Neurotransmitter interactions in smooth muscles from cat internal anal sphincter (in vitro experiments)

The contractile activity of strips (15 X 2 mm) isolated from cat internal anal sphincter (IAS) has been studied under isometric conditions. Noradrenaline (above 10(-10) mol/l) induces contraction of IAS, antagonized by phentolamine. Isoprenaline (10(-8)-10(-5) mol/l) has no effect on IAS. Acetylcholine and carbachol (above 10(-6) mol/l) result in relaxation of the IAS preparations, which is blocked only by simultaneous administration of atropin (10(-6) mol/l) and hexamethonium (10(-4) mol/l). The acetylcholine-induced relaxation is not influenced by guanetidine and reserpine, but it is antagonized by tetrodotoxin (TTX) (10(-7) g/ml). Nicotine (10(-6)-10(-4) mol/l) also causes IAS relaxation which is not influenced by atropin, being blocked by hexamethonium and antagonized by TTX. The IAS relaxation induced by acetylcholine and nicotine is not associated with adrenergic mechanisms; it is assumed to be the result of a release of noncholinergic, nonadrenergic inhibitory neurotransmitter by means of presynaptic postganglionic cholinergic receptors. Data are also presented concerning the release of noradrenaline through presynaptic N-cholinergic receptors, when nicotine and high acetylcholine concentrations are applied after treatment of the preparations with scorpion venom or after increasing the K+-concentration in the nutrient solution to 16-20 mmol/l. The complex neurotransmitter interactions at IAS level are discussed.