The role of angiotensin II to support arterial blood pressure following serial <scp>haemorrhage</scp> in the freshwater channel catfish (<scp><i>Ictalurus punctatus</i></scp>)

Reductions in blood volume and changes to environmental salinity are known regulators of the renin-angiotensin system (RAS) in marine and euryhaline species of fish. However, the mechanisms activating the RAS in freshwater stenohaline species remain poorly understood. The purpose of this study was to determine if reduction in blood volume stimulates the activation of the RAS in channel catfish (Ictalurus punctatus). To achieve this goal, this study sought to determine if angiotensin II (ANG II) functions to support arterial blood pressure following a moderate haemorrhage. Channel catfish (n = 12) were implanted with a dorsal aortic catheter to measure dorsal aortic pressure (P_DA), infuse drugs and sequentially withdraw four volumes of blood to produce an estimated 20% reduction in blood volume in a group of randomly sampled fish (n = 6), whereas the remaining fish (n = 6) were not haemorrhaged. Stepwise haemorrhage marginally reduced P_DA while significantly increasing heart rate. After haemorrhage or time control, infusion of the angiotensin-converting enzyme inhibitor captopril significantly reduced P_DA to a greater degree in haemorrhaged fish than non-haemorrhaged fish. Additionally, captopril reduced heart rate in haemorrhaged fish compared to non-haemorrhage fish. These results suggest that reduction in blood volume activates the RAS in the freshwater channel catfish. Also, these data suggest that the RAS of channel catfish may play a role in central neural mechanisms regulating heart rate.