A K+ single channel and whole-cell clamp study on the effects of levcromakalim in guinea pig portal vein cells

Abstract

Single channel cell-attached patch and whole-cell clamp experiments on the mode of action of the K+ channel opener (KCO), levcromakalim, were performed in guinea pig isolated portal vein cells. At +20 mV (135/23 mM K+ in bath/pipette), 10 microM levcromakalim activated K+ channels with a chord conductance of 23.2 pS (K(KCO)), which were sensitive to the blocker of ATP-dependent K+ channels (K(ATP)), glibenclamide. Voltage steps from -80 mV to +20 mV activated 4-aminopyridine-sensitive K+ channels of 6.5 pS with properties of delayed rectifier K+ channels (Kv). In patches which upon a previous voltage step had revealed the existence of Kv, levcromakalim reduced the open-probability of Kv, but it did not concomitantly activate K(KCO). During the course of the experiments, but unrelated to the presence of levcromakalim, large conductance K+ channels (BK(Ca)) appeared which could be inhibited by iberiotoxin, a selective blocker of BK(Ca), and by the membrane-permeant calcium buffer, BAPTA/AM, but not by glibenclamide. Whole-cell current-voltage (i-V) relations were established in response to voltage ramps from +50 mV to -100 mV; on subtraction of control i-V curves from i-V curves obtained in the presence of 10 microM levcromakalim, the KCO-induced K+ current remained which was proportional to voltage. This is not compatible with the upward-bent curvature predicted by the GHK current equation for purely resistive channels at high [K+]i versus low [K+]o. In conclusion, in the guinea pig portal vein cells, no evidence could be established for the hypotheses that KCOs may act via conversion of Kv to K(ATP) (Beech and Bolton 1989; Edwards et al. 1993) or by activation of BK(Ca) (Balwierczak et al. 1995). In these cells, mild inward rectification of the levcromakalim-induced current was observed which underlines their relationship to K(ATP) in other tissues.

Human Cardiac Inwardly-Rectifying K + Channel Kir 2.1b Is Inhibited by Direct Protein Kinase C-Dependent Regulation in Human Isolated Cardiomyocytes and in an Expression System

“…Single cells were enzymatically dispersed by use of collagenase and papain and were measured with the wholecell patch-clamp technique as described for portal vein smooth muscle cells. 18 Each single measurement within a series of experiments was done with a cardiomyocyte from a separate patient. The 2 microelectrode voltage-clamp configuration was used to record currents from Xenopus laevis oocytes.…”

Requirement for Functional BK Channels in Maintaining Oscillation in Venomotor Tone Revealed by Species Differences in Expression of the β1 Accessory Subunits

“…Previous work demonstrated that functional BK channels were detected in portal veins from rat, 1 rabbit, 2 guinea pig, 3 and BALB/c mice. 4 These studies are consistent with other studies showing that BK channels also modulate tone in human saphenous 7,8,9 and forearm veins.…”

“…Functional BK channels have also been detected in the large conduit portal vein from rat, 1 rabbit, 2 guinea pig, 3 and BALB/c mice, 4 and also rat inferior vena cava 5 and horse penile veins. 6 In these vessels, BK channels contribute to spontaneous phasic contractile activity and venous tone.…”

Vascular effects of class-III antiarrhythmic drugs: chromanol 293B, but not dofetilide blocks the smooth muscle delayed rectifier K + channel

“…However, the in situ hybridization experiments by Yuan et al (28) are in contrast with electrophysiological measurements on the single channel level: such electrophysiological measurements could only demonstrate the existence of a single delayed rectifier K + channel in portal vein smooth muscle. This channel has a fast activation kinetics and an inactivation time constant of 113.3 ms (12).…”