Physiological and pharmacological studies of the internal anal sphincter in the rat.

The rectoanal reflex in the rat consists of relaxation followed by contraction of the internal anal sphincter (IAS). Beta-adrenergic drugs produce prolonged relaxation of the IAS, where alpha-agonists cause brief relaxation followed by contraction. Muscarinic cholinergic drugs contract the IAS. ATP and related purines produce relaxation which is blocked by theophylline. The reflex persists during adregeneric, cholinergic, and purinergic block. The potassium-blocker apamin prevents the relaxation induced by rectal distension. This suggests that the reflex is not mediated by adrenergic, cholinergic, or purinergic transmitters. Blocking potassium conductance was apparently the only way to eliminate this reflex.