Cardiac α 1 ‐adrenoceptors stimulate a high‐affinity GTPase activity in sarcolemmal membranes from rabbit atrial and ventricular myocytes

The interaction between cardiac α 1 ‐adrenoceptors and GTP‐binding regulatory proteins was characterized in isolated rabbit cardiac myocytes (thereby avoiding interference by other cell types present in the myocardium) by examining the α 1 ‐adrenergic stimulation of GTPase activity in sarcolemma‐enriched membrane fractions. Stimulation of membrane‐associated GTPase activity in both atrial and ventricular myocyte preparations by the α 1 ‐adrenergic agonists 1‐noradrenaline and methoxamine (in the presence of propranolol) was observed to be both linear with time and saturable. α 1 ‐adrenergic stimulation did not change the K m for GTP (0.14–0.21 μM), but increased the V max by 39% and 72% above basal levels in atrial and ventricular membranes, respectively. Stimulation of GTPase activity by α 1 ‐agonists occurred in a concentration‐dependent fashion and was blocked in the presence of the α‐adrenoceptor antagonists phentolamine and prazosin, but not yohimbine. Prior treatment of myocytes with pertussis toxin had no effect on the α 1 ‐adrenergic stimulation of GTPase activity, but inhibited stimulation by muscarinic‐receptor activation with carbachol. Finally, photoaffinity labelling of an approximately 75‐kDa membrane‐bound protein with [α‐ 32 P]GTP was enhanced in the presence of the α 1 ‐agonist methoxamine and abolished by addition of excess nonlabelled GTP, suggesting that this GTP‐binding protein may interact with cardiac α 1 ‐adrenoceptors; a similar GTP‐binding protein which may be coupled to α 1 ‐adrenoceptors has been reported in rat liver plasma membranes (Im, M. J. & Graham, R. M. (1990) J. Biol. Chem. 265 , 18944–18951).