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

In the present study, effects of the alpha(2)- and beta-adrenoceptor agonists clonidine and isoproterenol on astrocytes in astroglial/neuronal cocultures from rat cerebral cortex were evaluated. The calcium- and potassium-sensitive dyes fura-2 and potassium-binding benzofuran isophtalate (PBFI) were used to study alterations in intracellular concentrations of calcium (Ca(2+)) and potassium (K(+)), respectively, while the perforated patch clamp technique was used to analyze transmembrane currents. Exposure to isoproterenol or clonidine elicited an immediate increase in Ca(2+) that was totally abolished in calcium-free extracellular media. Isoproterenol also decreased K(+), but clonidine did not. The reduction in K(+) was inhibited in Ca(2+)-free media. As evaluated with the perforated patch technique, isoproterenol (10(-6)-10(-4) M) induced a slowly developing and long lasting outward current that also was totally abolished in calcium-free buffer. This current was blocked by external tetraethylammonium (TEA, 10 mM) and charybdotoxin (ChTX, 10 nM), but was not affected by apamin (50 nM). The current-to-voltage (I-V) relationships for the isoproterenol-induced currents showed a markedly negative reversal potential, -96 mV+/-7, (mean+/-S.D., n=5). These results suggest that the stimulation of astroglial beta-adrenoceptors by isoproterenol opens calcium-activated potassium channels (K((Ca))). Preincubation with forskolin significantly increased the isoproterenol-induced currents compared with controls, indicating that the opening of astroglial K((Ca)) channels after beta-adrenergic stimulation not only depends on Ca(2+) but also synergistically involves the cAMP transduction system to which beta-adrenoceptors are known to be positively coupled.