Fig. 4. Branchial sodium flux in G. natalis acclimated to drinking seawater (50% SW) and then infused with either a saline carrier (control), dopamine or serotonin at 2x10^-4 mol l^-1 or dibutyryl cyclic AMP (db-cAMP; membrane-permeable cAMP) at 6.1x10^-4 mol l^-1. The branchial chambers were perfused with artificial urine labelled with ²²NaCl sufficient to provide 6000 c.p.m. in a 400 µl sample. Flux rates were determined over 90 min. Changes in total [Na] were used to provide rates of net uptake (J_net); uptake of ²²Na provided rates of unidirectional influx (J_in); and unidirectional loss was calculated for each crab as J_in–J_net=J_out. For details, see Materials and methods. N=8 for each treatment. * indicates significant depression compared with saline-infused control crabs; indicates significant difference to corresponding rate in crabs acclimated to freshwater in Fig. 3.