Receptor-mediated catecholamine release from chromaffin cells : the role of protein kinase C
This study reveals that bovine chromaffin cells express PKC-alpha, -beta, -epsilon and -iota and that they were differentially activated by nicotine receptors and a range of Gaq/11-coupled GPCRs. Thus, nicotinic receptor stimulation recruited PKC-alpha, -beta and -epsilon from the cytoplasm to the plasma membrane, indicative of activation. In contrast, activation of Gaq/11-coupled receptors with histamine activated all the expressed PKC isoforms and angiotensin II only activated PKC-alpha and -epsilon.;Inhibition of PKC using general or isoform-selective inhibitors potentiated catecholamine release in response to activation of Gaq/11-coupled receptors, most likely as consequence of potentiated phospholipase C-mediated signalling. However, inhibition of PKC, particularly PKCalpha, markedly inhibited nicotinic receptor-mediated catecholamine release. PKCalpha is a classical isoform of PKC, activated by Ca2+ and diacylglycerol (DAG).;The current study suggests that Ca2+ influx across the plasma membrane in response to nicotinic receptor activation is largely through the nicotinic receptors themselves. Furthermore, this Ca2+ entry activates PLC, generating both Ins(1,4,5)P3 and DAG. This activation of PLC contributes significantly to the activation of PKC. The mechanism through which PKC facilitates the release of catecholamines requires the PKC-dependent phosphorylation of myristoylated alanine-rich C protein kinase substrate (MARCKS), the subsequent disassembly of the cortical F-actin cytoskeleton and probably therefore, increased access of a reserve exocytotic vesicle pool to release sites at the plasma membrane. Moreover, this study also suggests that PLC-dependent generation of DAG recruits Munc13-1, a known vesicle priming agent. Thus, activation of Munc13-1 may also contribute to nicotinic receptor-mediated catecholamine release in a manner dependent on the activation of PLC.