Tolerance for ATP-insensitive K-ATP channels in transgenic mice

To examine the role of sarcolemmal K-ATP channels in cardiac function, we g enerated transgenic mice expressing GFP-tagged Kir6.2 subunits with reduced ATP sensitivity under control of the cardiac alpha -myosin heavy chain pro moter. Four founder mice were isolated, and both founders and progeny were all apparently normal and fertile. Electrocardiograms from conscious animal s also appeared normal, although mean 24-hour heart rate was approximately 10% lower in transgenic animals compared with littermate controls. In excis ed membrane patches, K-ATP channels were very insensitive to inhibitory ATP : mean K-1/2 ([ATP] causing half-maximal inhibition) was 2.7 mmol/L in high -expressing line 4 myocytes, compared with 51 mu mol/L in littermate contro l myocytes. Counterintuitively, K-ATP channel density was approximate to4-f old lower in transgenic membrane patches than in control. This reduction of total K-ATP conductance was confirmed in whole-cell voltage-clamp conditio ns, in which K-ATP was activated by metabolic inhibition. K-ATP conductance was not obvious after break-in of either control or transgenic myocytes, a nd there was no action potential shortening in transgenic myocytes. In mark ed contrast to the effects of expression of similar transgenes in pancreati c beta -cells, these experiments demonstrate a profound tolerance for reduc ed ATP sensitivity of cardiac K-ATP channels and highlight differential eff ects of channel activity in the electrical activity of the 2 tissues.