Pulmonary stretch receptors regulate heart rate and pulmonary blood flow in the turtle, Pseudemys scripta

Abstract

1. 1. The coupling of ventilatory and circulatory activity present during normal periodic breathing in turtles was produced experimentally during artificial ventilation via lung cannulae. Heart rates and left pulmonary minute flow of 25–30 beats/min and 5–6 ml/min associated with voluntary breathing changed to 3–10 beats/min and 0.1–0.5 ml/min during apnoea. Artificial tidal ventilations with air promptly brought heart rate and pulmonary flow back to 75% of the values during voluntary breathing. 2. 2. Step-wise imposed changes in lung volume caused reductions in heart rate and pulmonary flow on withdrawals and prompt restoration of initial values on air injection. 3. 3. Lung injection of N 2, O 2 or CO 2 enriched gas gave responses similar to injection of air and the involvement of pulmonary chemo-sensitive (CO 2) receptors cannot be important in eliciting the cardio-respiratory coupling. Measurement of intrapulmonary pressure revealed the circulatory changes to be intimately related to the recorded pressure and/or the associated pulmonary volume change. 4. 4. A change in intrapulmonary pressure and/or volume produced by placing turtles at variable depths in a water column caused graded changes in heart rate and pulmonary flow with depth. Atropinization completely abolished the coupling between pulmonary stretch and cardiovascular changes. 5. 5. The presence of a pulmonary stretch receptor function is discussed in a phylogenetical perspective. Its importance is amplified in periodic breathers like chelonians and especially in aquatic forms where lung volume changes occur with depth and during breathholding due to aquatic CO 2 elimination through the skin.