Effects of sympathomimetic drugs on reptilian atrial muscles

The effects of some sympathomimetic drugs on isolated atria of rainbow lizards (Agama agama Linn.) and land tortoises (Kinixys spp. Linn.) have been examined. Noradrenaline, adrenaline, isoprenaline, phenylephrine or dopamine (10(-9)-10(-5) M) induced concentration-dependent positive chronotropic and inotropic responses in spontaneously-beating right- and electrically-driven left atria of rainbow lizards or tortoises. These catecholamines produced similar responses in guinea-pig isolated atria. Phentolamine or propranolol (10(-10)-10(-6) M) competitively antagonised the positive chronotropic and/or positive inotropic responses induced by these sympathomimetic agonists in these reptilian (and mammalian) atrial muscles; probably suggesting the presence of alpha- and/or beta-adrenoceptors in reptilian atrial muscles (as in the mammalian myocardium). It is suggested that in reptilian atrial muscles, only one adrenoceptor subtype which is capable of converting its properties to either alpha- or beta-, or even alpha- and beta-adrenoceptor subtypes (as in the frog heart) depending on the environmental temperature, may exist. The use of rainbow lizard and tortoise atrial muscle preparations as laboratory models for routine pharmacological evaluation of adrenergic drugs, particularly in developing tropical African countries where these cheap, harmless reptiles abound, is strongly recommended.