Fig. 3. (A) Cumulative concentration–response curve showing the effect of 3-morpholinosydnonimine (SIN-1; from 10^–7 to 10^–5 mol l^–1) on stroke volume (VS) and stroke work (WS) in isolated and perfused icefish hearts. (B) Effects of 3-morpholinosydnonimine (SIN-1; 10^–5 mol l^–1) before and after treatment with superoxide dismutase (SOD; 10 i.u. ml^–1) on stroke volume (VS) and stroke work (WS) in isolated and perfused icefish hearts. Percentage changes were evaluated as means ± S.E.M. of five (A) and four (B) experiments. Asterisks indicate values significantly different from the control value: ^*P<0.05; ^**P<0.025.