Fig. 4. Upper trace shows a representative recording of venous pressure during a full swim protocol (30 min) in an untreated sea bass, Dicentrarchus labrax. Ventral aortic occlusions for MCFP measurements at the end of each swim speed (rest, 1 BL s^-1 and 2 BL s^-1, respectively) are marked with vertical arrows. Lower panels (A-D) show mean values (+ S.E.M.) of measured and calculated venous variables at rest and while swimming at 1 and 2 BL s^-1. Filled bars represent untreated fish (N=8-9) and open bars (N=7) represent fish after treatment with prazosin (1 mg kg^-1 M_b). The variables are A, central venous pressure (PCV); B, mean circulatory filling pressure (MCFP); C, pressure gradient for venous return (PV) and D, resistance to venous return (R_v). ^*Statistically significant difference (P0.05) from resting values within each treatment and statistically significant difference (P0.05) between values before and after prazosin treatment. For a detailed description on calculations of mean values, see Materials and methods section.