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.
Tiger
G
G
Department of Pharmacology, University of Umea, Sweden.
Björklund
P E
PE
Cowburn
R F
RF
Garlind
A
A
O'Neill
C
C
Wiehager
B
B
Fowler
C J
CJ
D000262:Adenylyl Cyclases / Q000378:metabolism; D000818:Animals; D002217:Carbachol / Q000494:pharmacology; D002540:Cerebral Cortex / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D019204:GTP-Binding Proteins / Q000378:metabolism*; D006868:Hydrolysis; D066298:In Vitro Techniques; D007295:Inositol Phosphates / Q000378:metabolism; D007477:Ions; D008094:Lithium / Q000494:pharmacology; D008297:Male; D019269:Phosphatidylinositol 4,5-Diphosphate; D010716:Phosphatidylinositols / Q000378:metabolism*; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D011976:Receptors, Muscarinic / Q000187:drug effects / Q000378:metabolism*; D012969:Sodium Fluoride / Q000494:pharmacology; D010738:Type C Phospholipases / Q000378:metabolism
D007295:Inositol Phosphates; D007477:Ions; D019269:Phosphatidylinositol 4,5-Diphosphate; D010716:Phosphatidylinositols; D011976:Receptors, Muscarinic; D002217:Carbachol; D012969:Sodium Fluoride; D008094:Lithium; D010738:Type C Phospholipases; D019204:GTP-Binding Proteins; D000262:Adenylyl Cyclases; D011188:Potassium