Calcium mobilization is required for peroxynitrite-mediated enhancement of spontaneous transient outward currents in arteriolar smooth muscle cells

Transiently local release of Ca2+ from the sarcoplasmic reticulum (SR) activates nearby Ca2+-activated K+ channels to produce spontaneous transient outward currents (STOCs) in smooth muscle cells. The purpose of the present study was to investigate the possible effect of peroxynitrite (ONOO−) on STOCs in mesenteric arteriolar smooth muscle cells (ASMCs) and decide whether Ca2+ mobilization was involved in STOCs alteration by ONOO−. STOCs were recorded and characterized using the perforated whole-cell patch-clamp configuration. The results demonstrated that STOCs activity was greatly suppressed by removal of extracellular Ca2+; by addition of nifedipine, a specific inhibitor of L-type voltage-gated Ca2+ channels (VGCCs); or by addition of ryanodine, a SR ryanodine receptors (RyRs) blocker. In contrast, both caffeine, a RyR activator, and 2-aminoethoxydiphenylborate (2-APB), a membrane-permeable inositol 1,4,5-trisphosphate receptors, (IP3R) antagonist, increased STOCs activity. 3-morpholinosydnonimine (SIN-1), an ONOO− donor, at concentrations of 20–200 μM, induced a dose-dependent enhancement of STOCs in ASMCs and led to conspicuous increases in STOCs frequency and amplitude, which were prevented by prior exposure to low external Ca2+ (200 nM), ryanodine (10 μM), or nifedipine (10 μM). In contrast, caffeine (0.5 mM) did not further stimulate STOCs in ASMCs preincubated with SIN-1, and pretreatment with 2-APB (50 μM) had little effect on ONOO−-induced STOCs activation. These findings suggest that complex Ca2+-mobilizing pathways, including external Ca2+ influx through VGCCs activation and subsequent internal Ca2+ release through RyRs but not IP3Rs, are involved in ONOO−-mediated STOCs enhancement in ASMCs.