Species dependent differences in the actions of sympathetic nerves and noradrenaline in the internal anal sphincter

Excitatory motor innervation to the internal anal sphincter (IAS) of the monkey, the rabbit and mouse were compared. Contractile responses to electrical field stimulation of nerves (EFS, atropine 1 mu mol L-1 and N(omega)-nitro-L-arginine 100 mu mol L-1 present throughout) were examined in isolated strips of IAS. In the monkey IAS, EFS caused frequency dependent (1-30 Hz) contractions which were abolished by guanethidine (10 mu mol L-1) or phentolamine (3 mu mol L-1). The sympathetic neurotransmitter noradrenaline (NA) also caused concentration-dependent (10 mu mol L-1 -100 mu mol L-1) contractions which were abolished by phentolamine revealing a small relaxation that was abolished by propranolol (3 mu mol L-1). In contrast, EFS caused only relaxation of the mouse and rabbit IAS which was not affected by guanethidine. Furthermore, NA relaxed these muscles and relaxation was nearly abolished by combined addition of phentolamine and propranolol. In conclusion, the monkey IAS is functionally innervated by sympathetic nerves that contract the muscle via excitatory alpha-adrenergic receptors. In contrast, no significant motor function could be identified for sympathetic nerves in the rabbit or mouse IAS although adrenergic receptors linked to muscle inhibition are present. These data reveal species dependent differences in sympathetic motor innervation and suggest that some species are more appropriate than others as models for motor innervation to the human IAS.