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CJ Lynch, V Prpic, PF Blackmore and JH Exton
Islet-activating protein (IAP, a Bordetella pertussis toxin) was employed to test the hypothesis that the inhibitory GTP-binding regulatory protein of adenylate cyclase (Ni) mediates GTP effects on the binding of Ca2+-mobilizing hormones to liver plasma membranes and is involved in calcium mobilization stimulated by these agonists. IAP added to normal liver plasma membranes catalyzed the incorporation of radioactivity from [32P]NAD into a 41,000-Da peptide (presumably the alpha-subunit of Ni). However, no such incorporation was observed in liver membranes prepared from rats 24 hr after intraperitoneal injection of IAP. Angiotensin II attenuated glucagon-stimulated increases in cAMP in hepatocytes prepared from control but not IAP- treated rats. In contrast, following IAP treatment, no changes were observed in the ability of glucagon, vasopressin, angiotensin II, or epinephrine to activate phosphorylase; nor did this treatment alter [3H]vasopressin binding or epinephrine displacement of [3H]prazosin binding. However, IAP treatment decreased [3H]angiotensin II binding affinity when studies were performed in the absence but not the presence of 5'-guanylylimidodiphosphate (GppNHp). This shift was small and represented only 5-8% of the shift in apparent Kd elicited by GppNHp in untreated membranes. In vitro studies with IAP confirmed the results of the radioligand binding studies using in vivo IAP treatment. The effects of NaCl on [3H]angiotensin II binding were also tested but were not typical of other receptors which couple to Ni. The data suggest that, although a small population of hepatic angiotensin II receptors couple to Ni and attenuate glucagon-stimulated increases in cAMP, vasopressin, alpha 1-adrenergic, and the majority of angiotensin II receptors do not interact significantly with Ni. Thus, although there is evidence that agonist-induced Ca2+ mobilization requires a GTP- binding regulatory protein, this protein does not appear to be Ni in rat liver.
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