One of the fundamental properties of heart cells is their ability to alter their electrical characteristics in response to neuro- hormonal stimulation. This property allows the heart to adapt its physiological function to the range of activity it must meet. The proposed work focuses upon one ionic channel in the membranes of mammalian ventricular cardiac muscle cells: the delayed rectifier potassium channel. This channel contributes outward current that regulates the duration of the action potential of heart ventricular muscle cells. Thus, changes in potassium current through this channel are critical to maintaining the proper relationship between systole (contraction) and diastole (filling time) as heart rate changes. In addition, because calcium ions enter heart cells during the period of depolarization of the action potential known as the plateau phase, changes in potassium current will indirectly affect calcium entry by controlling the action potential duration. With prior NSF support, this laboratory has demonstrated that the delayed rectifier potassium channel is regulated by two enzyme systems (protein kinase A and C) and that this regulation has a unique temperature requirement that distinguishes its control from that of the cardiac (L-type) calcium channel. The aim of the present proposal is to determine the mechanism(s) by which protein kinases A and C regulate the delayed rectifier potassium channel. Experiments will be designed to identify differences in the activity of channels modulated by the two kinase systems. Analysis of differences in expressed channel activity will be used to test for the location of the sites of protein phosphorylation by protein kinase A and protein kinase C. In addition, experiments are proposed to investigate the basis for the unique temperature-dependent response of the K channel protein to protein kinase stimulation. The opening and closing of specific ion channels in a precise sequence underlie the electrical activity of heart cells and ultimately are responsible for controlling the regular contraction and relaxation of the heart. The results of this research will be a more precise understanding of the biochemical mechanisms regulating one of these specific ion channels, the "delayed rectifier" potassium channel. Since similar ion channels occur in other cell types including neurons and certain gland cells, this contribution to our understanding of how ion channels are regulated will be of fundamental interest.
