Fig. 5. The effects of 30-min exposure to acute hypoxia (P_O2<5.99 kPa) on plasma (A,B) total catecholamine levels (filled bars; N=6) and (C,D) NO_X concentrations (filled bars; N=6). Unfilled bars are basal levels prior to hypoxia exposure. In all cases, fish were exposed twice-daily to acute hypoxia for a total of 4 days, but in one series (C,D) paired blood samples were withdrawn only once each on day 1 and 4. Values are shown as means ± 1 s.e.m. A dagger denotes a significant difference (P<0.5) between the pre-hypoxia values (unfilled bars) and hypoxia exposure (filled bars). An asterisk denotes a significant difference (P<0.5) in plasma catecholamine levels during hypoxia from value of the peak response (day 2, PM). A double dagger denotes a significant difference (P<0.5) in basal or hypoxia-evoked NO levels from the lowest values (Day 1, PM).