PHYSIOLOGY OF VOLTAGE DEPENDENT POTASSIUM CHANNELS IN COLONIC MUSCLES

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The goal of this Project is to clarify the mechanisms by which K+ channels control the electrical and mechanical activity of circular and longitudinal smooth muscle of the canine colon. In preliminary experiments we have found that what previously has been denoted as "delayed rectifier" K+ current is composed of at least three distinct channel types. In addition K/ATP channels and Ca2+-activated K+ channels (BK channels) are present in this tissue. (1) We will test the hypothesis that the diversity of electrical activity patterns in the gastrointestinal tract is due to regional differences in type and expression of these K+ channels. Accordingly, we will determine the pharmacological and kinetic properties of "delayed rectifier" components in voltage clamped colonic myocytes and their single channel properties in cell-attached and excised membrane patches. These data will be essential to link molecular biology studies of cloned and expressed K+ channels to observations in the intact tissue. Golenhofen has classified smooth muscle tissues according to their electrical activity patterns in phasic (spiking and non-spiking types) and tonic muscles. Our experiments will focus on tissues that present models of each of these three classes: We will utilize the protocols developed on circular layer colonic myocytes (phasic, non-spiking smooth muscle) to investigate the types and relative expression levels of K+ channels in longitudinal layer colonic myocytes (phasic, spiking smooth muscle) and the lower esophageal sphincter (tonic smooth muscle). (2) We will investigate the K+ conductances involved in nitric oxide and Beta-adrenergic mediated relaxation of the colonic musculature. For both of these inhibitory neurotransmitters evidence has been obtained on the tissue level that activation of K+ channels is essential for the membrane hyperpolarization and subsequent muscle relaxation. However the type of K+ channel involved in either of these actions is not known. We will investigate regulation of the K+ channel types identified in circular smooth muscle cells by cAMP and cGMP mediated pathways and protein kinase C. Single channel studies will clarify the mechanism of these regulatory pathways. These studies will advance our knowledge of the mechanisms of electrical and mechanical rhythmicity in the gastrointestinal tract.