On the mechanism of the differential effects of NS004 and NS1608 in smooth muscle cells from guinea pig bladder

Our recent study revealed that the reported BKCa channel openers NS004 (5-trifluoromethyl-(5-chloro-2-hydroxyphenyl)-1,3-dihydro-2H-benzimidazole-2-one) and its analog NS1608 (N-(3-(trifluoromethyl)phenyl)-N'-(2-hydroxy-5-chlorophenyl)urea) produced a differential pattern of action on the BKCa channel in porcine coronary arterial cells (Hu, S., H.S. Kim and C.A. Fink, 1995, Differential effects of the BKCa channel openers NS004 and NS1608 in porcine coronary arterial cells, Eur. J. Pharmacol. 294, 357). In this study, using the patch-clamp method on the smooth muscle cells from guinea pig bladder detrusor, the activity profile of NS004 and NS1608 on the whole-cell BKCa current (IBK) was examined and found to be similar to that in coronary arterial cells. NS004 did not significantly modify I-BK at concentrations below 5 mu M, but robustly augmented I-BK at concentrations greater than 20 mu M. With a minimum effective concentration of 0.5 mu M, NS1608 caused potentiation of I-BK with a bell-shaped concentration-response relationship. The activation was maximized between 5-10 mu M and substantially attenuated at higher concentrations. The behavior of single BKCa channels in the presence of NS004 and NS1608 was scrutinized to elucidate the mechanism underlying their distinct patterns of action. The channel open-state probability (NP0) was increased by NS004 at 0.5, 5 and 50 mu M to a respective 1.75 +/- 0.38, 4.05 +/- 0.90, and 15.01 +/- 3.66 fold (n = 7) of the control by increasing the open time and the frequency of opening while having no effect on the single BKCa channel conductance. NS1608 at 0.5 and 5 mu M increased NP0 to 1.69 +/- 0.43 and 18.03 +/- 6.64 fold of the control (n = 4), respectively. NS1608 at higher concentrations repeatedly blocked channel openings as evidenced by the highly flickering open state, though the overall open time and frequency of opening remained high. The diminished activation of I-BK by 50 mu M NS1608 manifested therefore a net result of these two opposing actions on the single channels. Thus, the two structurally related BKCa channel openers interact distinctly with the channel gating components.