation from a synaptosomal hypotonic lysate. First, a 5-fold increase in uptake over basal Na+ entry occurs with 50 microM veratridine. The veratridine-dependent Na+ uptake is partially inhibited by 2 microM tetrodotoxin with an apparent time dependency of action (half-maximal inhibition in approximately 20 min). Second, a larger Na+ accumulation (approximately 15-fold above basal) was observed with 2.5 mM ATP, this effect being dependent on internal K+ loading of vesicles although inhibited by high external K+. The two uptake processes are believed to represent operation of the plasma membrane voltage-sensitive Na+ channel, and the Na+-pumping (Na+ + K+)-ATPase, respectively. Both Na+ flux mechanisms appear to operate in a single population of vesicles since opening of the Na+ channel with veratridine diminishes the ATP-dependent accumulation of Na+ by over 75%. An inverted orientation of the plasma membrane vesicles is likely to account for the functioning of the ATP-dependent Na+ pump and may also account for the low sensitivity and time dependency of the inhibitory action of tetrodotoxin on Na+ channel-opening. Na+ accumulated by the Na+ pump was rapidly effluxed by 10 mM external Ca2+ via the Na+-Ca2+ exchange mechanism which (together with an ATP-dependent Ca2+-accumulating mechanism) was recently characterized in the vesicles (Gill, D. L., Grollman, E. F., and Kohn L. D. (1981) J. Biol. Chem. 256, 184-192). This result, together with the observed inhibition of Ca2+ influx via (Na+-Ca2+)-exchange due to veratridine-mediated Na+ flux, strongly suggests that the Na+ pump, Na+ channel, and both Ca2+ transport mechanisms function in a single population of inverted plasma membrane vesicles.