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Regulation of hepatocyte adenylate cyclase by amylin and CGRP: A single receptor displaying apparent negative cooperativity towards CGRP and simple saturation kinetics for amylin, a requirement for phosphodiesterase inhibition to observe elevated hepatocyte cyclic AMP levels and the phosphorylation of G i ‐2
Author(s) -
Houslay Miles D.,
Morris Nicholas J.,
Savage Anne,
Marker Alison,
Bushfield Mark
Publication year - 1994
Publication title -
journal of cellular biochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.028
H-Index - 165
eISSN - 1097-4644
pISSN - 0730-2312
DOI - 10.1002/jcb.240550008
Subject(s) - amylin , adenylate kinase , cyclase , endocrinology , medicine , chemistry , kinetics , cooperativity , saturation (graph theory) , receptor , phosphodiesterase , biology , biochemistry , enzyme , physics , islet , insulin , mathematics , quantum mechanics , combinatorics
Abstract Challenge of intact hepatocytes with amylin only succeeded in elevating intracellular cyclic AMP levels and activating phosphorylase in the presence of the cAMP phosphodiesterase inhibitor IBMX. Both amylin and CGRP similarly activated adenylate cyclase, around 5‐fold, although ∼ 400‐fold higher levels of amylin were required to elicit half maximal activation. Amylin activated adenylate cyclase though apparently simple Michaelin kinetics whereas CGRP elicited activation by kinetics indicative of apparent negative co‐operativity. Use of the antagonist CGPP(8–37) showed that both CGRP and amylin activated hepatocyte adenylate cyclase through a common receptor by a mnemonical mechanism where it was proposed that the receptor co‐existed in interconvertible high and low affinity states for CGRP. It is suggested that this model may serve as a paradigm for G‐protein linked receptors in general. Amylin failed to both stimulate inositol phospholipid metabolism in hepatocytes and to elicit the desensitization of glucagon‐stimulated adenylate cyclase. Amylin did, however, elicit the phosphorylation of the inhibitory guanine nucleotide regulatory protein G i ‐2 in hepatocytes and prevented the action of insulin in reducing the level of phosphorylation of this G‐protein. © 1994 Wiley‐Liss, Inc.