Phospholipase C-mediated signalling is not required for histamine-induced catecholamine secretion from bovine chromaffin cells

Potentiation of exocytosis by phospholipase C-coupled G-protein-coupled receptors requires the priming protein Munc13-1

The vesicle priming protein Munc13-1 is regulated by diacylglycerol (DAG) and is therefore hypothesized to play a role in the control of neurotransmitter release by phospholipase C (PLC)-coupled receptors. We combined voltage-clamp recordings of voltage-gated Ca2+ channels (VGCCs) and high-resolution capacitance measurements to investigate the mechanism of receptor-mediated modulation of exocytosis in bovine chromaffin cells. Activation of endogenous H1 G(q)-protein-coupled receptors (G(q)PCRs) by histamine potentiated stimulus-coupled secretion despite concurrently inhibiting Ca2+ influx through VGCCs. Histamine increased the size of the readily releasable pool of vesicles and in particular a subpool of fusion-competent vesicles localized in close proximity to VGCCs. Pharmacological characterization showed that potentiation of exocytosis depended on the activation of PLC but not protein kinase C. Overexpression of wild-type Munc13-1 by adenoviral infection had no effect on histamine-induced potentiation per se, whereas DAG-insensitive Munc13-1(H567K) completely abolished it. This is the first endogenous mammalian G(q)PCR signaling pathway identified that engages Munc13-1 to increase stimulus-coupled secretion by recruiting vesicles to the immediately releasable pool. G(q)PCRs are therefore able to control exocytosis at the level of SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex formation to produce rapid, short-term potentiation of the secretory output of neurons and endocrine cells.

Intracellular calcium involvement in pituitary adenylate cyclase-activating polypeptide stimulation of growth hormone and gonadotrophin secretion in goldfish pituitary cells

The involvement of intracellular Ca(2+) stores and their regulatory mechanisms in mediating pituitary adenylate cyclase-activating polypeptide (PACAP) stimulation of growth hormone (GH) and maturational gonadotrophin (GTH-II) secretion from goldfish pituitary cells was investigated using a cell column perifusion system. Pretreatment with caffeine abolished the GH and GTH-II responses to PACAP. Dantrolene attenuated PACAP-elicited GTH-II release but did not affect the GH response, whereas ryanodine and 8-bromo-cADP ribose did not alter PACAP-induced GH and GTH-II release. Two endoplasmic/sarcoplasmic reticulum Ca(2+) ATPase (SERCA) inhibitors, thapsigargin and cyclopiazonic acid, augmented PACAP-induced GTH-II release; similarly, thapsigargin elevated GH responses to PACAP. Treatment with carbonyl cyanide m-chlorophenylhydrazone, a mitochondrial uncoupler, reduced PACAP-stimulated GH release; however, inhibition of the mitochondrial Ca(2+) uniport by Ru360 did not affect GH and GTH-II responses. The phosphatidyl inositol (PI)-specific phospholipase C (PLC) inhibitor ET-18-OCH(3) inhibited, whereas the phosphatidyl-choline (PC)-specific PLC inhibitor D609 enhanced, PACAP-stimulated GH and GTH-II responses. On the other hand, the IP(3) receptor blocker xestospongin D had no effect on PACAP-induced GTH-II response and potentiated the GH response. These results suggest that, despite some differences between GH and GTH-II cells, PACAP actions in both cell types generally rely on a caffeine-sensitive, but a largely ryanodine receptor-independent, mechanism. PC-PLC and some SERCA negatively modulate PACAP actions but mitochondrial Ca(2+) stores per se are not important. A novel PI-PLC mechanism, which does not involve the traditional IP(3)/Ca(2+) pathway, is also suggested.

Opioid receptor stimulation suppresses the adrenal medulla hypoxic response in sheep by actions on Ca(2+) and K(+) channels

Before the preganglionic regulation of the adrenal medulla is established, hypoxia acts directly on the chromaffin cells to evoke the secretion of catecholamines. This direct action of hypoxia is suppressed by the gradual development of the preganglionic innervation and we have proposed that opioid peptides released from the adrenal splanchnic nerves may be responsible for this suppression. The effects of the specific opioid agonists DPDPE (delta-agonist), U-62066 (kappa-agonist) and DALDA (mu-agonist) on the hypoxia-evoked response were investigated in both a whole-gland preparation and in isolated adrenal chromaffin cells using amperometry, whole-cell patch clamping and measurement of cytosolic [Ca(2+)]. The combined application of mu- and kappa-type agonists abolished the hypoxia-evoked catecholamine secretion from whole perfused adrenal gland. In isolated chromaffin cells, mu- and kappa-opioid agonists reduced the rise in [Ca(2+)](i) that results from exposure to hypoxia. Both agonists decreased the voltage-dependent Ca(2+) current in these cells. The mu-agonist increased the conductance through SK-type K(+) channels and this action offset the decrease in K(+) conductance produced by exposure to hypoxia. The kappa-type agonist decreased the conductance through an action on BK-type K(+) channels, a class of channels that are not involved in initiating the direct response to hypoxia. These data suggest that opioids, through their action on SK channels and voltage-dependent Ca(2+) channels, may be responsible for the nerve-induced suppression of the hypoxic response of adrenal chromaffin cells and that these effects of endogenous opioids are mediated via mu- and kappa-type receptors.

Pasteurella multocida toxin as a tool for studying Gq signal transduction

Pasteurella multocida toxin (PMT) stimulates and subsequently uncouples phospholipase C (PLC) signal transduction through its selective action on the Galphaq subunit. This review summarizes what is currently known about the molecular action of PMT on Gq and the resulting cellular effects. Examples are presented illustrating the use of PMT as a powerful tool for dissecting the molecular mechanisms involving pertussis toxin (PT)-insensitive heterotrimeric G proteins.

Mechanisms in histamine-mediated secretion from adrenal chromaffin cells

The great majority of the sustained secretory response of adrenal chromaffin cells to histamine is due to extracellular Ca(2+) influx through voltage-operated Ca(2+) channels (VOCCs). This is likely to be true also for other G protein-coupled receptor (GPCR) agonists that evoke catecholamine secretion from these cells. However, the mechanism by which these GPCRs activate VOCCs is not yet clear. A substantial amount of data have established that histamine acts on H(1) receptors to activate phospholipase C via a Pertussis toxin-resistant G protein, causing the production of inositol 1,4,5-trisphosphate and the mobilisation of store Ca(2+); however, the molecular events that lead to the activation of the VOCCs remain undefined. This review will summarise the known actions of histamine on cellular signalling pathways in adrenal chromaffin cells and relate them to the activation of extracellular Ca(2+) influx through voltage-operated channels, which evokes catecholamine secretion. These actions provide insight into how other GPCRs might activate Ca(2+) influx in many excitable and non-excitable cells.