Muscarinic agents modify kinetics properties of membrane-bound guanylyl cyclase activity

Plasma membranes from bovine tracheal smooth muscle show guanylyl cyclase activity, which can be stimulated by muscarinic agonists such carbamylcholine and oxotremorine and blocked by atropine. This stimulation was observed in the presence of 150 mM NaCl. In the absence of this salt, guanylyl cyclase activity was considerably higher but was not affected by muscarinic agonists. Carbamylcholine decreased the apparent Km but did not change the Vmax of this enzyme. When plasma membrane fractions were extracted with 1% octylglucoside, guanylyl cyclase activity was preserved, however the muscarinic activation was abolished, despite a muscarinic receptor capable of [3H]quinuclidinylbenzilate binding being present in the extract. The detergent extraction changed the affinity of guanylyl cyclase for GTP but the Mn2+ kinetics was unaltered. Based on these findings and on current information in the literature, we propose that another component is required to restore the link between the muscarinic receptor and guanylyl cyclase, however the nature of this component remains to be established.