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 Ca2+‐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. © 1992 Wiley‐Liss, Inc.