Studies on muscarinic M2 receptors in smooth muscle.

Histamine after M3 receptor alkylation with 4-DAMP mustard does not serve simply as a spasmogen but facilitates visualization of the M2-mediated contraction induced by oxotremorine M. We speculate that M3 receptor activation has a similar role in untreated tissue. The facilitation appears to involve protein kinase C. The results with propranolol suggest that the phospholipase D pathway may also be important in the development of the M2-mediated response.