Adrenergic and Cholinergic Regulation of Intracardiac Shunting

Reciprocal variations in the heart rate (HR), pulmonary vascular resistance (Rpul) and pulmonary blood flow (Q̇pul) are associated with intermittent lung breathing in reptiles. During ventilation Rpul decreases and the HR and the Q̇pul increase. In contrast, Rpul increases and the HR and Q̇pul decrease during apnea. Besides these changes, intermittent ventilation is associated with changes in the distribution of blood flow between the pulmonary and systemic circulations. A right-to-left (R-L) intracardiac shunt predominates during apnea, while during ventilation a left-to-right intracardiac (L-R) shunt predominates. These changes in the HR, Q̇pul, Rpul, and intracardiac shunting may be under adrenergic and cholinergic control. Recent experiments in the turtle Pseudemys scripta support this hypothesis. In this species, cholinergic stimulation resulting from electrical stimulation of vagal efferent nerves or infusion of acetylcholine resulted in a bradycardia, an increased Rpul, a reduced Q̇pul, and the development of a net R-L intracardiac shunt. The net R-L shunt flow was 6 mL/min/kg and represented approximately 40% of the systemic blood flow. The changes in HR, Rpul, and Q̇pul were eliminated by atropine. The R-L shunt was also eliminated by atropine. In contrast, adrenergic stimulation, resulting from electrical stimulation of vagal afferent nerves or infusion of epinephrine resulted in a tachycardia, a decrease in Rpul, an increased Q̇pul, and the development of a net L-R shunt. The net L-R shunt flow was 28 mL/min/kg, representing 58% of the Q̇pul. Preliminary evidence suggested that the changes in the HR, Q̇pul, and Rpul during vagal afferent stimulation were reduced by propranolol. The results of this study support the hypothesis that the size and direction of the intracardiac shunt is determined by the relative resistances offered by the pulmonary and systemic circulations.