Analysis of Single KATP Channels in Mammalian Dentate Gyrus Granule Cells

ATP-sensitive potassium (KATP) channels are heteromultimer complexes of subunits from members of the inwardly rectifying K+ channel and the ATP-binding cassette protein superfamilies. KATP channels couple metabolic state to membrane excitability, are distributed widely, and participate in a variety of physiological functions. Understood best in pancreatic β cells, where their activation inhibits insulin release, KATP channels have been implicated also in postischemia cardio- and neuroprotection. The dentate gyrus (DG) is a brain region with a high density of KATP channels and is relatively resistant to ischemia/reperfusion-induced cell death. Therefore we were interested in describing the characteristics of single KATP channels in DG granule cells. We recorded single KATP channels in 59/105 cell-attached patches from DG granule cells in acutely prepared hippocampal slices. Single-channel openings had an E K close to 0 mV (symmetrical K+) and were organized in bursts with a duration of 19.3 ± 1.6 (SE) ms and a frequency of 3.5 ± 0.8 Hz, a unitary slope conductance of 27 pS, and a low, voltage-independent, probability of opening ( P open, 0.04 ± 0.01). Open and closed dwell-time histograms were fitted best with one (τopen = 1.3 ± 0.2 ms) and the sum of two (τclosed,fast = 2.6 ± 0.9 ms, τclosed,slow = 302.7 ± 67.7 ms) exponentials, respectively, consistent with a kinetic model having at least a single open and two closed states. The P open was reduced ostensibly to zero by the sulfonylureas, glybenclamide (500 nM, 2/6; 10 μM,11/14 patches) and tolbutamide (20 μM, 4/6; 100 μM, 4/4 patches). The blocking dynamics for glybenclamide included transition to a subconductance state (43.3 ± 2.6% of control I open channel). Unlike glybenclamide, the blockade produced by tolbutamide was reversible. In 5/5 patches, application of diazoxide (100 μM) increased significantly P open (0.12 ± 0.02), which was attributable to a twofold increase in the frequency of bursts (8.3 ± 2.0 Hz). Diazoxide was without effect on τopen and τclosed,fastbut decreased significantly τclosed,slow(24.4 ± 2.6 ms). We observed similar effects in 6/7 patches after exposure to hypoxia/hypoglycemia, which increased significantly P open (0.09 ± 0.03) and the frequency of bursts (7.1 ± 1.7 Hz) and decreased significantly τclosed,slow (29.5 ± 1.8 ms). We have presented convergent evidence consistent with single KATP channel activity in DG granule cells. The subunit composition of KATP channels native to DG granule cells is not known; however, the characteristics of the channel activity we recorded are representative of Kir6.1/SUR1, SUR2B-based channels.