Comparative effects of some autonomic drugs on lizard (Agama agama) and tortoise (Kinixys crosa) isolated rectum

The effects of some cholinomimetic and sympathomimetic drugs have been investigated on the isolated rectal muscles of West African rainbow lizard (Agama agama) and land tortoise (Kinixys crosa). Acetylcholine and its natural or synthetic analogues evoked concentration-related atropine-sensitive contractions of the muscle preparations, whereas all the catecholamines examined relaxed the lizard (and guinea-pig), but not the tortoise, isolated rectum in a dose-dependent fashion. The ganglionic stimulant drugs used, nicotine and DMPP, did not produce any effect on the isolated rectum of the two reptiles, although they contracted the guinea-pig isolated rectum in a concentration-dependent manner. Concentrations of physostigmine, which induced spasms or contractions in the isolated rectum of the guinea-pig, failed to excite reptilian rectal muscle preparations. Histamine induced biphasic responses in the lizard isolated rectum without affecting the isolated rectum of the tortoise. Serotonin (5-HT) which contracted the isolated rectum of the guinea-pig (like histamine) also provoked variable contractions of the lizard rectum but did not affect the tortoise isolated rectum. It is therefore concluded that although the rectal smooth muscles of these two terrestrial West African reptiles possess cholinergic innervation with mainly "muscarinic" cholinoceptors, their autonomic innervation differs from that of a conventional mammalian (e.g. guinea-pig) gastrointestinal tract smooth muscle.