Inhibitory prejunctional muscarinic receptors at sympathetic nerves do not operate through a cyclic AMP dependent pathway.

In mouse atria previously incubated with [3H]-noradrenaline, carbachol (1.0 mumo1/l) significantly inhibited the fractional stimulation-induced (S-I) outflow of radioactivity. The inhibitory effect of carbachol was greater in the presence of the alpha-adrenoceptor antagonist phentolamine (1.0 mumol/l), which by itself significantly increased the S-I outflow of radioactivity. In both cases the inhibitory effect of carbachol was blocked by atropine (0.3 mumol/l), suggesting that the effect was mediated through muscarinic receptors. 8-Bromo cyclic AMP (270 mumol/l) in the presence of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX, 100 mumol/l), was used to maximally enhance the S-I outflow of radioactivity through the cyclic AMP mechanism. The inhibitory effect of carbachol either in the presence or in the absence of phentolamine, was not reduced in the presence of 8-bromo cyclic AMP and IBMX. Similar results with carbachol in the presence of 8-bromo cyclic AMP and IBMX were also found in rat right atrial strips which had been incubated with [3H]-noradrenaline. These results suggest that the effects through inhibitory prejunctional muscarinic receptors are not mediated by cyclic AMP. The protein kinase inhibitor, staurosporine (0.1 mumol/l), significantly blocked the enhancing effects of 8-bromo cyclic AMP (270 mumol/l) plus IBMX (100 mumol/l) on the S-I outflow of radioactivity from rat atrial strips. The inhibitory effect of carbachol (1.0 mumol/l) however, was not reduced in the presence of staurosporine, suggesting that protein kinases affected by staurosporine (protein kinase A, protein kinase C) are not involved in the post-receptor mechanism for inhibitory prejunctional muscarinic receptors. This finding further rules out the involvement of cyclic AMP in muscarinic inhibition. The inhibitory effect of carbachol either by itself or in the presence of phentolamine, was not reduced in atria from mice that had been pretreated with pertussis toxin (1.5 or 3.0 micrograms). Furthermore, in rat atrial strips, the inhibitory effect of carbachol either in the presence or in the absence of phentolamine, was also not altered by pretreating the rats with pertussis toxin (8.4 micrograms). The results suggest that in both tissues the major mechanism for inhibition of noradrenaline release through muscarinic receptors does not involve a pertussis toxin sensitive G protein.