The L-type Ca²⁺ channel is involved in microvesicle-mediated glutamate exocytosis from rat pinealocytes

Pinealocytes, parenchymal cells of the pineal gland, secrete glutamate through microvesicle-mediated exocytosis upon depolarization by KCl in the presence of Ca²⁺, which is involved in a novel paracrine-like intercellular signal transduction mechanism in neuroendocrine organs. In the present study, we investigated whether or not the L-type Ca²⁺ channel is involved in the microvesicle-mediated glutamate secretion from cultured rat pinealocytes. Nifedipine, a specific antagonist of the L-type Ca²⁺ channel, inhibited the Ca²⁺-dependent glutamate exocytosis by 48% at 20 μM. Other L-type Ca²⁺ channel antagonists, such as nitrendipine, showed similar effects. 1,4-Dihydro-2,6-dimethyl-5-nitro-4[2-(trifluoromethyl)-phenyl]-3- pyridinecarboxylic acid methyl ester (BAY K8644), an agonist of the L-type Ca²⁺ channel, at 1 μM, on the other hand, stimulated the glutamate exocytosis about 1.6-fold. Consistently, these Ca²⁺ channel antagonists inhibited about 50% of the Ca²⁺ uptake, whereas BAY K8644 increased the uptake 5.3-fold. An antibody against the carboxyl-terminal region of the rabbit L-type Ca²⁺ channel recognized polypeptides of pinealocytes with apparent molecular masses of 250 and 270 kDa, respectively, and immunostained the plasma membrane region of the pinealocytes. These results strongly suggested that the entry of Ca²⁺ through L-type Ca²⁺ channel(s), at least in part, triggers microvesicle-mediated glutamate exocytosis in pinealocytes.