Mechanisms underlying the electrical and mechanical responses of the guinea-pig internal anal sphincter to field stimulation and to drugs

Abstract

The electrical membrane characteristics and the response of the circular muscle of the guinea-pig internal anal sphincter (i.a.s.) to field stimulation were studied in vitro using intracellular microelectrodes and conventional mechanical recording techniques. The i.a.s. developed its own tone (3-4 g), following initial stretch (1 g) and spontaneous spike potentials were evident. In the absence of spike potentials, tone declined and disappeared. Tone was not significantly reduced by phentolamine (1 X 10(-6)M). The resting membrane potential, measured between spontaneous spike potentials, was -45 +/- 3.0 mV (n = 224); the space constant (lambda) was 1.13 +/- 0.1 mm (n = 13). Spikes usually overshot by approximately 15 mV. The frequency of spike potential discharge (1-3 Hz) varied with the degree of membrane depolarization, being increased in K+-rich and decreased in K+-deficient solutions or by the presence of Mn2+. It was not significantly affected by C1-withdrawal but was increased in Na+-deficient solutions with or without tetrodotoxin (TTX; 1 X 10(-6)M). Field stimulation (1-20 Hz, 0.5 ms, supramaximal voltage) produced inhibitory junction potentials (i.j.ps) and relaxed tone; at high frequencies (50 Hz or greater), contractions were observed but excitatory junction potentials (e.j.ps) were not. I.j.ps and relaxations were inhibited by apamin (1 X 10(-6)M), TTX (1 X 10(-6)M) but not by atropine (1 X 10(-6)M), phentolamine (1 X 10(-6)M) or hexamethonium (1 X 10(-6)M). I.j.ps were reduced by hyperpolarization and enhanced by depolarization of the membrane by current pulses (15s). The mean equilibrium potential for the i.j.p. was -94 mV (correlation coefficient, gamma = 0.71, n = 5, p less than 0.001). I.j.ps were enhanced in K+-deficient solutions and reduced in K+-rich solutions. Together these results suggest that the i.j.p. is mediated by an increased GK. The absence of [Ca2+]o or the presence of Mn2+ (2 mM) abolished the i.j.p.; in contrast Na+-deficient or C1-free solutions were ineffective in this respect. Tetraethylammonium (5-50mM) abolished the i.j.p.; the accompanying relaxation was reduced by about 80%. The major aspect of the relaxation to nerve stimulation is mediated by membrane hyperpolarization.