Coupling of the alpha 1-adrenergic receptor to a guanine nucleotide-binding regulatory protein by a discrete domain distinct from its ligand recognition site

Enhanced alpha 1-adrenergic responsiveness in cardiomyopathic hamster cardiac myocytes. Relation to the expression of pertussis toxin-sensitive G protein and alpha 1-adrenergic receptors

The pathogenesis of the myopathy occurring in the heart of the cardiomyopathic strain of the Syrian hamster is not well understood but is believed to be associated with abnormal calcium handling by myopathic cells. The purpose of this study was to determine whether the cardiomyopathy occurring in strain BIO 14.6 animals is associated with an enhanced alpha 1-adrenergic receptor-mediated rise in cytosolic calcium, whether a pertussis toxin-sensitive G protein is involved in coupling the alpha 1-adrenergic receptor to changes in intracellular calcium and whether enhanced alpha 1 responsiveness is associated with an increase in the level of expression of the alpha 1-adrenergic receptor or in the pertussis toxin-sensitive G protein or proteins. To test the hypothesis that the cardiomyopathic state is associated with a greater alpha 1-receptor-mediated rise in cytosolic calcium, we studied the effect of phenylephrine (in the presence of propranolol) on time-averaged cytosolic calcium concentration ([Ca2+]i) in isolated cardiac myocytes from cardiomyopathic and age-matched control hamsters. Phenylephrine caused a greater increase both in time-averaged [Ca2+]i (an increase of 48 +/- 8% versus 12 +/- 3%, p less than 0.01) and in contractility (+181 +/- 22% versus +35 +/- 9%, p less than 0.01) in cardiomyopathic than in normal cardiac myocytes. Exposure to pertussis toxin (200 ng/ml for 3 hours) attenuated the alpha 1-adrenergic receptor-mediated increase in contractility and time-averaged [Ca2+]i in both cardiomyopathic and normal cells. The level of pertussis toxin-sensitive G protein, as determined by pertussis toxin-mediated [32P]ADP-ribosylation, was 1.6-fold higher in cardiomyopathic versus normal hamster hearts. The density of alpha 1-adrenergic receptors, as measured by the antagonist radioligand [3H]prazosin and the affinity of the receptor for agonist and antagonist were similar in myopathic and normal heart membranes. Thus, in cardiac myocytes from hamsters, the alpha 1-adrenergic receptor-mediated effects on [Ca2+]i and contractility appear to be mediated by a pertussis toxin-sensitive G protein or proteins. In myocytes from cardiomyopathic hamsters, these alpha 1-adrenergic effects were increased in magnitude, as was the level of pertussis toxin-sensitive G protein, but there was no measurable alteration in the density or ligand binding properties of alpha 1-adrenergic receptors.

Evidence for the convergence of beta-adrenergic and muscarinic signalling systems at a post-receptor site

The beta-adrenergic agonist isoproterenol stimulates inositol trisphosphate (IP3) formation and cytosolic Ca2+ [( Ca2+]i) mobilization in rat parotid acini via a cAMP-dependent process. Atropine, a muscarinic antagonist, inhibited these isoproterenol responses without affecting isoproterenol-induced amylase secretion or peak [Ca2+]i and IP3 responses elicited by alpha 1-adrenergic stimulation with epinephrine. Atropine had no effect on isoproterenol-induced [Ca2+]i responses in a cell line which lacked muscarinic receptors and did not alter beta-adrenoreceptor ligand binding. These results suggest that the inhibition by atropine results from a post-receptor effect on cAMP-mediated stimulation of phosphatidylinositol 4,5 bisphosphate (PIP2) hydrolysis.

Formation of the high-affinity agonist state of the alpha 1-adrenergic receptor at cold temperatures does not require a G-protein

Two methods were employed to uncouple hepatic alpha 1-adrenergic receptors from their associated G-protein (termed Gp) in order to determine whether locking of the alpha 1-receptor in a high-affinity agonist state at cold temperatures (2 degrees C) represents formation of a ternary complex. Uncoupling is defined as the inability to observe the GppNHp-sensitive, high-affinity agonist state of the receptor in [3H]prazosin competition binding studies performed at 25 degrees C. The first method for achieving uncoupling involved brief alkalinization and resulted in greater than 95% loss of several G-proteins. The second method involved proteolytic cleavage of either part or all of the alpha 1-receptor coupling domain from the binding domain. Following either treatment, receptors were converted to the high-affinity agonist state at 2 degrees C. Thus, while formation of the high-affinity state of the receptor at higher temperatures may require Gp, formation of this state at 2 degrees C does not require Gp or even the entire alpha 1-adrenergic receptor.