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Rapid desensitization of adrenaline‐ and neuropeptide Y‐stimulated Ca 2+ mobilization in HEL‐cells
Author(s) -
Michel Martin C.
Publication year - 1994
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1994.tb13101.x
Subject(s) - homologous desensitization , thapsigargin , endocrinology , medicine , desensitization (medicine) , neuropeptide y receptor , receptor , agonist , chemistry , neuropeptide , biology , endoplasmic reticulum , biochemistry
1 Desensitization of G s ‐coupled receptors, the β 2 ‐adrenoceptor for example, involves rapid and slower components but little is known regarding the existence of rapid desensitization of G i ‐coupled receptors and its possible mechanisms. In HEL‐cells stimulation of α 2A ‐adrenoceptors by adrenaline or Y 1 ‐like neuropeptide Y receptors by neuropeptide Y, transiently mobilizes Ca 2+ from intracellular stores via a G i ‐protein. We have used this model to study the existence and possible mechanisms of rapid desensitization of a G i ‐mediated cellular response. 2 Following stimulation by adrenaline or neuropeptide Y Ca 2+ levels returned towards baseline a few minutes after agonist addition and were refractory to a second agonist exposure demonstrating rapid desensitization. Cross‐desensitization experiments with neuropeptide Y, adrenaline and moxonidine demonstrated the presence of homologous (both receptors) and heterologous desensitization (neuropeptide Y receptors only), and that the α 2A ‐adrenoceptor desensitization was not specific for phenylethylamine (adrenaline) or imidazoline agonists (moxonidine). 3 The protein kinase C activator, phorbol ester, rapidly desensitized the hormonal Ca 2+ responses and inhibitors of protein kinase C enhanced the hormonal responses inconsistently. The tyrosine kinase inhibitor, herbimycin, enhanced Ca 2+ mobilization by adrenaline and neuropeptide Y, whereas the protein phosphatase inhibitor, okdadaic acid, did not affect Ca 2+ mobilization or its desensitization. 4 In the absence of extracellular Ca 2+ the endoplasmic reticulum Ca 2+ ‐ATPase inhibitor, thapsigargin, reduced hormone‐stimulated Ca 2+ elevations, demonstrating that mobilization occurs from a thapsigargin‐sensitive pool in the endoplasmic reticulum. The inositol phosphate‐independent Ca 2+ release modulator, ryanodine, significantly enhanced adrenaline‐ and neuropeptide Y‐stimulated Ca 2+ elevations. Blockade of the endoplasmic reticulum Ca 2+ ‐ATPase by thapsigargin in the presence of extracellular Ca 2+ enhanced hormone‐stimulated Ca 2+ increases, demonstrating the importance of this enzyme for the termination of the Ca 2+ signal. 5 It is concluded that adrenaline and neuropeptide Y‐stimulated Ca 2+ mobilization in HEL‐cells occurs from a thapsigargin‐ and ryanodine‐sensitive store in the endoplasmic reticulum and desensitizes rapidly; this appears to involve multiple mechanisms including protein kinases, possibly acting on receptors, and Ca 2+ release and sequestration mechanisms.