• adrenergic;
  • autonomic;
  • gastrointestinal;
  • internal anal sphincter;
  • motility;
  • sympathetic

Abstract  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 μmol L−1 and N(ω)-nitro-l-arginine 100 μmol 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 μmol L−1) or phentolamine (3 μmol L−1). The sympathetic neurotransmitter noradrenaline (NA) also caused concentration-dependent (10 nmol L−1–100 μmol L−1) contractions which were abolished by phentolamine revealing a small relaxation that was abolished by propranolol (3 μmol 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 α-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.