K⁺ currents underlying the action of endothelium‐derived hyperpolarizing factor in guinea‐pig, rat and human blood vessels

Abstract

Of the endothelium-derived relaxing and hyperpolarizing factors, the nature of endothelium-derived hyperpolarizing factor (EDHF) and the ionic mechanisms underlying its actions are least understood (Mombouli & Vanhoutte, 1997;Hecker, 2000). The nature of EDHF as a chemical factor has been explored in a number of studies (Campbell et al. 1996;Popp et al. 1996;Randall et al. 1996;Plane et al. 1997;Chataigneau et al. 1998;Fisslthaler et al. 1999) with a lack of consensus. Some studies suggest that the EDHF-induced hyperpolarization of smooth muscle cells results from electrotonic spread of activity originating in the endothelial cells (Little et al. 1995; B eny, 1997;Chaytor et al. 1998;Yamamoto et al. 1998Yamamoto et al. , 1999 Edwards et al. 1999), while other studies provide evidence against coupling (B eny et al. 1997;Welsh & Segal, 1998). Identification of the nature of EDHF, whether as a diffusible factor or as electrotonic current spread, and the Journal of Physiology (2001) 1. Membrane currents attributed to endothelium-derived hyperpolarizing factor (EDHF) were recorded in short segments of submucosal arterioles of guinea-pigs using single microelectrode voltage clamp. The functional responses of arterioles and human subcutaneous, rat hepatic and guinea-pig coronary arteries were also assessed as changes in membrane potential recorded simultaneously with contractile activity. 2. The current-voltage (I-V) relationship for the conductance due to EDHF displayed outward rectification with little voltage dependence. Components of the current were blocked by charybdotoxin (30-60 nÒ) and apamin (0·25-0·50 ìÒ), which also blocked hyperpolarization and prevented EDHF-induced relaxation. 3. The EDHF-induced current was insensitive to Ba¥ (20-100 ìÒ) andÏor ouabain (1 ìÒ to 1 mÒ). 4. In human subcutaneous arteries and guinea-pig coronary arteries and submucosal arterioles, the EDHF-induced responses were insensitive to Ba¥ andÏor ouabain. Increasing [K¤]ï to 11-21 mÒ evoked depolarization under conditions in which EDHF evoked hyperpolarization. 5. Responses to ACh, sympathetic nerve stimulation and action potentials were indistinguishable between dye-labelled smooth muscle and endothelial cells in arterioles. Action potentials in identified endothelial cells were always associated with constriction of the arterioles. 6. 18â-Glycyrrhetinic acid (30 ìÒ) and carbenoxolone (100 ìÒ) depolarized endothelial cells by 31 ± 6 mV (n = 7 animals) and 33 ± 4 mV (n = 5), respectively, inhibited action potentials in smooth muscle and endothelial cells and reduced the ACh-induced hyperpolarization of endothelial cells by 56 and 58%, respectively. 7. Thus, activation of outwardly rectifying K¤ channels underlies the hyperpolarization and relaxation due to EDHF. These channels have properties similar to those of intermediate conductance (IKCa) and small conductance (SKCa) Ca¥-activated K¤ channels. Strong electrical coupling between endothelial and smooth muscle cells implies that these two layers function as a single electrical syn...

Enhanced spontaneous Ca2+ events in endothelial cells reflect signalling through myoendothelial gap junctions in pressurized mesenteric arteries

“…In this regard, carbenoxolone was more effective than 18· -GA, which is consistent with several studies showing 18· -GA to be less effective than carbenoxolone [44][45][46][47]. However, caution should be exercised in interpreting these findings; although these uncouplers have inhibitory effects on gap junctions, they are also known to have non-specific effects directly on endothelial cells [44,47,48]. Although carbenoxolone had little effect on endothelial cell resting membrane potential in rat mesenteric arteries which had been cut open and pinned out, it did appear to reduce hyperpolarization to ACh.…”

Red blood cell K+ could be a marker of K+ changes in other cells involved in blood pressure regulation

“…K + fluxes triggered by the opening of K Ca produce vasodilatation in the VSMC 1 and underlie the hyperpolarization and relaxation due to EDHF. 3,4 In the present study, we found that hypertensives and their offspring had significantly lower RBC Ki compared to normotensives and that these levels were inversely related to DBP. Our data are consistent with previous studies showing a decreased RBC Ki in hypertensives and offspring of hypertensives [9][10][11] and a significant inverse relation- ship of RBC Ki and blood pressure.…”

“…1 In the endothelium, K + efflux, through intermediate conductance K Ca (IK Ca ), hyperpolarize VSMC via electrical coupling between endothelium and VSMC, 2 producing hyperpolarization and closing voltage-dependent Ca 2+ channels. Stimulation of endothelial cells results in an outwardly rectifying K + current 3 and the combination of the K + channel inhibitors apamin and charybdotoxin completely prevents the hyperpolarizing and vasodilating action of endothelium-derived hyperpolarizing factor (EDHF) in the rat hepatic artery. 4 This observation is evidence that K Ca with the pharmacological characteristics of small and intermediate conductance K Ca are involved in the EDHF vasodilatation.…”

EDHF-mediated vasodilation involves different mechanisms in normotensive and hypertensive rat lungs

“…Our results showed that 18␣-glycyrrhetinic acid, but not palmitoleic acid, inhibited the hypoxic response in both normotensive and hypertensive lungs. Whereas it is tempting to speculate the blunting by 18␣-glycyrrhetinic acid was due to interference with hetero-and/or homocellular gap junction communication, the failure of this agent to inhibit the thapsigargin-induced, EDHF-mediated vasodilation, and its reported nonspecific effects (12,27,51), raise a note of caution. More direct measurements and more specific inhibitors are needed to rigorously test a possible role of gap junction intercellular communication in the mechanism of hypoxic vasoconstriction.…”

“…EDHF hyperpolarizes the smooth muscle, inhibits voltage-gated Ca 2ϩ influx, and elicits vasodilation. EDHF-mediated vasodilation is blocked by the combined treatment with the Ca 2ϩ -activated K ϩ channel blockers charybdotoxin and apamin, and it is now generally believed that the blockade is due to inhibition of endothelial cell hyperpolarization and generation of the EDHF signal (12,17,19). Whereas there is evidence that epoxyeicosatrienoic acids (EETs) (5,26,44), potassium ions (3,19), and hydrogen peroxide (36) can act as an EDHF in some arteries, the identity of EDHF remains controversial and may differ among species and vascular segments (6,15,20,22,55).…”