Endothelium-dependent hyperpolarization and intercellular electrical coupling in guinea-pig mesenteric arterioles

1. Using the conventional whole-cell clamp method, the electrical responses of individual smooth muscle and endothelial cells to acetylcholine (ACh) w ere observed in multicellular preparations where the two types of cells rem ained in close apposition. 2. In both types of cells, ACh induced similar hyperpolarizing responses wh ich, when recorded in current clamp mode, had two phases (an initial fast a nd a second slower phase). 3. After Mocking gap junctions, including myoendothelial junctions, with 18 beta-glycyrrhetinic acid, ACh induced an outward current with two phases i n voltage-clamped endothelial cells. The outward current appeared around -9 0 mV and increased linearly with the membrane depolarization. 4. In smooth muscle cells, ACh failed to induce a membrane current after ga p junctions had been blocked with 18 beta-glycyrrhetinic acid. The inhibiti on of ACh-induced response by 18 beta-glycyrrhetinic acid was observed usin g either sharp or patch electrodes. 5. Nominally Ca2+-free solution reduced the initial phase and abolished the second phase of ACh-induced responses of endothelial cells. Both phases we re also reduced by charybdotoxin (CTX). 6. Our results indicate that in guinea-pig mesenteric arterioles, ACh hyper polarizes endothelial cells by activating Ca2+-activated K+ channels which are sensitive to CTX. On the other hand, hyperpolarizing responses detected in smooth muscle cells seem to originate in endothelial cells and conduct to the muscle layer via myoendothelial gap junctions.