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

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 degrees C to 26.5 degrees 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.