Effects of cholinomimetic drugs on reptilian atrial muscles.

The effects of some cholinomimetic drugs have been examined on isolated atria of rainbow lizards (Agama agama Linn.) and land tortoises (Kinixys spp. Linn.). Acetylcholine (ACh, 10(-8) - 5 X 10(-5) M) caused concentration-dependent, atropine-sensitive, negative chronotropic or inotropic responses in isolated spontaneously-beating right or electrically-driven left atria of rainbow lizards and land tortoises. This prototype cholinergic drug (ACh, 10(-8) 5 X 10(-5) M) also induced atropine-sensitive, concentration-related negative chronotropic or inotropic responses in isolated atrial muscles taken from guinea-pigs. Histamine (10(-8) - 5 X 10(-5) M) produced concentration-related positive chronotropic or inotropic responses in isolated spontaneously-beating right or electrically-driven left atria of rainbow lizards and guinea-pigs without significantly affecting (P greater than 0.05) the tortoise atria. 5-Hydroxytryptamine (5-HT, 10(-7) - 5 X 10(-5) M) induced concentration-dependent positive chronotropic or inotropic responses in isolated, spontaneously-beating right or electrically-driven left atria of rainbow lizards, tortoises and guinea-pigs. Nicotine (10(-8) - 5 X 10(-5) M) had no effect on the rate of beating of the lizard or tortoise isolated right atria, but caused concentration-dependent positive chronotropic responses in spontaneously-beating isolated right atria of the guinea-pig. Furthermore, nicotine (10(-8) - 5 X 10(-5) M) had no effect on contractile tension of isolated, electrically-driven left atria of the tortoise; it induced a biphasic inotropic response in driven left atria of the rainbow lizard and produced a concentration-related positive inotropic response in electrically-driven isolated left atria of the guinea-pig.(ABSTRACT TRUNCATED AT 250 WORDS)