Nor epinephrine induces Ca²⁺ release from intracellular stores in rat pinealocytes

Sáez JC, Moreno AP, Spray DC. Nor epinephrine induces Ca ²⁺ release from intracellular stores in rat pinealocytes. J. Pineal Res. 1994: 16: 57–64. In rat pinealocytes, an increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) due to Ca²⁺ influx in response to nor epinephrine (NE) is a well recognized event involved in regulating several metabolic functions. Since NE also stimulates the metabolism of phosphatidyl inositols in rat pineal gland, it is conceivable that Ca²⁺ release from intracellular stores also contributes to the NE‐induced increase in [Ca²⁺]_i. In this communication, we report that in rat pinealocytes loaded with fura‐2, a Ca²⁺ indicator, NE induced a transient increase in [Ca²⁺]_i that preceded the known Ca²⁺ influx. This novel [Ca²⁺]_i response to NE was detected in pinealocytes bathed with Ca²⁺‐free saline and prevented by TMB‐8, a blocker of Ca²⁺ release from intracellular stores, supporting the notion that the transient NE‐induced Ca²⁺ response was due to Ca²⁺ release from intracellular stores. In addition, after an extended exposure to NE a new addition of this neurotransmitter did not elicit the phasic Ca²⁺ response, and application of increasing amounts of NE induced a Ca²⁺ response that was progressively smaller, suggesting desensitization. Thus, NE is proposed to increase [Ca²⁺]_i in rat pinealocytes by two mechanisms: (1) phasic release from intracellular stores and (2) tonic influx through a mechanism activated by larger applications of NE than required to evoke the phasic release.