Activation of beta-adrenoceptors opens calcium-activated potassium channels in astroglial cells

In the present study, effects of the alpha (2)- and beta -adrenoceptor agon ists clonidine and isoproterenol on astrocytes in astroglial/neuronal cocul tures from rat cerebral cortex were evaluated. The calcium- and potassium-s ensitive dyes fura-2 and potassium-binding benzofuran isophtalate (PBFI) we re used to study alterations in intracellular concentrations of calcium ([C a2+](i)) and potassium ([K+](i)), respectively, while the perforated patch clamp technique was used to analyze transmembrane currents. Exposure to iso proterenol or clonidine elicited an immediate increase in [Ca2+](i) that wa s totally abolished in calcium-free extracellular media. Isoproterenol also decreased [K+](i) but clonidine did not. The reduction in [K+](i) was inhi bited in Ca2+-free media. As evaluated with the perforated patch technique, isoproterenol (10(-6)-10(-4) M) induced a slowly developing and long lasti ng outward current that also was totally abolished in calcium-free buffer. This current was blocked by external tetraethylammonium (TEA, 10 mM) and ch arybdotoxin (ChTX, 10 nM), but was not affected by apamin (50 nM). The curr ent-to-voltage (I-V) relationships for the isoproterenol-induced currents s howed a markedly negative reversal potential, -96 mV +/- 7, (mean +/- S.D., n = 5). These results suggest that the stimulation of astroglial beta -adr enoceptors by isoproterenol opens calcium-activated potassium channels (K-( Ca)). Preincubation with forskolin significantly increased the isoprotereno l-induced currents compared with controls, indicating that the opening of a stroglial K-(Ca) channels after beta -adrenergic stimulation not only depen ds on [Ca2+](i) but also synergistically involves the cAMP transduction sys tem to which beta -adrenoceptors are known to be positively coupled. (C) 20 01 Elsevier Science Ltd. All rights reserved.