Turtles exhibit intermittent ventilatory patterns in which brief periods of ventilation are interspersed with apneas of variable duration. Associated with this ventilatory pattern are cyclic changes in pulmonary and systemic blood flow. Due to the anatomy of the heart, these changes in blood flow may be associated with both right-to-left (R-L; pulmonary bypass) and left-to-right (L-R; pulmonary recirculation) intracardiac shunting. These cardiovascular changes can influence pulmonary gas exchange, affecting both CO2 and O2 transport. The functional significance of intracardiac shunting has remained primarily speculative. This study will determine the physiological role of intracardiac shunting during intermittent ventilation in turtles. Specifically the study will determine; 1) the flow ratios between the pulmonary and systemic circuits (i.e. R-L and L-R shunts) during ventilation and apnea and 2) the effects of cholinergic and adrenergic blockade on the direction and magnitude of intracardiac shunt and correlate these effects with changes in ventilatory patterns, pulmonary gas exchanged and blood gas transport. Studies will be conducted in the turtle, Pseudemys scripta, in vivo at 25oC. All objectives will be accomplished in chronically instrumented, freely diving, spontaneously ventilating animals. Continuous measurements of pulmonary and systemic blood flows, ventilation, gas exchange, lung gases and blood gases will be made. The results will be analyzed within the context of several current hypotheses addressing the respiratory "benefits" of intracardiac shunting. The hypotheses presented are not necessarily novel, however, the experiments outlined in this proposal will for the first time specifically examine the functional significance of this distinctive type of cardiovascular regulation.
