Responses to phenethylamines and nicotine and histology of turtle atria

¹ Trinity College, Washington, D.C.

Epinephrine (E), norepinephrine (NE), tyramine (T) and nicotine were tested at various dosage levels on different atrial regions of the heart of the eastern painted turtle, Chrysemys picta picta (Schneider), and the median effective dose determined. The three phenethylamines acted according to the following pattern in their effect on rate increase, amplitude increase, and inhibition of tonus waves: E > NE > T, except for amplitude increase in the left atrium and tonus wave inhibition in both atria, where E and NE were equal. The right sino-atrium and left atrium differed in sensitivity to the drugs. The various responses, including treppe, are consonant with the theory of the release from the tissues of a potentiating substance and a depressing substance. Histological stains for chromaffin tissue, a possible source of potentiating substance, were negative, but a differentially stained type of cardiac muscle, a possible conducting tissue, is present in regions of the sinus venosus and both atria and at the base of the interatrial septum.