Inhibition of Ca²⁺ channels and adrenal catecholamine release by G protein coupled receptors

Catecholamines and other transmitters released from adrenal chromaffin cells play central roles in the "fight-or-flight" response and exert profound effects on cardiovascular, endocrine, immune, and nervous system function. As such, precise regulation of chromaffin cell exocytosis is key to maintaining normal physiological function and appropriate responsiveness to acute stress. Chromaffin cells express a number of different G protein coupled receptors (GPCRs) that sense the local environment and orchestrate this precise control of transmitter release. The primary trigger for catecholamine release is Ca²⁺ entry through voltage-gated Ca²⁺ channels, so it makes sense that these channels are subject to complex regulation by GPCRs. In particular G protein βγ heterodimers (Gβγ) bind to and inhibit Ca²⁺ channels. Here I review the mechanisms by which GPCRs inhibit Ca²⁺ channels in chromaffin cells and how this might be altered by cellular context. This is related to the potent autocrine inhibition of Ca²⁺ entry and transmitter release seen in chromaffin cells. Recent data that implicate an additional inhibitory target of Gβγ on the exocytotic machinery and how this might fine tune neuroendocrine secretion are also discussed.