Functional involvement of sulphonylurea receptor (SUR) type 1 and 2B in the activity of pig urethral ATP-sensitive K+ channels

1 We have investigated the possible roles of sulphonylurea receptor (SUR) type 1 and 2B in the activity of pig urethral ATP-sensitive K+ channels (K-ATP channels) by use of patch-clamp and reverse transcriptase - polymerase chain reaction (RT - PCR) techniques. 2 In voltage-clamp experiments, not only diazoxide, a SUR1 and weak SUR2B activator, but also pinacidil, a selective SUR2 activator, caused an inward current at a holding potential of -50 mV in symmetrical 140 mM K+ conditions. 3 Gliclazide (less than or equal to1 muM), a selective SUR1 blocker, inhibited the 10 muM pinacidil-induced currents (K-i = 177 muM) and the 500 muM diazoxide-induced currents (high-affinity site, K-i1 = 5nM; low-affinity site, K-i2 = 108 muM) at -50 mV. 4 Application of tolbutamide (less than or equal to100 muM) reversibly caused an inhibition of the 500 muM diazoxide-induced current at - 50 mV. 5 MCC-134, a SUR type-specific K-ATP channel regulator (1 - 100 muM), produced a concentration-dependent inward K+ current, which was suppressed by the application of glibenclamide at -50 mV. The amplitude of the MCC-134 (100 muM)-induced current was approximately 50% of that of the 100 muM pinacidil-induced currents. 6 Using cell-attached configuration, MCC-134 activated a glibenclamide- sensitive K-ATP channel which was also activated by pinacidil. 7 RT - PCR analysis demonstrated the presence of SUR1 and SUR2B transcripts in pig urethra. 8 These results indicate that both SUR1 and SUR2B subunits play a functional role in regulating the activity of pig urethral K-ATP channels and that SUR1 contributes less than 25% to total K-ATP currents.