Fig. 3. (A) The effects of acute hypercarbia (indicated by the shaded area) on cardiac frequency (fH) in control (open circles; N=16) and atropinised (shaded symbols; N=7) rainbow trout (Oncorhynchus mykiss). The data are presented as means ± 1 S.E.M.; significant differences (P<0.05) from pre-hypercarbic values (time=0) are denoted by a horizontal line. (B) Relationships between water P_CO2 (Pw_CO2) and fH in intact (open circles; r²=0.97), atropinised (shaded circles; r²=0.75) and gill-extirpated (filled circles; r²=0.90) rainbow trout. The data represent mean values for 2 min intervals immediately prior to, and during, the 20 min period of hypercarbia. The non-linear regression (four-parameter exponential decay) for the intact fish was generated using iterative curve-fitting software (Sigmaplot 2001; SPSS Inc.). 1 mmHg=0.133 kPa.