The involvement of L-type Ca2+ channels in the relaxant effects of the ATP-sensitive K+ channel opener ZD6169 on pig urethral smooth muscle

1 The effects of ZD6169, a novel ATP-sensitive K+ channel (K-ATP channel) o pener, were investigated on membrane currents in isolated myocytes using pa tch-clamp techniques. Tension measurement was also performed to study the e ffects of ZD6169 on the resting tone of pig urethral smooth muscle. 2 Levcromakalim was more potent than ZD6169 in lowering the resting urethra l tone. Relaxation induced by low concentrations of ZD6169 (less than or eq ual to3 muM) was completely suppressed by additional application of glibenc lamide (1 muM). In contrast, glibenclamide (1-10 muM) only partially inhibi ted the relaxation induced by higher concentrations of ZD6169 (greater than or equal to 10 muM). 3 Bay K8644 (1 muM) reduced the maximum relaxation produced by ZD6169 (grea ter than or equal to 10 muM). 4 In whole-cell configuration, ZD6169 suppressed the peak amplitude of volt age-dependent Ba2+ currents in a concentration- and voltage-dependent manne r, and at 100 muM, shifted the steady-state inactivation curve of the volta ge-dependent Ba2+ currents to the left at a holding potential of - 90 mv. 5 In cell-attached configuration, open probability of unitary voltage-depen dent Ba2+ channels (27 pS, 90 mM Ba2+) was inhibited by 100 muM ZD6169 and by 10 muM nifedipine. 6 Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis reveale d the presence of the transcript of the alpha (1C) subunit of L-type Ca2+ c hannels in pig urethra. 7 These results demonstrate that ZD6169 causes urethral relaxation through two distinct mechanisms, activation of KATP channels at lower concentration s and inhibition of voltage-dependent Ca2+ channels at higher concentration s (about 10 muM).