Electrical coupling and release of K+ from endothelial cells co-mediate ACh-induced smooth muscle hyperpolarization in guinea-pig inner ear artery

• Published in: The Journal of physiology 2005-04, Vol.564 (2), p.475-487
• Main Authors: Jiang, Zhi‐Gen, Nuttall, Alfred L., Zhao, Hui, Dai, Chun‐Fu, Guan, Bing‐Cai, Si, Jun‐Qiang, Yang, Yu‐Qin
• Format: Article
• Language: English
• Subjects: Acetylcholine - pharmacology; Acetylcholine - physiology; Animals; Arteries - drug effects; Arteries - physiology; Cell Physiology; Cochlea - blood supply; Cochlea - drug effects; Cochlea - physiology; Dose-Response Relationship, Drug; Ear, Inner - blood supply; Ear, Inner - drug effects; Ear, Inner - physiology; Endothelial Cells - drug effects; Endothelial Cells - physiology; Guinea Pigs; Muscle, Smooth, Vascular - drug effects; Muscle, Smooth, Vascular - physiology; Potassium - physiology; Potassium Channels, Inwardly Rectifying - physiology; Sodium-Potassium-Exchanging ATPase - physiology
• ISSN: 0022-3751; 1469-7793
• DOI: 10.1113/jphysiol.2004.080960

The physiological basis of ACh-elicited hyperpolarization in guinea-pig in vitro cochlear spiral modiolar artery (SMA) was investigated by intracellular recording combined with dye labelling of recorded cells and immunocytochemistry. We found the following. (1) The ACh-hyperpolarization was prominent only in cells that had a low resting potential (less negative than −60 mV). ACh-hyperpolarization was reversibly blocked by 4-DAMP, charybdotoxin or BAPTA-AM, but not by N ω -nitro- l -arginine methyl ester, glipizide, indomethacin or 17-octadecynoic acid. (2) Ba 2 + (100 μ m ) and ouabain (1 μ m ) each attenuated ACh-hyperpolarization by ∼30% in smooth muscle cells (SMCs) but had only slight or no inhibition in endothelial cells (ECs). A combination of Ba 2 + and 18β-glycyrrhetinic acid near completely blocked the ACh-hyperpolarization in SMCs. (3) High K + (10 m m ) induced a smaller hyperpolarization in ECs than in SMCs, with an amplitude ratio of 0.49 : 1. Ba 2 + blocked the K + -induced hyperpolarization by ∼85% in both cell types, whereas ouabain inhibited K + -hyperpolarization differently in SMCs (19%) and ECs (35%) and increased input resistance. 18β-Glycyrrhetinic acid blocked the high K + -hyperpolarization in ECs only. (4) Weak myoendothelial dye coupling was detected by confocal microscopy in cells recorded with a propidium iodide-containing electrode for longer than 30 min. A sparse plexus of choline acetyltransferase-immunoreactive (ChAT) fibres was observed around the SMA and its up-stream arteries. (5) Evoked excitatory junction potentials (EJP) were partially blocked by 4-DAMP in half of the cells tested. We conclude that ACh-induced hyperpolarization originates from ECs via activation of Ca 2 + -activated potassium channels, and is independent of the release of NO, cyclo-oxygenase or cytochrome P450 products. ACh-induced hyperpolarization in smooth muscle cells involves two mechanisms: (a) electrical spread of the hyperpolarization from the endothelium, and (b) activation of inward rectifier K + channels (K ir ) and Na + –K + pump current by elevated interstitial K + released from the endothelial cells, these being responsible for about 60% and 40% of the hyperpolarization, respectively. The role ratio of K ir and pump current activation is at 8 : 1 or less.