Rilmenidine elevates cytosolic free calcium concentration in suspended cerebral astrocytes

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 intra- cellular 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.