Potassium efflux triggered by P2Y purinoceptor activation in cultured pituicytes

We have previously investigated the effects of extracellular ATP on the con centration of free cytosolic calcium ([Ca2+](i)) from rat cultured neurohyp ophysial astrocytes (pituicytes). We demonstrated that ATP acts via a P-2Y receptor to increase [Ca2+](i). In the present study, we examine the effect of ATP on K+ efflux using Rb-86(+) as an isotopic tracer, in order to char acterize the possible presence of a Ca2+-activated K+ conductance and to es tablish the implications of pituicytes in the regulation of stimulus-secret ion coupling. ATP evoked an increase in Rb-86(+) emux from cultured pituicy tes. This effect was Ca2+ dependent, as indicated by the unresponsiveness o f cells loaded with BAPTA/AM (20 muM). Furthermore, the effect of ATP was m imicked by 2-methylthio-adenosine-5'-triphosphate (2MeSATP), a Pt purinocep tor agonist, and abolished by Reactive Blue 2 (RB-2), a selective P-2Y anta gonist, implying a role for the P-2Y purinoreceptor. A pharmacological stud y revealed that Ba2+ and tetraethylammonium (TEA), two inhibitors of K+ cha nnels, both strongly reduced the ATP-stimulated Rb-86(+) efflux. In additio n, the effect of ATP was modulated by different peptidic toxins. Apamin (10 0 nM), an inhibitor of the small-conductance Ca2+-activated K+ channels, pa rtly blocked ATP-induced Rb-86(+) efflux. Leiurus quinquestriatus hebraeus (LQH) scorpion venom (20 mug/ml) and Buthus tamulus (BT) scorpion venom (20 -200 mug/ml) inhibited ATP-induced Rb-86(+) efflux. The specificity of the effects of the crude venoms was checked using charybdotoxin (100 nM) and ib eriotoxin (1 muM), which are the active toxins extracted from the LQH and B T venoms, respectively. These data indicate the involvement of several type s of Ca2+-activated K+ channels in the ATP-dependent K+ efflux, and lead to the proposal that, in the neurohypophysis, extracellular ATP released by n erve terminals may act directly on the pituicytes and induce a K+ efflux vi a a P-2Y purinoreceptor.