Sci. STKE, 6 September 2005
SECRETION Gβ Decreases Quantal Size
In a study of rat adrenal chromaffin cells, which have dense-core secretory granules, Chen et al. reported that release of Gβ in response to activation of Gi/o-coupled receptors inhibited secretion by reducing quantal size. The reduction in quantal size was measured by amperometry in response to application of adenosine triphosphate (ATP), opioids, or somatostatin. Although each agonist partially inhibited calcium signaling, each also inhibited secretion induced by caffeine, which bypasses plasma membrane calcium channels to promote release of calcium directly from intracellular stores. Thus, application of ATP, for example, which can inhibit plasma membrane calcium channels, does not affect calcium transients induced by caffeine, yet still inhibits secretion. The agonists did not inhibit secretion stimulated by activation of muscarinic acetylcholine receptors (mAChR), but pharmacological inhibition of protein kinase C (PKC), which is downstream of the Gq-coupled mAChR, restored ATP-, opioid,-, and somatostatin-mediated inhibition of secretion. ATP was confirmed to act through the Gi/o-coupled receptor P2Y. Somatostatin and the opioid agonist tested also act through a Gi/o-coupled receptor; however, the effects of these agonists appeared to be mediated through the β and not the α subunits. The mechanism by which β inhibited secretion appeared not to involve the known kinases downstream of β, because pharmacological inhibition of phosphoinositide 3-kinase (PI3K), phospholipase β2, serine-threonine kinases, or tyrosine kinases did not affect caffeine-induced secretion. Sequestration of β with a membrane-permeable peptide blocked ATP-induced inhibition of caffeine-induced secretion, and whole-cell dialysis of Gβ12 into the cells inhibited depolarization-induced secretion. The authors propose that Gβ limits the open time of the fusion pore, but the exact target of β for this effect remains to be determined.
X.-K. Chen, L.-C. Wang, Y. Zhou, Q. Cai, M. Prakriya, L.-K. Duan, Z.-H. Sheng, C. Lingle, Z. Zhou, Activation of GPCRs modulates quantal size in chromaffin cells through Gβ and PKC. Nat. Neurosci. 8, 1160-1168 (2005). [PubMed]
Citation: Gβ Decreases Quantal Size. Sci. STKE 2005, tw322 (2005).
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