Mechanisms of serotonin-induced Ca2+ responses in mesangial cells

Smooth muscle cell-like mesangial cells play an important role in the regulation of glomerular blood flow and are involved in renal inflammatory reactions, rhc reb,: interacting with circulating cells, The platelet products serotonin (5-HT) and ATP induce similar, e.g. mitogenic. effects in mesangial cells. but differentially activate and induce inflammation-related genes, To get an insight into intracellular signaling steps. a very early step in the signaling cascade, the biphasic Ca2+ signal elicited by 5-HT and ATP in rat mesangial cells was investigated. Both phases of the Ca2+ signal, release from internal stores as well as influx of extracellular Ca2+, were dependent on phospholipase C activation as shown by the specific inhibitor U73122 (complete inhibition at 10 mu M U73122). There was no evidence for voltage-gated L-type channels in these cells, suggesting that Ca2+ in-flux was mediated by Ca2+ release-activated channels. The L-type channel blocker verapamil. however, dose-dependently (0.1-10 mu M) and specifically inhibited 5-HT-elicited Ca2+ signals by interference with binding of 5-HT to 5-HT2A receptors. 5-HT-mediated Ca2+ release was reduced by 80% when protein kinase C was activated by the phorbolester TPA (0.1 mu M). Interaction of 5-HT2A receptors with phospholipase C was also inhibited by genistein (30%) at 5 mu M: 100% at 50 mu M. an inhibitor of tyrosine kinases. Binding of 5-HT to its receptor reduced subsequent ATP-mediated Ca2+ signaling. The cross talk between the receptors was sensitive to genistein. ATP-mediated Ca2+ signaling was attributed to different types of P-2y receptors and/or multiple G-proteins coupled, because the signal was partially inhibited by pertussis toxin (50%). In accordance, modulation of the ATP-mediated signaling by phosphorylation was less tightly controlled than 5-HT-mediated Ca2+ release, These data indicate that although the Ca2+ responses elicited by the two stimuli are comparable, interactions between receptors. G-proteins and target enzymes are regulated differentially.