Anterior pituitary corticotropes secrete adrenocorticotropic hormone (ACTH) and beta-endorphin in response to emotional or physical stress. Two important stimulants of ACTH secretion are corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP). ACTH release is also regulated by feedback inhibition via corticosterone, a hormone that is released from the adrenal cortex. ACTH secretion is a Ca2+ dependent process that is evoked in part by agonist evoked Ca2+ influx. Sodium dependent action potentials and Ca2+ uptake through voltage dependent Ca2+ channels play a role in ACTH secretion since CRH and AVP stimulated secretion are attenuated by compounds that inhibit Na+ and Ca2+ channel activity. In this proposal the long term hormonal regulation of membrane electrical excitability and its influence on rapid hormonal regulation of Ca2+ influx through voltage dependent Ca2+ channels will be examined. The studies will be performed on a highly purified population of corticotropes isolated from rat pituitary.
The first aim will be to determine if corticotropes cultured in the presence of low levels of CRH, AVP or glucocorticoid undergo long lasting changes in calcium or sodium channel expression. In situ hybridization, northern blot analyses, and whole cell voltage clamp will be performed to assess for changes in Ca2+ or Na+ channel mRNA content and membrane channel current density.
The second aim i s to study the effects of changes in Na+ and Ca2+ channel expression on membrane excitability and basal ACTH secretion.
The third aim i s to determine the rapidly induced effects of hormones on corticotropes. CRH, AVP, and glucocorticoids, in combination with pharmacological inhibitors of Na+, Ca2+ and K+ channels, will be tested on corticotropes for their immediate effects on ionic currents, action potential generation, cytosolic calcium levels, and ability to evoke ACTH secretion.
The fourth aim i s to determine how changes in Na+ or Ca2+ channel expression influence hormonal regulation of cytosolic calcium levels and ACTH secretion. The study will provide information on the receptor transduction mechanisms that participate in hormonal regulation of secretion and long term changes in excitability that may be involved in the adaptation to stress.
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