Regulation of cytoplasmic calcium levels by two nitric oxide receptors

We examined the effects of dissolved nitric oxide (NO) gas oncytoplasmic calcium levels ([Ca²⁺]_i) in C6glioma cells under anoxic conditions. The maximum elevation (27 ± 3 nM) of [Ca²⁺]_i was reached at 10 µM NO. Asecond application of NO was ineffective if the first was >0.5 µM.The NO donor diethylamine/NO mimicked the effects of NO. Acute exposureof the cells to low calcium levels was without effect on the NO-evokedresponse. Thapsigargin (TG) increased [Ca²⁺]_iand was less effective if cells were pretreated with NO. Hemoglobin inhibited the effects of NO at a molar ratio of 10:1. 8-Bromo-cGMP waswithout effect on the NO-evoked response. If cells were pretreated withTG or exposed chronically to nominal amounts of calcium, NO decreased[Ca²⁺]_i. The results suggest that C6 gliomacells have two receptors for NO. One receptor (NO_A)elevates [Ca²⁺]_i and resides on theendoplasmic reticulum (ER). The other receptor (NO_B)decreases [Ca²⁺]_i and resides on theplasmalemma or the ER. The latter receptor dominates when the level ofcalcium within intracellular stores is diminished.