Reactive oxygen species regulate cardiovascular and renal function in health and 
disease. Superoxide participates in acute calcium signaling in afferent 
arterioles and renal vasoconstriction produced by angiotensin II, endothelin, 
thromboxane, and pressure-induced myogenic tone. Known mechanisms by which 
superoxide acts include quenching of nitric oxide and increased ADP ribosyl 
cyclase/ryanodine-mediated calcium mobilization. The effect(s) of superoxide on 
other calcium signaling pathways in the renal microcirculation is poorly 
understood. The present experiments examined the acute effect of superoxide 
generated by paraquat on calcium entry pathways in isolated rat afferent 
arterioles. The peak increase in cytosolic calcium concentration caused by KCl 
(40 mmol/L) was 99±14 nmol/L. The response to this membrane depolarization was 
mediated exclusively by L-type channels because it was abolished by nifedipine 
but was unaffected by the T-type channel blocker mibefradil. Paraquat increased 
superoxide production (dihydroethidium fluorescence), tripled the peak response 
to KCl to 314±68 nmol/L (P<0.001) and doubled the plateau response. These 
effects were abolished by tempol and nitroblue tetrazolium, but not by catalase, 
confirming actions of superoxide and not of hydrogen peroxide. Unaffected by 
paraquat and superoxide was calcium entry through store-operated calcium 
channels activated by thapsigargin-induced calcium depletion of sarcoplasmic 
reticular stores. Also unresponsive to paraquat was ryanodine receptor-mediated 
calcium-induced calcium release from the sarcoplasmic reticulum. Our results 
provide new evidence that superoxide enhances calcium entry through L-type 
channels activated by membrane depolarization in rat cortical afferent 
arterioles, without affecting calcium entry through store-operated entry or 
ryanodine receptor-mediated calcium mobilization.