Direct and indirect actions of 5-hydroxytryptamine in the rat anococcygeus muscle

The contractile effect of 5-hydroxytryptamine on the rat isolated anococcygeus muscle was analysed. 5-Hydroxytryptamine (10(-6) g ml-1 - 1.6 X 10(-5) g ml-1) concentration-dependently contracted the anococcygeus muscle. The contractions were not competitively antagonized by methysergide (2.5 X 10(-8) g ml-1 - 10(-6) g ml-1) as the contractile response to higher concentrations of 5-hydroxytryptamine were not much reduced. The methysergide-resistant contractions to 5-hydroxytryptamine were not reduced by hyoscine (6 X 10(-7) g ml-1), tetrodotoxin (2 X 10(-7) g ml-1), but were completely abolished by phentolamine (3 X 10(-7) g ml-1). 5-Hydroxytryptamine-induced contractions were considerably reduced but not completely abolished by reserpinization. It is suggested that 5-hydroxytryptamine contracted the rat anococcygeus muscle directly (by activating tryptaminergic 'D' receptors) and indirectly by releasing noradrenaline at the nerve endings.