Differential enhancement by potassium ions of M1-type and M2-type muscarinic receptor-mediated phosphoinositide breakdown in the rat brain.

Raising the assay [K+] from 6 to 18 mM enhances the inositol phospholipid breakdown response to carbachol in rat brain miniprisms. In the frontal cortex, the degree of enhancement by K+ was independent of the carbachol concentration used, whereas in the striatum, a significantly higher degree of enhancement was seen at 1000 than at 50 microM carbachol. The carbachol-stimulated inositol phospholipid breakdown was antagonized by pirenzepine at both [K+] with potencies suggesting involvement of M1-type muscarinic receptors in the frontal cortex and both M1- and M2-type muscarinic receptors in the striatum. It is suggested that the response mediated by the M1-type receptors is enhanced to a greater degree by raised [K+] than that mediated by the M2-type receptors.