Adrenergic and cholinergic response of ventricular muscle from the turtle, Trachemys (Pseudemys) scripta

Muscle strips were dissected from the ventricle of the turtle, Trachemys (Pseudemys) scripta. Individual strips were placed in a muscle bath, set at a resting tension of three grams and electrically paced at 24 min(-1) for a control period of 1 hr. Cumulative dose-response curves were constructed using the adrenergic agonist, epinephrine (Epi) and the cholinergic agonist, metacholine (MCh). The administration of Epi to the muscle bath resulted in a dose-dependent increase in active tension development. The maximum tension developed was 40% above the control values and the concentration producing a half-maximal response (EC50) was 7.5 x 10(-7) M. The positive inotropic effect of Epi was diminished following the administration 1 x 10(-7) M propranolol. This reduced the maximum active tension by over 80% at the highest dose of Epi. In addition, the EC50 was increased to 8.9 x 10(-6) M. The administration of MCh resulted in a dose-dependent decrease in active tension development. The maximum decrease in active tension was 35% below the control values and the EC50 was 2.5 x 10(-6) M. The negative inotropic effect of MCh was diminished by the application of 1 x 10(-6) M atropine, with the EC50 increasing to 2.5 x 10(-4) M. These findings show that the isolated, in vitro turtle ventricle responds to adrenergic and cholinergic agonist. The results of this study support the hypothesis that the ventricular function, in reptiles, is controlled by both subdivisions of the autonomic nervous system.