Effects of acute cooling and bradycardia on central venous pressure and cardiac function in Nile tilapia (Oreochromis niloticus).

We developed and validated a surgical technique to measure central venous pressure (CVP) in Nile tilapia, and investigated the effects of an acute temperature decrease (from 30 vs. 24 °C) and changes in heart rate (f_H) using zatebradine hydrocholoride, which decreases intrinsic f_H, on this species' cardiac function. As predicted, f_H and cardiac output ( $\dot{Q}$ ) were ~ 40% lower in the acutely cooled fish, and both groups had very comparable (i.e., within 10%) values for stroke volume (V_S)_. The CVP of fish acutely exposed to 24 °C was consistently ~ 0.04 kPa higher than in those measured at 30 °C across all concentrations of zatebradine (i.e., CVP increased from 0.04 to 0.11 kPa vs. - 0.01-0.07 kPa for 24 vs. 30 °C tilapia, respectively, as f_H was reduced). However, this did not result in an increase in V_S due to a right-shifted relationship between CVP and V_S for the 24 °C fish. These data suggest that the V_S of tilapia is less sensitive to changes/increases in CVP when temperature is acutely lowered, and that regardless of increases in preload (CVP), $\dot{Q}$ is primarily modulated by f_H in this species.