Effects of hypoxia on the venous circulation in rainbow trout (Oncorhynchus mykiss)

Hypoxia in fish is generally associated with bradycardia while cardiac output (Q˙) remains unaltered or slightly increased due to a compensatory increase in stroke volume (SV). Rainbow trout (Oncorhynchus mykiss) were subjected to severe (PWO2=7.3±0.2 kPa) or mild (PWO2=11.5±0.2 kPa) hypoxia. Central venous pressure (Pven), dorsal aortic pressure (Pda), heart rate (fH) and Q˙, were recorded in vivo. Both levels of hypoxia triggered a significant increase in Pven. Severe hypoxia was associated with bradycardia and unaltered Q˙, whereas mild hypoxia was associated with a small but significant increase in Q˙ and no bradycardia. These findings indicate that an increase in Pven promotes an increase in SV during hypoxia. Since mild hypoxia increased Pven, Q˙ and SV without bradycardia or reduced systemic resistance (Rsys), we hypothesize that an active increase in venous tone serving to mobilize blood to the central venous compartment in order to increase cardiac preload and consequently SV, is an important cardiovascular trait associated with hypoxia. Pharmacological pre-treatment with prazosin (1 mg kg-1) did not conclusively reveal the underlying mechanisms to the observed changes in Pven. This study discusses the influence of venous pooling, reduced Rsys and altered venous tone on changes in Pven observed during hypoxia.