Smooth muscle mediates circumferential conduction of hyperpolarization and relaxation to focal endothelial cell activation in large coronary arteries

Endothelium-Derived Hyperpolarizing Factor and Myoendothelial Coupling: The in vivo Perspective

The endothelium controls vascular tone adopting blood flow to tissue needs. It releases chemical mediators [e.g., nitric oxide (NO), prostaglandins (PG)] and exerts appreciable dilation through smooth muscle hyperpolarization, thus termed endothelium-dependent hyperpolarization (EDH). Initially, EDH was attributed to release of a factor, but later it was suggested that smooth muscle hyperpolarization might be derived from radial spread of an initial endothelial hyperpolarization through heterocellular channels coupling these vascular cells. The channels are indeed present and formed by connexins that enrich in gap junctions (GJ). In vitro data suggest that myoendothelial coupling underlies EDH-type dilations as evidenced by blocking experiments as well as simultaneous, merely identical membrane potential changes in endothelial and smooth muscle cells (SMCs), which is indicative of coupling through ohmic resistors. However, connexin-deficient animals do not display any attenuation of EDH-type dilations in vivo, and endothelial and SMCs exhibit distinct and barely superimposable membrane potential changes exerted by different means in vivo. Even if studied in the exact same artery EDH-type dilation exhibits distinct features in vitro and in vivo: in isometrically mounted vessels, it is rather weak and depends on myoendothelial coupling through connexin40 (Cx40), whereas in vivo as well as in vitro under isobaric conditions it is powerful and independent of myoendothelial coupling through Cx40. It is concluded that EDH-type dilations are distinct and a significant dependence on myoendothelial coupling in vitro does not reflect the situation under physiologic conditions in vivo. Myoendothelial coupling may act as a backup mechanism that is uncovered in the absence of the powerful EDH-type response and possibly reflects a situation in a pathophysiologic environment.

Intact rat superior mesenteric artery endothelium is an electrical syncytium and expresses strong inward rectifier K+ conductance

BACKGROUND AND PURPOSE

Vascular endothelial and smooth muscle cell phenotypes may change dramatically after isolation and in cell cultures. This study was designed to investigate gap junctions coupling in an integrated intact preparation and to test if K(IR) channels modulate resting membrane conductance in "in situ" endothelial cells (EC), and acetylcholine (ACh)-evoked relaxation of the rat superior mesenteric artery.

EXPERIMENTAL APPROACH

Whole cell blind patch recordings of ionic currents from in situ EC, dye-coupling experiments, and functional studies were performed in rat superior mesenteric artery.

KEY RESULTS

EC were dye-coupled through gap junctions. 18β-glycyrretinic acid (25 μM) decreased outward and inward currents, the 80% decay of time and time constant of the capacitative transients, capacitance, and increased input resistance. Barium chloride (30 μM) decreased resting and ACh-evoked inward currents, the sensitivity of ACh-evoked relaxation, and decreased both the sensitivity and the maximal relaxation to S-nitroso-N-acetyl penicillamine in arteries with, but not in arteries without endothelium.

CONCLUSIONS

The present results suggest that the EC layer of this large artery is electrically coupled, and that K(IR) channels regulate resting inward conductance, hence suggesting that they are of importance for resting membrane potential in in situ EC. Moreover, EC K(IR) channels are involved in ACh-evoked relaxation.

Connexins and gap junctions in the EDHF phenomenon and conducted vasomotor responses

It is becoming increasingly evident that electrical signaling via gap junctions plays a central role in the physiological control of vascular tone via two related mechanisms (1) the endothelium-derived hyperpolarizing factor (EDHF) phenomenon, in which radial transmission of hyperpolarization from the endothelium to subjacent smooth muscle promotes relaxation, and (2) responses that propagate longitudinally, in which electrical signaling within the intimal and medial layers of the arteriolar wall orchestrates mechanical behavior over biologically large distances. In the EDHF phenomenon, the transmitted endothelial hyperpolarization is initiated by the activation of Ca(2+)-activated K(+) channels channels by InsP(3)-induced Ca(2+) release from the endoplasmic reticulum and/or store-operated Ca(2+) entry triggered by the depletion of such stores. Pharmacological inhibitors of direct cell-cell coupling may thus attenuate EDHF-type smooth muscle hyperpolarizations and relaxations, confirming the participation of electrotonic signaling via myoendothelial and homocellular smooth muscle gap junctions. In contrast to isolated vessels, surprisingly little experimental evidence argues in favor of myoendothelial coupling acting as the EDHF mechanism in arterioles in vivo. However, it now seems established that the endothelium plays the leading role in the spatial propagation of arteriolar responses and that these involve poorly understood regenerative mechanisms. The present review will focus on the complex interactions between the diverse cellular signaling mechanisms that contribute to these phenomena.

Normothermic and hypothermic models for studying the deleterious effects of hypoxia-reoxygenation on EDHF-mediated relaxation in isolated porcine coronary arteries

INTRODUCTION

The vasomotor response of the coronary artery is altered by hypoxia-reoxygenation (H-R) induced damage. The aim of our study was to compare and evaluate normothermic and hypothermic models which are suitable for future drug studies of vasoprotective action against H-R injury.

METHODS

Porcine coronary arterial rings were isolated and placed in Krebs-Henseleit (K-H) solution. Rings were exposed to normoxic conditions (control group) and two different H-R conditions: the first induced by a 95% N(2)-5% CO(2) gas mixture (40- and 60-min hypoxia) in a normothermic protocol, and the second induced by hypothermic (4 degrees C) hypoxia-reoxygenation in an air-tight beaker filled with K-H solution (24- and 48-hours hypoxia). Reoxygenation was applied by introducing K-H solution aerated with a 95% O(2)-5% CO(2) mixture under normothermic (37 degrees C) conditions. To test the EDHF-mediated relaxation by substance P, rings were first incubated in L-NNA, nitric oxide synthase inhibitor, and indomethacin, cyclooxygenase inhibitor, and then pre-contracted with thromboxane analogue U-46619. Analysis of the maximum relaxation of the arterial rings was performed by one-way ANOVA, followed by Bonferroni's post-test.

RESULTS

Distal segments of the coronary artery responded faster to contraction induced by U-46619 and were relaxed by substance P to a greater extent than proximal segments. Maximal relaxations of arterial rings induced by a 10 nM solution of substance P were significantly reduced (p<0.001) from the values for normoxic rings (81.0+/-1.0%, n=30) after 40-min H-R (50.5+/-5.3%, n=30), 60-min H-R (32.1+/-3.5%, n=30), 24-hours hypothermic H-R (56.0+/-2.3%, n=30) and after 48-hours hypothermic H-R (38.5+/-5.1%, n=30).

CONCLUSIONS

The model employing 40-min normothermic H-R is as effective as 24-hours hypothermic H-R, and 60-min normothermic H-R as 48-hours hypothermic H-R for studying the deleterious effects of H-R on EDHF-mediated relaxation.