Evidence of a respiratory role for the hypoxic bradycardia in the dogfish Scyliorhinus canicula L.

Exposure to moderate hypoxia (P1, O2 50 mmHg) at 15 degrees C caused a bradycardia, with fH falling to approximately 50% of the normoxic level. Injection of atropine abolished the hypoxic bradycardia. Ventilation rate did not change during hypoxia. Atropinization caused significant reductions in the mean hypoxic levels of Pa, O2, Pv, O2 and Cv, O2. Calculated diffusive conductance for oxygen (DO2) was reduced by 28% following abolition of the hypoxic bradycardia, despite an assumption that cardiac output was unchanged. Factors, other than relative flow rates of water and blood, are clearly implicated in determining the effectiveness of gas exchange over the counter-current of blood and water at the gills of the dogfish.