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

We have investigated the effects of histamine on the intracellular calcium concentration ([Ca2+]i) of cultured rat cerebellar astrocytes using fura-2-based Ca2+ imaging microscopy. Most of the cells responded to the application of histamine with an increase in [Ca2+]i which was antagonized by the H1 receptor blocker mepyramine. When histamine was applied for several minutes, the majority of the cells displayed a biphasic Ca2+ response consisting of an initial transient peak and a sustained component. In contrast to the initial transient [Ca2+]i response, the sustained, receptor-activated increase in [Ca2+]i was rapidly abolished by chelation of extracellular Ca2+ or addition of Ni2+, Mn2+, Co2+ and Zn2+, but was unaffected by nifedipine, an antagonist of L-type voltage-activated Ca2+ channels. These data indicate that the sustained increase in [Ca2+]i was dependent on Ca2+ influx. When intracellular Ca2+ stores were emptied by prolonged application of histamine in Ca2+-free conditions, Ca2+ re-addition after removal of the agonist did not lead to an 'overshoot' of [Ca2+]i indicative of store-operated Ca2+ influx. However, Ca2+ stores were refilled despite the absence of any substantial change in the fura-2 signal. Depletion of intracellular Ca2+ stores using cyclopiazonic acid in Ca2+-free saline and subsequent re-addition of Ca2+ to the saline resulted in an increase in [Ca2+]i that was significantly enhanced in the presence of histamine. The results suggest that besides capacitative mechanisms, a non-capacitative, voltage-independent pathway is involved in histamine-induced Ca2+ entry into cultured rat cerebellar astrocytes.