Cholesterol Regulation of Atrial GIRk Channels

In recent years, cholesterol emerged as a major regulator of ion channel function. The most common effect of cholesterol on ion channels is a decrease in channel activity. Here we focus on G-protein gated inwardly rectifying potassium (GIRK or Kir3) channels that play an important role in regulating membrane excitability in cardiac, neuronal and endocrine cells. We have recently shown that unexpectedly cholesterol enrichment up-regulates GIRK activity in atrial myocytes. In accordance, we also observed elevated GIRK currents in cholesterol-enriched Xenopus oocytes expressing the GIRK1/GIRK4 heteromers, the two pore-forming subunits expressed in the heart. In this study, we addressed two questions: (1) is there a correlation between cholesterol and atrial GIRK currents in diet-induced hypercholesterolemia in-vivo and (2) what is the biophysical basis of cholesterol-induced increase in GIRK activity. Our results show that feeding rats high-cholesterol diet for 21-22 weeks resulted in ∼2.5-fold increase in serum LDL levels without any change in HDL and ∼1.8-fold increase in cholesterol level in the atrial tissue. Furthermore, this increase results in up to 3-fold increase in atrial GIRK currents. We also demonstrate here that cholesterol enrichment in-vitro has no effect on the surface expression of the GFP-tagged GIRK channels expressed in Xenopus oocytes, as measured by fluorescent microscopy. This observation was confirmed in HEK293 cells using TIRF microscopy. Most importantly, using planar lipid bilayers we show that cholesterol significantly increases the open probability of the GIRK channels. No change was observed in the unitary conductance. Thus, taken together, our data indicate that up-regulation of GIRK channels by cholesterol is not a result of an increase in their surface expression but is due to the increase in their open probability.