Regulation of central vascular blood flow in the turtle

The purpose of this study was to determine the effects of vagal nerve stimulation on the pulmonary and systemic circulations in the turtle Pseudemys scripta. The heart rate (HR), systemic vascular resistance (Rsys), pulmonary vascular resistance (Rpul), total pulmonary blood flow (Qpul), and total systemic blood flow (Qsys) were measured during electrical stimulation of the vagal efferent and the vagal afferent nerves. Vagal efferent nerve stimulation resulted in a bradycardia, increased Rsys and Rpul, and a 60% reduction in the Qpul and Qsys. These cardiovascular changes were eliminated after an intravenous injection of atropine. In contrast, vagal afferent nerve stimulation resulted in a tachycardia, a twofold increase in Rsys, a reduction in Rpul, and an 85% increase in Qpul. These changes were eliminated after pretreating the animals with bretylium tosylate (10 mg/kg). An intravenous infusion of epinephrine (0.1 micrograms/kg) produced cardiovascular changes similar to vagal afferent stimulation. The cardiovascular changes resulting from afferent and efferent nerve stimulation were similar to the cardiovascular adjustments often associated with intermittent lung ventilation in reptiles. The results of our study suggest that such cardiovascular changes are under cholinergic and adrenergic control.