ATP-sensitive potassium channels (KATP) consist of four pore-forming Kir6.2 subunits and four sulfonylurea receptor subunits (SUR). The SUR subunit mediates the stimulatory effect of MgADP. Transgenic mice expressing the mutant Kir6.2 [▓Dgr│N2-30] (~10-fold decrease in ATP sensitivity, Koster et al. 1999) in pancreatic β-cells develop severe neonatal diabetes (Koster et al. 2000), while transgenic mice expressing the more severe mutation Kir6.2 [▓Dgr│N2-30, K185Q] (> 100-fold decrease in ATP sensitivity) expressed in cardiac cells are essentially normal (Koster et al. 2001). One difference between cardiac and pancreatic β-cell expression of KATP channels is the SUR isoform that is co-expressed with the Kir6.2 subunit – pancreatic β-cells express SUR1, while cardiac cells express SUR2A.
Wild-type Kir6.2 subunits were co-expressed with either SUR1 or SUR2A subunits in transiently transfected COSm6 cells. In excised membrane patches, sensitivity of channels to inhibition by ATP was similar in both SUR1 and SUR2A, with a half-maximal inhibition of ~10 µM. In the maintained presence of 100 µM ATP, MgADP stimulation was more potent with SUR1, with half-maximal stimulation by MgADP of 250 µM for SUR1, and 1100 µM for SUR2A. In the maintained presence of 100 µM inhibitory ATP, MgGDP stimulation was also more potent for SUR1, with 500 µM MgGDP increasing current from 13 to 70 % of maximum (i.e. in zero nucleotides) in SUR1 (n = 13), but from 7 % to only 17 % of maximum in SUR2A (n = 12). The time course of activation with MgADP was also faster in SUR1 than SUR2A (t 1/2 of 1.8 ± 0.9 s (± S.E.M. throughout abstract) versus 17.1 ± 5.3 s, respectively, n = 12, 11 patches, respectively) while the time course of deactivation was similar (t 1/2 of 1.1 ± 0.3 versus 1.5 ± 0.4 s). The rate of activation by MgGDP, however, was similar between SUR1 and SUR2A (t 1/2 of 0.7 ± 0.2 versus 1.1 ± 0.6 s, n = 7, 4 patches, respectively), and deactivation was also similar (with a t 1/2 of 1.4 ± 0.5 versus 2.5 ± 1.3 s).
The relative inefficiency of MgADP to activate cardiac versus pancreatic KATP channels may provide part of the explanation for the physiologically relevant dynamic activity of the KATP channel in the pancreas versus the heart. This may also explain the tolerance for ATP-insensitive KATP channels expressed transgenically in the heart versus the severe diabetic phenotype that is seen with less severe mutants transgenically expressed in pancreatic β-cells (Koster et al. 2000).
- Koster, J.C., Knopp, A., Markova, K., Sha, Q., Yamada, K. & Nichols, C.G. (2001). J. Physiol. 535.P, 9P.
Koster, J.C., Marshall, B.A., Ensor, N., Corbett, J.A. & Nichols, C.G. (2000). Cell 100, 645-654.
Koster, J.C., Sha, Q., Shyng, S. & Nichols, C.G. (1999). J. Physiol. 515, 19-30. abstract