Norepinephrine induces Ca2+ release from intracellular stores in rat pinealocytes

Area of Science:

• Neuroendocrinology
• Cellular Physiology
• Calcium Signaling

Background:

• Norepinephrine (NE) is known to increase intracellular calcium ([Ca2+]i) in rat pinealocytes, primarily through calcium influx, regulating metabolic functions.
• NE also stimulates phosphatidylinositol metabolism, suggesting a potential role for intracellular calcium release in NE-mediated signaling.

Purpose of the Study:

• To investigate the contribution of intracellular calcium stores to the NE-induced increase in [Ca2+]i in rat pinealocytes.
• To elucidate the mechanisms underlying NE-induced calcium signaling in pinealocytes.

Main Methods:

• Fura-2 loaded rat pinealocytes were used to monitor intracellular calcium concentrations ([Ca2+]i).
• Experiments were conducted in Ca2+-free saline to distinguish between influx and release.
• TMB-8, a known blocker of intracellular calcium release, was used to assess the source of calcium.

Main Results:

• NE induced a transient increase in [Ca2+]i that preceded calcium influx, observed even in Ca2+-free conditions.
• This initial NE-induced calcium response was abolished by TMB-8, confirming release from intracellular stores.
• Prolonged NE exposure led to desensitization, with reduced responses to subsequent NE additions.
• Higher NE concentrations activated a secondary, sustained calcium influx mechanism.

Conclusions:

• NE increases [Ca2+]i in rat pinealocytes through a dual mechanism: an initial rapid release from intracellular stores followed by a sustained influx.
• The phasic intracellular calcium release is triggered by lower NE concentrations and is subject to desensitization.
• A tonic influx mechanism, activated by higher NE concentrations, contributes to sustained calcium elevation.