Fig. 16. Model depicting the signal transduction events leading to potentiation of gill muscle contraction. The coupling between the 5HT receptor is unknown, possibly via a G protein or the receptor being part of a calcium channel. The presence of calcium channels is inferred because the calcium channel blocker verapamil inhibits contraction (L. F. Gainey, personal observation). The existence of a neuronal-like NOS activated by calmodulin is based upon sequence data (L. L. Moroz and B. Untch, personal communication). The protein target(s) of phosphorylation by protein kinase G (PK-G) are also unknown, but based upon data from other molluscs, the target may be an L-type calcium channel, but the 5HT receptors or contractile proteins may also be phosphorylated. -, enzyme inhibitors; + stimulators;?, uncertainty (also broken lines). CaM, calmodulin; nNOS, neuronal-like nitric oxide synthase; NO, nitric oxide; cGMP, cyclic GMP.