Rilmenidine Elevates Cytosolic Free Calcium Concentration in Suspended Cerebral Astrocytes

Authors


Address correspondence and reprint requests to Dr. D. F. Cechetto at Department of Anatomy and Cell Biology, University of Western Ontario, London, Ontario, Canada, N6A 5C1.

Abstract

Abstract: Rilmenidine, a ligand for imidazoline and α2-adrenergic receptors, is neuroprotective following focal cerebral ischemia. We investigated the effects of rilmenidine on cytosolic free Ca2+ concentration ([Ca2+]i) in rat astrocytes. Rilmenidine caused concentration-dependent elevation of [Ca2+]i, consisting of a transient increase (1–100 µM rilmenidine) or a transient increase followed by sustained elevation above basal levels (1–10 mM rilmenidine). A similar elevation in [Ca2+]i was induced by the imidazoline ligand cirazoline. The transient response to rilmenidine was observed in Ca2+-free medium, indicating that rilmenidine evokes release of Ca2+ from intracellular stores. However, the sustained elevation of Ca2+ was completely dependent on extracellular Ca2+, consistent with rilmenidine activating Ca2+ influx.Pretreatment with thapsigargin, an inhibitor of the endoplasmic reticulum Ca2+-ATPase, abolished the response to rilmenidine, confirming the involvement of intracellular stores and suggesting that rilmenidine and thapsigargin activate a common Ca2+ influx pathway. The α2-adrenergic antagonist rauwolscine attenuated the increase in [Ca2+]i induced by clonidine (a selective α2 agonist), but not the response to rilmenidine. These results indicate that rilmenidine stimulates both Ca2+ release from intracellular stores and Ca2+ influx by a mechanism independent of α2-adrenergic receptors. In vivo, rilmenidine may enhance uptake of Ca2+ from the extracellular fluid by astrocytes, a process that may contribute to the neuroprotective effects of this agent.

Abbreviations used: [Ca2+]i, cytosolic free Ca2+ concentration; I1, I2, and I3, subtype 1, 2, and 3 imidazoline receptor, respectively.

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