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

Abstract

1The 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. 2 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 (I x 10-6M). The resting membrane potential, measured between spontaneous spike potentials, was -45 ± 3.0 mV (n = 224); the space constant (A) was 1 .13 ± 0.1 mm (n = 13). Spikes usually overshot by approximately 15 mV. 3 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 Cl --withdrawal but was increased in Na'-deficient solutions with or without tetrodotoxin (TTX; I x 10-6M). 4 Field stimulation (1-20 Hz, 0.5 ms, supramaximal voltage) produced inhibitoryjunction potentials (i.j.ps) and relaxed tone; at high frequencies (50 Hz or greater), contractions were observed but excitatory junction potentials (ej.ps) were not. Ij.ps and relaxations were inhibited by apamin (1 X 10-6M), TTX (I X 10-6M) but not by atropine (1 x 10-6M), phentolamine (I x 10-6M) or hexamethonium (1 x 10-6M). 5 Ij.ps were reduced by hyperpolarization and enhanced by depolarization of the membrane by current pulses (15 s). The mean equilibrium potential for the ij.p. was -94 mV (correlation coefficient, y = 0.71, n = 5, p <0.001). Ij.ps were enhanced in K+-deficient solutions and reduced in K+-rich solutions. Together these results suggest that the ij.p. is mediated by an increased GK. The absence of [Ca2"0 or the presence of Mn2+ (2 mM) abolished the ij.p.; in contrast Na+-deficient or Cl -free solutions were ineffective in this respect. 6 Tetraethylammonium (5-50 mM) 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.

Functional evidence for purinergic inhibitory neuromuscular transmission in the mouse internal anal sphincter

“…Our results suggest that a purinergic pathway significantly contributes to inhibitory motor innervation in this animal model. This conclusion is in agreement with earlier microelectrode studies of the guinea pig IAS (35,39) as well as previous contractile studies of the rabbit and rat IAS (14,32). A purinergic inhibitory neural pathway has been described for the human colon (20,28), and there is preliminary evidence that this pathway may contribute to neurally evoked relaxations in the human IAS as well (5).…”

“…Membrane hyperpolarization in adjacent electrically coupled smooth muscle cells leads to the closing of voltage-dependent Ca 2ϩ channels and contractile inhibition (43). To date, the electrical events underlying inhibitory transmission to the IAS have only been described in one species, i.e., the guinea pig (35,39). Inhibitory nerves were reported to generate a fast IJP that could be reduced by purinergic antagonists and a slower IJP reduced by antagonists of the NO pathway.…”

Mitochondrial regulation of the cytosolic Ca2+ concentration and the InsP3‐sensitive Ca2+ store in guinea‐pig colonic smooth muscle

“…A longitudinal incision was made in the colon from the anal end, the submucosa removed and the circular muscle dissected from the longitudinal layer (Lim & Muir, 1983). Unless otherwise stated all experimental procedures were carried out at room temperature (20-22°C).…”

Role of alpha adrenoceptors in opossum internal anal sphincter.

“…In vitro studies from different laboratories in different species (2)(3)(4) have suggested that resting tone in the IAS is primarily due to the myogenic properties of the IAS smooth muscle. Relaxation of IAS on the other hand is neural and involves the release of noncholinergic, nonadrenergic inhibitory neurotransmitter (2,5) from the myenteric inhibitory neurons.…”