An inhibitory effect of histamine on the rat anococcygeus muscle

Abstract

The rat anococcygeus muscle contracts in response to noradrenaline, 5-hydroxytryptamine (5-HT), acetylcholine and carbachol (Gillespie 1972). It is also contracted by high doses of histamine. However, the responses to histamine showed tachyphylaxis after a single dose. In a similar preparation in the cat (Gillespie & McGrath 1974) and the rabbit (Creed et a1 1977), histamine-induced contractions were abolished by mepyramine suggesting an action on histamine HIreceptors. It was also observed that metiamide, an H,-receptor antagonist (Black et a1 1973) potentiated histamine-induced contractions in the rabbit anococcygeus which might suggest the presence of histamine H,-receptors in this species. This communication, reports the inhibitory effect of histamine on the rat isolated anococcygeus muscle.Adult male rats (ca. 250 g) were killed by a blow on the head and then bled. The two anococcyeus muscles were removed according to Gillespie (1972) and suspended in a 20.0 ml organ bath containing aerated Tyrode solution (NaCI 137; KCI 2.7; CaCl, 1.8; MgCI, 0.9; NaH,PO, 0.3; NaHCO, 11.9 and glucose 5.6 mmol litre-') at 37 "C. Isotonic contractions (under a tension of 0.5 g) were recorded. Drugs used were: histamine acid phosphate; dimaprit; mepyramine maleate; impromidine; cimetidine; metiamide; acetylcholine chloride; carbachol ; ( -)-noradrenaline bitartrate and 5-hydroxytryptamine creatinine sulphate. All drug concentrations refer to the base.The muscle has no intrinsic tone, hence inhibitory effects of histamine and two selective H,-receptor agonists-dimaprit (Parsons et al 1977) and impromidine (Durant et a1 1978) were tested against agonistinduced contractions in the presence of mepyramine (1 p~) to block HI-receptors. In preliminary experiments when noradrenaline, carbachol and ACh were used as agonist, very high concentrations of histamine (> M) were needed to demonstrate an inhibitory effect. However, much lower doses (approx. 2 x M)