Fig. 4. (Ai,Bi) The effects of rapidly switching from control saline to saline containing nicotine (10^–5 mol l^–1; hatched bars; N=18) or nicotine plus the VPAC receptor antagonist VIP_6-28 (10^–6 mol l^–1; filled bars; N=18) on catecholamine secretion in an in situ perfused posterior cardinal vein preparation of rainbow trout Oncorhynchus mykiss. After 10 min, the preparations were electrically stimulated using a frequency of either (Aii) 20 Hz or (Bii) 1 Hz and maximal catecholamine secretion rates were determined under the various conditions. An asterisk denotes a statistically significant difference from the `pre-switch' (Pre) value; a dagger represents a significant difference from the secretion rate after 10 min; a double dagger represents a significant difference from the control (salineperfused) group.