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

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Abstract
Pinealocytes, parenchymal cells of the pineal gland, secrete glutamate through microvesicle‐mediated exocytosis upon depolarization by KC1 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 uM. 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 uM, 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.