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

Abstract

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 nM) and apamin (0.25-0.50 μM), which also blocked hyperpolarization and prevented EDHF-induced relaxation. 3 The EDHF-induced current was insensitive to Ba²⁺ (20-100 μM) and/or ouabain (1 μM to 1 mM). 4 In human subcutaneous arteries and guinea-pig coronary arteries and submucosal arterioles, the EDHF-induced responses were insensitive to Ba²⁺ and/or ouabain. Increasing [K⁺]_o to 11-21 mM 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 18β-Glycyrrhetinic acid (30 μM) and carbenoxolone (100 μM) 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 (IK_Ca) and small conductance (SK_Ca) Ca²⁺-activated K⁺ channels. Strong electrical coupling between endothelial and smooth muscle cells implies that these two layers function as a single electrical syncytium. The non-specific effects of glycyrrhetinic acid precludes its use as an indicator of the involvement of gap junctions in EDHF-attributed responses. These conclusions are likely to apply to a variety of blood vessels including those of humans.