PROPERTIES OF K+ CURRENT THAT CONTROLS SECRETION

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In mammals, corticosteroid secretion by cells of the adrenal gland is controlled by peptide hormones including adrenocorticotropic hormone (ACTH) and angiotensin II (AII). Glucocorticoids such as cortisol act on cells of the liver, muscle and adipose tissue to enhance glucose synthesis-and to promote the breakdown of fat and proteins. Aberrant corticosteroid secretion is responsible for serious pathology including Cushing's and Addison's diseases. The mineralocorticoid aldosterone acts to maintain fluid and electrolyte balance. Inappropriate aldolsterone secretion is manifested as hypo- or hypertension. The cellular mechanisms by which peptide hormones regulate corticosteroid secretion are not understood. In many secretory cells, hormone production is coupled to membrane depolarization through activation of voltage-gated Ca2+ channels. Bovine adrenocortical cells possess a novel K+ channel (IAC) that sets the membrane potential of these cells. Importantly, both ACTH and AII inhibit IAC and depolarize adrenal cells at equivalent subnanomolar concentrations. The convergent inhibition of IAC by these two peptides suggests a physiological mechanism whereby biochemical signals at the cell membrane can be coupled to depolarization-dependent Ca2+ entry and steroid hormone secretion. Because IAC appears to act pivotally in transducing biochemical signals to electrical events involved in secretion, a detailed characterization of this current, the underlying channels and the signalling pathways that regulate IAC activity will be essential to understanding adrenal cortical physiology. In the proposed studies, IAC in bovine and human adrenal zona fasciculata cells will be examined with patch voltage clamp techniques. Functional expression of IAC in Xenopus oocytes will be used to clone IAC channel cDNA. The aims of the proposed research will be: 1) To characterize IAC with respect to biophysical properties, pharmacology, and modulation by metabolic factors and enzymes; 2) To identify and characterize the signalling pathways and molecular mechanisms by which ACTH inhibits IAC; 3) To determine whether IAC is present and regulated by AII and ACTH in human adrenocortical cells; 4) To clone and sequence the cDNA for IAC by expression cloning in Xenopus oocytes.