Two potassium channels: one, K(ATP) sensitive to ATP, and the other, K(Ca, V), sensitive to calcium and voltage, play a pivotal role in excitation-secretion coupling in beta cells of the endocrine pancreas. Both channels are glucose sensitive, implicating them as part of the cell glucose sensing mechanism, linking glucose stimulation to insulin secretion. Previous patch clamp studies have shown that the K(ATP) channel modulation by glucose is mediated via inhibition of a protein kinase A- dependent phosphorylation process. The proposed patch clamp and biochemical studies primarily aim to demonstrate that glucose induced K(Ca, V) channel inhibition is mediated via protein kinase C (PKC) and G protein activity. From our preliminary data it is hypothesized that the K(Ca, V) channel possesses a G protein binding site that is occupied either by an active, non- phosphorylated, alpha subunit of Gk (alpha k), or following an increase of PKC activity, by the phosphorylated inactive form (alpha k.P), the binding of alpha k.P resulting in channel inhibition. It is further proposed that activation of PKC following production of diacylglycerol in response either to increased glucose metabolism or to activation of phospholipase C by alpha k, is essential for K channel modulation. This finding of PKC and alpha k regulation of K channel activity will permit the formulation of a model which will integrate the function of various insulin secretagogues that modulate beta-cell function via protein kinases and G proteins. Investigation of the role of the individual components of the second messenger cascade, particularly the "G protein"-protein kinase C interaction in potassium channel regulation will be essential for the understanding of the chain of events linking glucose stimulation to insulin secretion in pancreatic beta cells. The results of these studies promise, in addition, to shed new light on the regulation of ion channels by second messenger molecules in a wide variety of other cell types.
