Properties of Ion Channels that Control Secretion

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DESCRIPTION (provided by applicant): The adrenal cortex of mammals is divided into an outer glomerulosa and inner fasciculata that secrete different corticosteroid hormones, including the mineralocorticoid aldosterone and the glucocorticoid cortisol. These hormones function critically in regulating electrolyte, water, and energy balance, and are therefore vital for maintaining blood pressure and plasma glucose within normal limits. Precise control of blood glucose by cortisol is essential because this sugar is the brain's primary energy source. Hypoglycemia rapidly leads to brain damage and death. Aberrant secretion of corticosteroids causes endocrine pathology, including Cushing's and Addison's diseases. At the cellular level, cortisol secretion by adrenal zona fasciculata (AZF) cells is controlled primarily by the pituitary peptide ACTH, while zona glomerulosa (AZG) cells secrete aldosterone in response to the peripheral peptide Angiotensin II (All). Although many of the physiological stimuli and intracellular messengers that regulate secretion of corticosteroids have been identified, the signaling pathways that link these stimuli to secretion are incompletely understood. In particular, the role of electrical activity and specific ion channels in the secretion of corticosteroids has not been clarified. Bovine AZF and AZG cells express several ion channels that determine their electrical properties and the ionic events involved in secretion. The K+ channel bTREK-1 sets the resting potential, is activated by ATP and inhibited by ACTH and All. Thus, TREK-1 channels act pivotally in integrating hormonal and metabolic signals and coupling these to depolarization-dependent calcium (Ca2+) entry and secretion. A specific model has been developed for ACTH- and All-stimulated cortisol and aldosterone secretion that depends on the generation of Ca2+-dependent action potentials. Specifically, the inhibition of bTREK-1 K+ channels by ACTH and All leads to action potentials driven by opposing voltage-gated Ca2+ and K+ currents. Experiments described in this proposal test this hypothesis and identify signaling pathways that control function and expression of adrenocortical ion channels. Specific Aims will be: 1) To demonstrate that All inhibits bTREK-1 K+ channels in bovine AZF and AZG cells by separate Ca2+ - and ATP hydrolysis-dependent signaling pathways, and to identify their molecular mechanisms. To demonstrate that bTREK-1 channels set the resting potential of bovine AZG cells and that inhibition of these channels by All is coupled to depolarization-dependent aldosterone secretion; 2) To demonstrate that ACTH exerts short and long term control over the electrical properties of bovine AZF cells by regulating both the function and expression of AZF cell ion channels and to characterize the molecular mechanisms involved; and 3) To determine whether the ATP sensitivity of native bTREK-1 channels allows them to function as sensors whose activity varies with the plasma glucose concentration and the metabolic state of the AZF cell.