Influence of intermittent breathing on ventricular depolarization patterns in chelonian reptiles.

1. Alterations between two quite distinct patterns of epicardial depolarization are associated with the normal occurrence of intermittent lung ventilation in both lightly anaesthetized and unrestrained, conscious turtles (Pseudemys scripta) and tortoises (Testudo graeca). 2. During apnoea depolarization sweeps from the left to the right over the ventricular surface at a conduction velocity of 0.15 m/sec. With the onset of lung ventilation the direction of depolarization propagation over the ventricle is reversed, and conduction velocity in the epicardium falls to 0.09 m/sec. 3. Vagal stimulation and acetylcholine produce a shift from the apnoea to the breathing pattern of depolarization in intact animals, while vagal sectioning and atropine abolish all shifts. Acetylcholine reduces conduction velocity but has no effect on the strength of contraction of isolated cardiac muscle strips from turtle hearts. Changes between the two patterns of ventricle depolarization are likely produced by vagal innervation of the rudimentary conduction system of the chelonian heart. 4. Experimental induction of the depolarization pattern of the ventricle normally evident during lung ventilation produces an improved separation of oxygenated and deoxygenated blood within the anatomically undivided chelonian ventricle. It is suggested that changes in ventricle depolarization patterns during intermittent lung ventilation may be an active component of the cardiovascular responses controlling intracardiac blood shunting in reptiles.