Effects of drugs and electrical stimulation on rainbow lizard (Agama agama Linn.) isolated gastrointestinal tract smooth muscles.

The effects of some autonomic drugs and transmural electrical stimulation have been investigated on different isolated regions (oesophagus, stomach, duodenum, ileum, colon, and rectum) of the rainbow lizard (Agama agama Linn.) alimentary canal. Acetylcholine and its natural or synthetic analogues (5 X 10(-10)-10(-6)M) contracted the rainbow lizard isolated oesophagus, duodenum, ileum, colon, and rectum (but not the stomach) in a concentration-dependent manner. Acetylcholine cholinoceptors on the lizard gut were classified as "muscarinic" since acetylcholine-induced contractions of the muscle preparations were inhibited or abolished by relatively low concentrations of atropine (10(-9)-10(-6)M), but not by d-tubocurarine or hexamethonium (at the same concentration levels. Atropine exhibited characteristics of competitive antagonism and gave a high mean pA2 value (9.40 +/- 0.25) against acetylcholine on lizard isolated rectum. Physostigmine (eserine, 10(-8)-10(-4)M) did not contract any isolated region of the lizard gut. However, low to medium concentrations of eserine (10(-8)-10(-6)M) potentiated acetylcholine-but not carbachol-induced contractions of the isolated smooth muscle segments taken from different regions of the rainbow lizard alimentary canal. The selective potentiation of ACh-elicited contractions of the muscle preparations by eserine is therefore thought to suggest the presence of acetylcholinesterases in the tissues, and their inhibition by physostigmine. Dimethylphenylpiperazinium (DMPP) or nicotine (10(-8)-10(-5)M) did not contract any of the isolated segments from the different regions of the rainbow lizard alimentary canal. Adrenaline and all its analogues examined (10(-8)-10(-5)M) dose-dependently relaxed isolated segments from different regions (except the stomach) of the rainbow lizard gastrointestinal tract. The catecholamine-induced relaxations of the lizard gut smooth muscles were inhibited or abolished by low concentrations of phentolamine (10(-7)-2.5 X 10(-6)M). This observation probably suggests the presence of "alpha" adrenoceptors on rainbow lizard gastrointestinal tract. Repetitive transmural electrical excitation induced tetrodotoxin- (and atropine-) resistant, frequency-dependent contractions of all isolated segments taken from different regions (except the stomach) of the lizard alimentary canal. The findings that medium to high concentrations of physostigmine (10(-6)-10(-4)M), DMPP and nicotine (10(-6)-10(-5)M) failed to elicit intrinsic nerve-mediated contractions, coupled with the fact that tetanic transmural electrical stimulation caused tetrodotoxin-resistant contractions of the rainbow lizard isolated gut muscles, strongly suggest that the autonomic innervation of this reptilian alimentary canal differs from that of a conventional mammalian (e.g. guinea-pig) gastrointestinal tract...