Similarity of relaxations evoked by BRL 34915, pinacidil and field‐stimulation in rat oesophageal tunica muscularis mucosae

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Abstract
1 In the rat oesophageal tunica muscularis mucosae (TMM) the potassium channel openers, BRL 34915 and pinacidil, raised the threshold for concentration-dependent K+ contractions, suppressed contractions evoked by field stimulation of the TMM in the presence of tetrodotoxin (TTX) and tetraethylammonium (TEA), and relaxed tonic contractions resulting from muscarinic cholinoceptor stimulation. 2 BRL 34915 and both (+)− and (−)-pinacidil increased 86Rb efflux from tracer-loaded tissues; nifedipine abolished this effect. 3 Relaxations produced by potassium channel openers were inhibited by a temperature drop from 37°C to 26.5°C, an increase in extracellular K+ concentration to 64 mm, and treatment with the calcium channel antagonist, nifedipine. The same treatments also blocked field stimulation-evoked TTX-insensitive relaxations. 4 It is concluded that field stimulation of rat oesophageal smooth muscle in the presence of cholinoceptor-induced tone results in an increase in K+ permeability that is directly or indirectly coupled to Ca2+ influx through potential-operated channels.