Rilmenidine elevates cytosolic free calcium concentration in suspended cerebral astrocytes

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.