Calyculin A and outward K+ channel currents in rat tail artery smooth muscle cells

Interaction of acetylcholine with Kir6.1 channels heterologously expressed in human embryonic kidney cells

Kir6.1 subunit is one of the pore-forming components of K(ATP) channel complex. The endogenous modulation of Kir6.1 subunit function has been largely unknown. Whether acetylcholine modulated the function of Kir6.1 subunit stably expressed in human embryonic kidney (HEK-293) cells was examined in the present study using the whole-cell patch-clamp technique. Acetylcholine from 1-100 microM concentration-dependently stimulated the heteologously expressed and PNU-37883A sensitive Kir6.1 channels (p<0.05). Co-expression of sulphonylurea receptor 1 subunit with Kir6.1 significantly inhibited the stimulatory effect of acetylcholine on K(ATP) currents. Pretreatment of the transfected HEK-293 cells with atropine, alpha-bungarotoxin, mecamylamine, prazocine, propranolol, or dihydro-beta-erythroidine hydrobromide did not alter the stimulatory effect of acetylcholine on Kir6.1 currents. When intracellular ATP was increased from 0.3 mM to 5 mM, acetylcholine at 10 microM still exhibited its stimulatory effect (-16.4+/-2.3 to -25.5+/-3.8 pA/pF, n=8, p<0.05). In conclusion, we have demonstrated an excitatory effect of acetylcholine on Kir6.1 channels, which is mediated neither by an acetylcholine receptor-dependent mechanism, nor by alteration in ATP metabolism.

Altered vascular reactivity and KATP channel currents in vascular smooth muscle cells from deoxycorticosterone acetate (DOCA)-salt hypertensive rats

This study was designed to evaluate the contribution of ATP-dependent potassium (KATP) channels to the changes in vascular reactivity and spontaneous tone observed in vessels isolated from deoxycorticosterone acetate (DOCA)-salt hypertensive rats. In phenylephrine preconstricted aortic rings, cromakalim induced concentration-dependent, glibenclamide-sensitive relaxation. The concentration response curve to cromakalim was shifted to the right in DOCA-salt hypertensive rats (EC50: 0.850 +/- 0.100 microM) compared with SHAM-normotensive rats (0.108 +/- 0.005 microM), and the maximum relaxation (Emax) evoked by cromakalim was significantly lower in aortic rings from the DOCA group (68 +/- 2%) compared with the SHAM group (108 +/- 5%). The results were similar in endothelium-denuded rings. Spontaneous tone was observed in aortic rings (5 g preload) from DOCA-salt but not SHAM rats. Cromakalim abolished spontaneous tone and the effect was blocked by glibencamide. In whole cell patch clamp studies, increasing extracellular K concentrations from 5.4 to 140 mM and the administration of cromakalim evoked dramatic increases in KATP channel currents in aortic cells isolated from SHAM rats. In contrast, in aortic cells from DOCA-salt hypertensive rats, KATP channel currents were either absent or weak in response to challenges by elevated extracellular K and by cromakalim. These findings suggest that the function of KATP channels is impaired in smooth muscle cells from aorta of DOCA-salt hypertensive rats, which may contribute to the impaired vasodilatation and spontaneous tone observed in these rats.

Mediation of the Effect of Nicotine on Kir6.1 Channels by Superoxide Anion Production

KATP channels are a complex of regulatory sulfonylurea receptor subunits and the pore-forming inward rectifiers such as Kir6.1. Using the whole-cell patch-clamp technique, we investigated the interaction of nicotine with the Kir6.1 subunit as well as the underlying mechanism. Stable expression of Kir6.1 in HEK-293 cells yielded a detectable inward rectifier KATP current. This inward current was significantly inhibited by PNU-37883A and by a specific anti-Kir6.1 antibody. Nicotine at 30 and 100 microM increased Kir6.1 currents by 42 +/- 11.8% and 26.2 +/- 14.6%, respectively (n = 4-6, P < 0.05). In contrast, nicotine at 1-3 mM inhibited Kir6.1 currents (P < 0.05). Nicotine at 100 microM increased the production of superoxide anion (O2) by 20.3 +/- 5.7%, whereas at 1 mM it significantly decreased the production of O2 by 37.7 +/- 4.3%. Coapplication of hypoxanthine (HX) and xanthine oxidase (XO) to the transfected HEK-293 cells resulted in a significant and reproducible increase in Kir6.1 currents (P < 0.05). The stimulatory effect of HX/XO on Kir6.1 current was abolished by tempol, a scavenger of O2. Tempol also abolished the stimulatory effect of 30 muM nicotine on Kir6.1 currents. In conclusion, nicotine stimulates Kir6.1 channel at least in part through the production of O2.

Calyculin-A disrupts subplasmalemmal junction and recurring Ca2+ waves in vascular smooth muscle

Excitation-contraction coupling (E-C coupling) in phenylephrine(PE)-stimulated rabbit inferior vena cava (IVC) depends on the generation of asynchronous recurring Ca2+ waves in the in situ vascular smooth muscle cells (VSMC). Previous studies by our group have implicated a putative non-selective cationic store-operated channel and the reverse-mode Na+-Ca2+ exchange in refilling of the intracellular Ca2+ store via the sarco/endoplasmic reticulum ATPase (SERCA) and the maintenance of the recurring Ca2+ waves. We hypothesize that for the proper functioning of these three Ca2+ translocators in the process of SR refilling, the plasma membrane (PM) and the underlying superficial sarcoplasmic reticulum (SR) form specialized PM-SR junctions, which are essential for the maintenance of the recurring Ca2+ waves. In order to test this hypothesis, calyculin-A, a serine/threonine phosphatase inhibitor that has been demonstrated to result in the disruption of the PM-SR junctions was used. In the control rabbit IVC, electron microscopy of the in situ VSMC indicates that 14.2+/-0.7% of the PM is closely apposed by the prominent superficial SR network, forming numerous flattened PM-SR junctional cytoplasmic spaces. In the control IVC stimulation with 5 microM PE resulted in sustained recurring Ca2+ waves with a frequency of 0.42+/-0.02 Hz. In calyculin-A treated rabbit IVC, a concentration-dependent dissociation of the superficial SR and loss of PM-SR junctions was observed. This progressive loss of the PM-SR junctions occurs over the same concentration range as the inhibition of PE-induced recurring Ca2+ waves. These findings offer support for the hypothesis that the presence of the PM-SR junctions is required for the generation of asynchronous recurring Ca2+ waves, which underlie excitation-contraction coupling in the VSMC of the rabbit IVC.

Selective expression of Kir6.1 protein in different vascular and non-vascular tissues

K(ATP) channels are composed of pore-forming subunits Kir6.x and auxiliary subunits SURx. These channels play important roles in modulating the contractility of vascular smooth muscle cells (SMCs) by altering membrane potentials. The molecular basis of K(ATP) channels in vascular SMCs is unclear and the expression of different K(ATP) channel subunits at protein level in various tissues still undetermined. In this study, using an anti-Kir6.1 antibody, we detected the expression of Kir6.1 proteins in rat vascular tissues including mesenteric artery, pulmonary artery, aorta, and tail artery. Kir6.1 proteins were also identified in heart and other non-vascular tissues including spleen and brain, but they were undetectable in liver and kidney. Immunocytochemical study revealed the expression of Kir6.1 proteins in cultured rat thoracic aortic SMCs. Using the whole-cell patch-clamp technique, it was found that the intracellularly applied anti-Kir6.1 antibody significantly inhibited K(ATP) channel currents in HEK-293 cells that were stably transfected with Kir6.1 cDNA. A better understanding of differential expression of Kir6.1 proteins in various vascular and non-vascular tissues may help discern different molecular basis and functions of K(ATP) channel complexes in these tissues.