Effect of monovalent ions upon G proteins coupling muscarinic receptors to phosphoinositide hydrolysis in the rat cerebral cortex.

It has been suggested that K+, Li+ and Fl- affect the function of G proteins coupled to signal transducing enzymes. Lithium, at concentrations which were found to reduce forskolin-stimulated adenylate cyclase activity, was without effect on either membrane [3H]phosphatidylinositol-4,5-bisphosphate ([3H]PIP2) hydrolysis measured in the absence or presence of 5'-guanylyl-imidodiphosphate (Gpp(NH)p), or (at greater than or equal to 2.3 mM Li+) upon the stimulation of rat cerebral cortical inositol phospholipid breakdown by either carbachol, noradrenaline or NaF measured at either 6 or 18 mM K+. The increase in assay [K+] greatly enhanced the inositol phospholipid response to carbachol but not to NaF. The inhibitory effect of carbachol upon forskolin-stimulated adenylate cyclase was not affected by raising the [K+] from 6 to 18 mM. At 6 mM K+ (both in the absence and presence of 15 microM AlCl3), the effects of carbachol and NaF upon inositol phospholipid breakdown were essentially additive, whereas at 18 mM K+, the breakdown response to carbachol (antagonised by pirenzepine with a pA2 value of 7.6) was similar in the absence and presence of NaF. It is concluded that in the rat cerebral cortex: (a) Li+ does not affect the function of either the phosphoinositide-specific phospholipase C enzyme itself or the Gp coupled to this enzyme; (b) the difference between the additivity between NaF and carbachol seen at different assay [K+] may reflect the K(+)-dependent changes in the tetrodotoxin-resistant and tetrodotoxin-sensitive pathways of carbachol stimulation of inositol phospholipid breakdown reported by Gurwitz and Sokolovsky (1987, Biochemistry 26, 633); and (c) the effect of K+ on muscarinic receptor-coupled inositol phospholipid breakdown is not found for muscarinic receptors inhibitorily coupled to adenylate cyclase. Evidence is also presented to suggest that NaF affects the dephosphorylation of the formed [3H]inositol polyphosphates.