Fig. 2. Experimental designs used to construct internal and external contraction ratios. In general, internal contraction ratios were used to ascertain elements of the signaling pathway leading to potentiation e.g. enzyme inhibitors that block potentiation, whereas external contraction ratios were used to ascertain elements that mimic potentiation e.g. DEANO, and to determine if any of the enzymes are involved in the initial response to 5HT. Traces represent contraction of the gill muscles in response to 5-hydroxytryptamine (5HT; upward arrows) followed by flushing of the organ bath and relaxation of the muscle (downward arrows). The enzymes inferred for the signaling pathway are boxed. -, enzyme inhibitors; +, stimulators; h_c, height of control contraction; h_t, height of treatment contraction; nNOS, neuronal nitric oxide synthase; NO, nitric oxide; L-NAME, a NOS inhibitor; sGC, soluble guanylate cyclase; ODQ, a sGC inhibitor; cGMP, cyclic GMP; PKG, protein kinase G; Rp-8-CPT-cGMPS, a PK-G inihibitor; DEANO, an NO generator; db-cGMP, dibutyryl cGMP, a cell permeable cGMP analog.

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