An Examination of the Mechanisms Underlying Acute and Chronic Hypoxic Regulation of L-Type Ca2+ Channel a 1CSubmits

Abstract

L-type Ca2+ channels, found in vascular smooth muscle cells, function to control Ca2+ influx, which directly regulates the degree of tension in the vasculature. An influx of Ca2+ causes these cells to contract while inhibition of this channel causes muscle relaxation, a major goal in treating hypertension. Acute hypoxia inhibits, and chronic hypoxia enhances, Ca2+ channel currents. The mechanisms underlying these hypoxic responses were examined in HEK 293 cells by altering cellular levels of proposed mediators of 0 2 sensing which have previously been shown to be involved in the redox model of 02 sensing in various cell types. In these studies I investigated the roles of mitochondrial complexes and NADPH oxidase function, and changes in cellular ROS levels, on the acute and chronically hypoxic regulation of recombinant L-type Ca2+ channels. An increase in H202, a form of ROS, by exogenous application was found to enhance Ca2+ currents. However neither catalase nor H202 affected the acute hypoxic response. In contrast superoxide dismutase (SOD) abolished hypoxic inhibition of recombinant L-type Ca2+ channels, suggestive of a role of 02- production in 02 sensing. Altered production of this ROS during hypoxia may occur within the mitochondria since acute 02 sensing was abolished in mitochondria-depleted p0 cells. Alterations in NADPH oxidase activity via application of NADPH oxidase inhibitors such as DPl and P AO did not mediate the acute hypoxic response. Hypoxic regulation of mitochondrial complex I may also mediate the response to chronic hypoxia since current enhancement by this stimulus was abolished by rotenone. These findings support the involvement of altered mitochondrial function in the 0 2 sensing pathway which mediates the hypoxic responses of recombinant L-type Ca2+ channel a1c subunits.