Modulation of intracellular calcium by potassium channel openers in vascular muscle

Differential antivasoconstrictor effects of levcromakalim and rilmakalim on the isolated human mammary artery and saphenous vein

It is well established that spasm of an arterial and venous graft conduit may occur during harvesting or after coronary artery bypass grafting (CABG). The antivasoconstrictor effect of levcromakalim and rilmakalim, K(+) channel openers (KCOs), was studied in isolated human internal mammary artery (HIMA) and human saphenous vein (HSV) prepared for CABG. HIMA and HSV rings were contracted by electrical field stimulation (EFS, 20 Hz ) or with exogenous noradrenaline (NA). Levcromakalim induced a concentration-dependent and equipotent inhibition of contraction of HIMA and HSV preconstricted by EFS and exogenoulsy applied NA, while rilmakalim produced a stronger inhibition of EFS- than NA-evoked contractions. Glibenclamide, a selective ATP-sensitive K(+) channel (K(ATP) channel) blocker, significantly antagonized levcromakalim-induced inhibition of EFS- and NA-evoked contractions, as well as rilmakalim-induced inhibiton of EFS-evoked contractions on HIMA and HSV. However, glibenclamide failed to antagonize rilmakalim-induced inhibition of NA-evoked contractions. The results suggest that the antivasoconstrictor effect of levcromakalim occurs postsynapticaly by the opening K(ATP) channels in the vascular smooth muscle cells. They also suggest that the effect of rilmakalim on EFS-evoked contractions involves K(ATP) channels located pre-synaptically. However, the mechanism by which rilmakalim inhibits NA-evoked contraction seems to be K(ATP) channel independent and warrants further elucidation.

Basal nitric oxide release differentially modulates vasodilations by pinacidil and levcromakalim in goat coronary artery

In the current investigation, the role of basal nitric oxide (NO) in modulating the vasorelaxant responses to pinacidil and levcromakalim was examined in goat isolated coronary artery. Pinacidil (10(-8) 10(-4) M) elicited concentration-dependent relaxations of the coronary artery ring segments (with intact endothelium) constricted with 30 mM K+ saline solution. The EC50 of the vasodilator was 2.57 x 10(-6) M (95% CL, 1.9-3.46 x 10(-6) M). The removal of endothelium by mechanical rubbing caused a rightward shift in the concentration-response curve of pinacidil with a corresponding increase in EC50 value (1.90 x 10(-5) M; 95% CL, 1.12-3.23 x 10(-5) M). Similar to endothelium removal, treatment of endothelium-intact rings either with the NO synthesis inhibitor L-NAME (NG-nitro-L-arginine methyl ester; 3 x 10(-5) M) or the guanylate cyclase inhibitor, methylene blue (3 x 10(-6) M) resulted in a marked inhibition in the relaxant responses to pinacidil. Hence, the EC50 values of the potassium channel opener were significantly higher in tissues treated either with L-NAME (7.41 x 10(-6) M; 95% CL, 6.02-9.12 x 10(-6) M) or methylene blue (2.29 x 10(-5) M; 95% CL, 1.58-3.31 x 109-5) M) as compared to untreated controls. The ATP-sensitive potassium (KATP) channel blocker glibenclamide, which caused a significant rightward shift in the concentration-relaxation curve of pinacidil in control tissues, was found to be less potent in antagonising the relaxant responses of the KATP channel opener in endothelium-denuded rings and in rings with intact endothelium but treated with either L-NAME or methylene blue. In contrast to the observations made with pinacidil, the vasodilator responses to another KATP channel opener, levcromakalim, were potentiated in the absence of basal NO. Thus, the EC50 of levcromakalim was 1.33 x 10(-8) M (95% CL, 0.8-2.21 x 10(-8) M) in control tissues with intact endothelium, which was significantly higher than those obtained in endothelium-deprived rings (4.81 x 10(-9) M; 95% CL, 4.04-5.73 x 10(-9) M) or endothelium intact rings treated either with L-NAME (2.63 x 10(-9) M; 95% CL, 1.58-4.36 x 10(-9) M) or methylene blue (2.82 x 10(-9) M; 95% CL, 1.7-4.68 x 10(-9) M). The selective modulation by basal NO of the arterial relaxations elicited with the KATP channel openers was evident from the findings that papaverine-induced relaxations were not affected in the absence of basal NO. Taken together, the results of the present study suggest that basal NO differentially modulates the interaction of pinacidil and levcromakalim with the KATP channels in goat coronary artery through a cGMP-dependent pathway.

Activation of large-conductance potassium channels in pregnant human myometrium by pinacidil

OBJECTIVE

The aim was to investigate the effects of the potassium-channel opener pinacidil on single uterine potassium channels and the contribution of the latter to pinacidil-induced myometrial relaxation.

STUDY DESIGN

Myometrial strips and freshly dispersed uterine myocytes were prepared from the myometrial biopsy samples of women undergoing elective, nonlabor caesarean section at term gestation.

RESULTS

In isometric tension experiments pinacidil potently relaxed pregnant nonlabor human myometrial strips, with an agonist concentration yielding the half maximal response of 0.4 +/- 0.1 micromol/L. This effect was antagonized by 500 nmol/L charybdotoxin. Application of 10 micromol/L glibenclamide also inhibited the pinacidil-induced relaxation. Coapplication of charybdotoxin (500 nmol/L) and glibenclamide (10 micromol/L) produced a biphasic curve, which was fitted to a two-site model with values for agonist concentration yielding the half maximal response of 0.6 +/- 0.2 micromol/L and 189.7 +/- 0.8 micromol/L. Large-conductance calcium-dependent potassium channel activity was dramatically increased after application of pinacidil (between 10 and 100 micromol/L) to both inside-out and outside-out patches. The activation required the presence of calcium ions at the intracellular aspect of the membrane. Charybdotoxin but not glibenclamide blocked pinacidil-induced unitary large-conductance calcium-dependent potassium channel activity.

CONCLUSION

Pinacidil-mediated relaxation of human pregnant myometrial strips may be partially attributable to the opening of uterine large-conductance calcium-dependent potassium channels in addition to adenosine triphosphate potassium channel activation. Drugs with specific potassium channel-activating properties may have important clinical application as novel tocolytics in the treatment of preterm labor.

K(ATP) channels do not mediate vasodilation by 3-morpholinosydnonimine in goat coronary artery

The present study investigated the role of ATP-sensitive potassium (K(ATP)) channels in mediating relaxation to the nitric oxide (NO) donor, 3-morpholinosydnonimine (SIN-1) in goat coronary arteries. SIN-1 (10(-8)-10(-5) M) caused concentration-dependent relaxations of the coronary artery ring segments contracted with K+ (30 mM) with an EC50 of 6.61 x 10(-7) M. Methylene blue (3 x 10(-6) M) caused a rightward shift in the concentration-response curve of SIN-1 (10(-8)-3 X 10(-5) M) with a corresponding increase in the EC50 (3.62 x 10(-6) M) of the nitrovasodilator. While the K(ATP) channel blocker, glibenclamide (1 and 3 x 10(-6) M) caused dose-dependent inhibition of vasorelaxations produced by pinacidil (10(-8)-10(-4) M), it had no effect on the vasodilations elicited by SIN-1 (10(-8)-10(-5) M) in the coronary arterial smooth muscle. Increasing the extracellular K+ concentration from 30 mM to 80 mM to reduce the K+ gradient across the cell membrane, inhibited the relaxations elicited by pinacidil (10(-8)-10(-4) M). On the other hand, SIN-1 (10(-8)-10(-5) M)-induced relaxations were potentiated in high K+ (80 mM) compared to those observed at K+ (30 mM). These results suggest that goat coronary artery vasodilations caused by the NO donor, SIN-1, do not involve K(ATP) channels.

Role of membrane potential in endothelium-dependent relaxation of guinea-pig coronary arterial smooth muscle

1. Membrane potential and tension were measured simultaneously in ring segments of main coronary artery of guinea-pigs. The synthetic thromboxane A2 analogue U46619 depolarized the tissues from -58 +/- 2 to -40 +/- 1 mV and increased tension by 12 +/- 1 mN mm-1. Nitric oxide (NO) and Iloprost, the stable analogue of prostacyclin, evoked hyperpolarization and relaxation. 2. The concentration of NO required to evoke half-maximal hyperpolarization (EC50 of 2 x 10(-5) M) was 40-fold higher than that which was required to induce relaxation (EC50 of 5 x 10(-7) M). The EC50 for Iloprost-induced hyperpolarization (3 x 10(-8) M) was similar to that for relaxation (4 x 10(-8) M). 3. Glibenclamide (10(-6) M) abolished the hyperpolarization in response to both NO and Iloprost but was without effect on the amplitudes of the relaxations over the complete concentration-response curves. 4. Acetylcholine evoked concentration-dependent hyperpolarization and relaxation in the presence of N omega-nitro-L-arginine methyl ester (NAME; 10(-5) M) and indomethacin (10(-6) M), and these responses were attributed to endothelium-derived hyperpolarizing factor (EDHF). The hyperpolarization produced by EDHF always preceded relaxation, and relaxation never occurred at concentrations of acetylcholine that were insufficient to evoke hyperpolarization. 5. The concentration-hyperpolarization and concentration-relaxation curves in response to acetylcholine were not affected by glibenclamide or barium (1-3 mM) but were shifted to the right 4- and 5-fold, respectively, by 1 mM tetraethylammonium. The hyperpolarization and relaxation evoked by acetylcholine were also reduced in a parallel manner when the potassium concentration in the superfusate was increased. 6. Hyperpolarizing current steps, applied to spiral strips of coronary artery denuded of endothelium and depolarized and constricted with U46619, caused relaxation. The relationship between hyperpolarization and relaxation evoked electronically was similar to that which was due to EDHF in intact tissues stimulated with acetylcholine. 7. It is concluded that the ability of NO or Iloprost to relax guinea-pig coronary artery does not depend upon hyperpolarization of the smooth muscle. In contrast, hyperpolarization is likely to play a major, if not the only, role in the relaxation in response to EDHF in this tissue.

Dynamic effects and pharmacokinetics of lemakalim in the isolated guinea-pig heart

Myocardial effects of lemakalim were studied in the isolated retrogradely perfused and spontaneously beating guinea-pig heart. Pharmacokinetic analysis of drug accumulation and disposition showed two-compartment characteristics with phasic half-times of about 0.27 and 2.1 min., respectively. Myocardial clearance was 6.8 ml min.-1, and myocardial drug accumulation was 5-fold. Dynamic parameters were studied during increasing lemakalim concentrations from 1.4 nM up to 10 microM. Dynamic steady-states developed within 3-4 min. Increase of coronary flowrate showed an Emax of 185% with an EC50 of 60 nM. Lemakalim produced biphasic inhibitory actions on contraction velocity and amplitude. Emax and EC50 for the two phases were: 48% and 47% and 100 nM and 65 nM, respectively, for the first phase and 100% in both cases and 1.7 microM and 1.6 microM for the second phase. Heart frequency increased to a maximum of 15% above baseline at 0.3 microM. Oxygen consumption increased progressively at concentrations above 7 nM and reached a maximum of 175% at 0.3 microM. The frequency-corrected QT-interval was biphasically shortened to a maximal extent of 30% at the highest concentration. Myocardial efficiency expressed as the ratio of amplitude times frequency to oxygen consumption exhibited a progressive decline to about 20% of control values. PQ- and QRS-intervals showed only minor or no changes, respectively. No arrythmogenic effects were observed. The study demonstrated a rapid and very moderate accumulation of lemakalim in the guinea-pig heart accompanied by a marked increase in coronary flowrate and a progressive negative inotropic effect followed by reduced myocardial efficiency.