Activation of facilitation calcium channels in chromaffin cells by D1 dopamine receptors through a cAMP/protein kinase A-dependent mechanism

Facilitation calcium channels in unstimulated bovine chromaffin cells are normally quiescent but are activated by large pre-depolarizations or by repetitive depolarization in the physiological range. The activation of these 27-pS dihydropyridine-sensitive channels by repetitive stimulation, such as by increased splanchnic nerve activity, can lead to an almost twofold increase in Ca2+ current in these cells. This increase in Ca2+ current is of probable physiological importance in stimulating rapid catecholamine secretion in response to danger or stress. We have identified D1 dopaminergic receptors on bovine chromaffin cells by fluorescence microscopy. Here we show that stimulation of the D1 receptors activates the facilitation Ca2+ currents in the absence of pre-depolarizations or repetitive activity, and that activation by D1 agonists is mediated by cyclic AMP and protein kinase A. The recruitment of facilitation Ca2+ channels by dopamine may form the basis of a positive feedback loop mechanism for catecholamine secretion.     

Sphingosine-1-phosphate induces Ca2+ mobilization via TRPC6 channels in SH-SY5Y cells and hippocampal neurons

Sphingosine-1-phosphate (S1P) is a widely expressed biologically active sphingolipid that plays an important role in cell differentiation, migration, proliferation, metabolism and apoptosis. S1P activates various signaling pathways, some of which evoke Ca2+ signals in the cytosol. Few studies have focused on the mechanism by which S1P evokes Ca2+ signals in neurons. Here, we show that S1P evokes global Ca2+ signals in SH-SY5Y cells and hippocampal neurons. Removal of extracellular calcium largely abolished the S1P-induced increase in intracellular Ca2+, suggesting that the influx of extracellular Ca2+ is the major contributor to this process. Moreover, we found that S1P-induced Ca2+ mobilization is independent of G protein-coupled S1P receptors. The TRPC6 inhibitor SAR7334 suppressed S1P-induced calcium signals, indicating that the TRPC6 channel acts as the downstream effector of S1P. Using patch-clamp recording, we showed that S1P activates TRPC6 currents. Two Src tyrosine kinase inhibitors, Src-I1 and PP2, dramatically inhibited the activation of TRPC6 by S1P. Taken together, our data suggest that S1P activates TRPC6 channels in a Src-dependent way to induce Ca2+ mobilization in SH-SY5Y cells and hippocampal neurons.