Elimination of GTP biphasic regulation of synaptosomal adenylate cyclase by manganese and solubilization.

Abstract

Effects of divalent cations and solubilization with Lubrol-PX were studied on guanine nucleotide regulation of synaptosomal adenylate cyclase activity of the rat caudate nucleus. In the presence of Mg2+, both GTP and Gpp(NH)p exerted biphasic actions on the membrane-bound adenylate cyclase activity. The K0.5 value for the GTP stimulation of the cyclase was 47 nM, and the value for the GTP inhibition was 4.5 microM. One hundred microM dopamine selectively enhanced the stimulatory phase of the GTP action, whereas 10 microM morphine selectively enhanced the inhibitory phase of the GTP action. When Mg2+ was replaced by Mn2+, the inhibition of the membrane-bound adenylate cyclase by these nucleotides and morphine was completely abolished; but the catalytic activity of adenylate cyclase was not impaired. These results suggest that the inhibitory action of GTP is responsible for the morphine inhibition of synaptosomal adenylate cyclase. Lubrol-solubilized adenylate cyclase prefered Mn2+ to Mg2+ for its activity. The stimulation of adenylate cyclase by either GTP or Gpp (NH)p was eliminated in the Sepharose 6B-fractionated solubilized preparation in the presence of either Mg2+ or Mn2+. Ten mM NaF also failed to activate the fractionated adenylate cyclase. In the fractionated solubilized preparation, GTP and Gpp(NH)p failed to inhibit adenylate cyclase. These results indicate that GTP and Gpp(NH)p are unable to inhibit the resolved catalytic unit of the synaptosomal adenylate cyclase.