DUAL REGULATION OF L-TYPE CA2+ CHANNELS BY SEROTONIN-2 RECEPTOR STIMULATION IN VASCULAR SMOOTH-MUSCLE CELLS

The purpose of the present study was to investigate regulation of voltage-dependent Ca2+ channels by serotonin in rat aortic smooth muscle cells in primary culture. L- and T-type Ca2+ currents (I-ca) were recorded using the whole cell voltage-clamp method. Without pretreatment, in 25 of 30 cells examined, 10 mu M serotonin decreased L-type I-ca to various extents (-14 to -72%). However, in the remaining five cells, serotonin increased L-type I-Ca 21 +/- 4%. Thus, in 30 cells, serotonin decreased L-type Ic, an average of 22 +/- 5%. In the presence of intracellular heparin (100 mu g/ml), a blocker of inositol 1,4,5-trisphosphate binding to its receptor, serotonin increased L-type I-Ca in all cells 29 +/- 3% (n = 6). When stored Ca2+ was depleted by pretreatment either with 20 mu M ryanodine and 20 mM caffeine or with 100 nM A-23187, serotonin also increased L-type I-Ca in all cells 30 +/- 5 (n = 4) or 37 +/- 5% (n = 12), respectively. In the presence of heparin, the serotonin-induced increase of L-type I-Ca was prevented by 100 nM staurosporine (2 +/- 3%; n = 6, P < 0.01). The serotonin-induced decrease of L-type I-Ca was significantly augmented by 100 nM staurosporine (-43 +/- 10%; n = 5). Phorbol 12,13-dibutylate (PDBu; 1 mu M) increased L-type I-Ca 29 +/- 3% (n = 6), and serotonin did not further increase L-type I-Ca after its potentiation by PDBu. Ketanserin (3 mu M) inhibited both serotonin-induced increase and decrease of L-type I-Ca. T-type I-Ca was affected by neither serotonin nor PDBu. We suggest dual regulation of L-type but not T-type Ca2+ channels by serotonin 2 receptor stimulation: 1) inhibition of L-type Ca2+ channels mediated by elevated cytosolic Ca2+ via release of intracellular Ca2+ and 2) potentiation of L-type Ca2+ channels mediated by protein kinase C activation.