Cardiac alpha 1-adrenoceptors stimulate a high-affinity GTPase activity in sarcolemmal membranes from rabbit atrial and ventricular myocytes.
The interaction between cardiac alpha 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 alpha 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 alpha 1-adrenergic agonists 1-noradrenaline and methoxamine (in the presence of propranolol) was observed to be both linear with time and saturable. alpha 1-adrenergic stimulation did not change the Km for GTP (0.14-0.21 microM), but increased the Vmax by 39% and 72% above basal levels in atrial and ventricular membranes, respectively. Stimulation of GTPase activity by alpha 1-agonists occurred in a concentration-dependent fashion and was blocked in the presence of the alpha-adrenoceptor antagonists phentolamine and prazosin, but not yohimbine. Prior treatment of myocytes with pertussis toxin had no effect on the alpha 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 [alpha-32P]GTP was enhanced in the presence of the alpha 1-agonist methoxamine and abolished by addition of excess non-labelled GTP, suggesting that this GTP-binding protein may interact with cardiac alpha 1-adrenoceptors; a similar GTP-binding protein which may be coupled to alpha 1-adrenoceptors has been reported in rat liver plasma membranes (Im, M. J. & Graham, R. M. (1990) J. Biol. Chem. 265, 18944-18951).
Braun
A P
AP
MRC Group in Signal Transduction, Faculty of Medicine, University of Calgary, Canada.
Walsh
M P
MP
D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000345:Affinity Labels; D000818:Animals; D002462:Cell Membrane / Q000201:enzymology; D002478:Cells, Cultured; D004789:Enzyme Activation; D020558:GTP Phosphohydrolases / Q000378:metabolism*; D019204:GTP-Binding Proteins / Q000378:metabolism; D006325:Heart Atria; D006352:Heart Ventricles; D066298:In Vitro Techniques; D007700:Kinetics; D009206:Myocardium / Q000166:cytology / Q000201:enzymology*; D037342:Pertussis Toxin; D011817:Rabbits; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects / Q000502:physiology*; D013379:Substrate Specificity; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
D000317:Adrenergic alpha-Antagonists; D000345:Affinity Labels; D011942:Receptors, Adrenergic, alpha; D010566:Virulence Factors, Bordetella; D037342:Pertussis Toxin; D020558:GTP Phosphohydrolases; D019204:GTP-Binding Proteins