Histamine‐induced calcium entry in rat cerebellar astrocytes: evidence for capacitative and non‐capacitative mechanisms

1 We have investigated the effects of histamine on the intracellular calcium concentration ([Ca 2+ ] i ) of cultured rat cerebellar astrocytes using fura‐2‐based Ca 2+ imaging microscopy. 2 Most of the cells responded to the application of histamine with an increase in [Ca 2+ ] i which was antagonized by the H 1 receptor blocker mepyramine. When histamine was applied for several minutes, the majority of the cells displayed a biphasic Ca 2+ response consisting of an initial transient peak and a sustained component. In contrast to the initial transient [Ca 2+ ] i response, the sustained, receptor‐activated increase in [Ca 2+ ] i was rapidly abolished by chelation of extracellular Ca 2+ or addition of Ni 2+ , Mn 2+ , Co 2+ and Zn 2+ , but was unaffected by nifedipine, an antagonist of L‐type voltage‐activated Ca 2+ channels. These data indicate that the sustained increase in [Ca 2+ ] i was dependent on Ca 2+ influx. 3 When intracellular Ca 2+ stores were emptied by prolonged application of histamine in Ca 2+ ‐free conditions, Ca 2+ re‐addition after removal of the agonist did not lead to an ‘overshoot’ of [Ca 2+ ] i indicative of store‐operated Ca 2+ influx. However, Ca 2+ stores were refilled despite the absence of any substantial change in the fura‐2 signal. 4 Depletion of intracellular Ca 2+ stores using cyclopiazonic acid in Ca 2+ ‐free saline and subsequent re‐addition of Ca 2+ to the saline resulted in an increase in [Ca 2+ ] i that was significantly enhanced in the presence of histamine. 5 The results suggest that besides capacitative mechanisms, a non‐capacitative, voltage‐independent pathway is involved in histamine‐induced Ca 2+ entry into cultured rat cerebellar astrocytes.