Activation of endothelin receptors by sarafotoxin regulates Ca2+ homeostasis in cerebellar astrocytes.
We carried out experiments designed to investigate the effects of sarafotoxin-6B (SFTx) on [Ca2+]i in cerebellar astrocytes using the Ca2+ indicator fura-2. Both endothelin-1 and sarafotoxin-6B increased [Ca2+]i in individual cerebellar astrocytes in cell culture. The shape of the response was variable but usually consisted of an initial peak of [Ca2+]i followed by an extended plateau increase in [Ca2+]i. In Ca(2+)-free medium only the initial peak was observed. If Ca2+ was subsequently readmitted to the external medium a plateau was now formed. When external Ca2+ was removed during a plateau, [Ca2+]i rapidly declined; replacing the external Ca2+ reversed this decline. The plateau was also reversibly reduced by addition of Ni2+ (5 mM) to the external medium. Addition of 50 mM K+ produced a small increase in [Ca2+]i in most cells. This response was blocked by nimodipine. However, nimodipine only slightly blocked the plateau increase in [Ca2+]i that was formed following activation of endothelin receptors. Furthermore, perfusion of cells with 50 mM K+ during the plateau portion of a response to SFTx reduced [Ca2+]i. In some cells addition of a phorbol ester produced a sustained increase in [Ca2+]i that was blocked by nimodipine. In conclusion, activation of endothelin receptors by SFTx in cerebellar astrocytes produces both Ca2+ mobilization and Ca2+ influx. The pathway for Ca2+ influx is predominantly a non-voltage-dependent one, although some entry through a dihydropyridine-sensitive pathway also appears to occur. Furthermore, activation of protein kinase C in cerebellar astrocytes activates voltage-sensitive Ca2+ channels.
Holzwarth
J A
JA
Department of Pharmacological and Physiological Sciences, University of Chicago, Illinois 60637.
Glaum
S R
SR
Miller
R J
RJ
D000818:Animals; D000831:Animals, Newborn; D001253:Astrocytes / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D002531:Cerebellum / Q000378:metabolism*; D016232:Endothelins / Q000378:metabolism / Q000494:pharmacology*; D016257:Fura-2; D006706:Homeostasis; D007700:Kinetics; D008856:Microscopy, Fluorescence; D009498:Neurotoxins / Q000494:pharmacology; D009532:Nickel / Q000494:pharmacology; D015240:Phorbol 12,13-Dibutyrate / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D011956:Receptors, Cell Surface / Q000187:drug effects / Q000502:physiology*; D017466:Receptors, Endothelin; D014757:Viper Venoms / Q000494:pharmacology*
D016232:Endothelins; D009498:Neurotoxins; D011956:Receptors, Cell Surface; D017466:Receptors, Endothelin; D014757:Viper Venoms; C057354:sarafotoxins s6; D015240:Phorbol 12,13-Dibutyrate; D009532:Nickel; D011188:Potassium; D002118:Calcium; D016257:Fura-2