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Arginine Vasopressin Triggers Intracellular Calcium Release, a Calcium-Activated Potassium Current and Exocytosis in Identified Rat Corticotropes¹

Department of Pharmacology, University of Alberta, Edmonton, Alberta, Canada T6G 2H7

Address all correspondence and requests for reprints to: Amy Tse, Department of Pharmacology, 9–70 Medical Science Building, University of Alberta, Edmonton, Alberta, Canada T6G 2H7. E-mail: amy.tse{at}ualberta.ca

Arginine vasopressin (AVP) stimulates the secretion of ACTH from pituitary corticotropes. We investigated the action of AVP in single corticotropes of male rats. Corticotropes were identified with the reverse hemolytic plaque assay using antibodies against ACTH. Using the whole-cell recording technique in conjunction with the fluorescent Ca²⁺ indicator, indo-1 to measure the concentration of cytosolic free Ca²⁺ ([Ca²⁺]_i), we show that AVP triggers a transient and plateau pattern of Ca²⁺ signal. The [Ca²⁺]_i elevation activates the apamin-sensitive Ca²⁺-activated K⁺ current, which, in turn, causes membrane hyperpolarization. The Ca²⁺ signal can be elicited in the absence of extracellular Ca²⁺ and is mimicked by intracellular inositol 1,4,5-trisphosphate (IP₃). Both GDP-ß-S and heparin inhibit the AVP response. Thus, AVP triggers intracellular Ca²⁺ release from the (IP₃)-sensitive store via a GTP binding protein-coupled phosphoinositide pathway. Using the high temporal resolution capacitance measurement to detect exocytosis in single corticotropes, we show that a burst of exocytosis is evoked during the AVP-triggered [Ca²⁺]_i elevation. Exocytosis can also be triggered when Ca²⁺ is released directly from the IP₃-sensitive store via flash photolysis of caged IP₃. We conclude that AVP-stimulated ACTH secretion in rat corticotrophs is closely coupled to intracellular Ca²⁺ release from the IP₃-sensitive store.

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