Fig. 8. The seasonal effect of dopamine (DA) on 5-hydroxytryptamine (5-HT)-induced clearance rates. To induce maximal clearance during the initial run, gill tissues were first exposed to 10^–5 mol l^–1 5-HT for 5 min. Then, between the initial and treatment runs, the gills were exposed to DA for 5 min. Data are the means (± s.e.m.) of the ratios of the treatment rate to the maximal rate. Sample sizes ranged from 4 to 7 at each concentration of DA. The regression lines are significantly different (P<0.05).