Fig. 6. The effect of L-NAME on internal contraction ratios. (A) Contractions of the same demibranch in response to 2x10^-5 mol l^-1 5HT (applied at the upward arrows). The response to each dose of 5HT was terminated by flushing the organ bath (downward arrows). L-NAME (10^-4 mol l^-1) was applied for 15 min before the second contraction. The resulting internal contraction ratio was 0.92. (B) Various concentrations of L-NAME were applied to isolated demibranchs for 15 min between their first and second contractions in response to 2x10^-5 mol l^-1 5HT Values are means ± 1 S.E.M., N=3 for all concentrations of L-NAME except 10^-4 mol l^-1 where N=4.