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+)](i)) and potassium ([K(+)](i)), 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+)](i) that was totally abolished in calcium-free extracellular media. Isoproterenol also decreased [K(+)](i), but clonidine did not. The reduction in [K(+)](i) 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+)](i) but also synergistically involves the cAMP transduction system to which beta-adrenoceptors are known to be positively coupled.
Muyderman
H
H
Institute of Neurobiology and Institute of Clinical Neuroscience, Göteborg University, Box 420, SE-405 30, Göteborg, Sweden.
Sinclair
J
J
Jardemark
K
K
Hansson
E
E
Nilsson
M
M
D058647:Adrenergic alpha-2 Receptor Agonists; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D000818:Animals; D001030:Apamin / Q000494:pharmacology; D001253:Astrocytes / Q000166:cytology / Q000378:metabolism*; D001572:Benzofurans; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002478:Cells, Cultured; D018999:Charybdotoxin / Q000494:pharmacology; D003000:Clonidine / Q000494:pharmacology; D018920:Coculture Techniques; D004305:Dose-Response Relationship, Drug; D004988:Ethers, Cyclic; D005456:Fluorescent Dyes; D016257:Fura-2; D007424:Intracellular Fluid / Q000378:metabolism; D015640:Ion Channel Gating / Q000187:drug effects; D007545:Isoproterenol / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000378:metabolism; D026902:Potassium Channel Blockers; D015221:Potassium Channels / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D018341:Receptors, Adrenergic, alpha-2 / Q000378:metabolism; D011943:Receptors, Adrenergic, beta / Q000378:metabolism
D058647:Adrenergic alpha-2 Receptor Agonists; D000316:Adrenergic alpha-Agonists; D000318:Adrenergic beta-Agonists; D001572:Benzofurans; D004988:Ethers, Cyclic; D005456:Fluorescent Dyes; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D018341:Receptors, Adrenergic, alpha-2; D011943:Receptors, Adrenergic, beta; C073886:potassium-binding benzofuran isophthalate; D018999:Charybdotoxin; D001030:Apamin; D007545:Isoproterenol; D003000:Clonidine; D011188:Potassium; D002118:Calcium; D016257:Fura-2