Differential effects of alkylating agents on the multiple muscarinic receptor subtypes linked to activation of phospholipase C by carbachol in rat brain cortical membranes.

Muscarinic cholinergic receptor function in rat brain cortex was characterized by performing binding assays with [3H](-)quinuclidinyl benzilate ([3H]QNB) in parallel with assays of phospholipase C (PLC) activation by carbachol using membrane preparations and exogenous [3H]-phosphatidylinositol 4,5-bisphosphate ([3H]PIP2). Competitive binding studies revealed high- and low-affinity binding sites for the receptor antagonists, pirenzepine, methoctramine and the p-fluoro analog of hexahydro-sila-difenidol (p-F-HHSiD). Carbachol-stimulated [3H]-phosphatidylinositol 4,5-biphosphate breakdown was specifically inhibited by pirenzepine and p-F-HHiSD. The inhibition curves for these antagonists were best described by interactions at two sites. There was quantitative agreement between the antagonist affinity constants and the proportion of high- and low-affinity sites derived in functional and binding studies. The characteristics of the putative subtypes of muscarinic receptors and their stimulation of phospholipase C was examined after treatment with two alkylating agents, N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline and propylbenzilylcholine mustard. Loss of receptors was closely correlated with loss of PLC activation by carbachol, without alteration of the EC50 value (21 microM) of this agonist, clearly demonstrating a lack of receptor reserve. When both alkylating treatments were adjusted to induce a decrease of 60% in the maximal number of [3H]QNB binding sites, a similar (60%) reduction in the maximal effect of carbachol on PLC activation was found. However, the characteristics of the remaining receptors after the treatment with the two alkylating agents differ markedly as determined by competition of pirenzepine, p-F-HHSiD and methoctramine for [3H]QNB binding, and for inhibition of carbachol-stimulated phospholipase C by pirenzepine and p-F-HHSiD.(ABSTRACT TRUNCATED AT 250 WORDS)