Identification of Nitric Oxide‐Sensitive and ‐Insensitive Forms of Cytoplasmic Guanylate Cyclase

Cytoplasmic, nitric oxide‐activated guanylate cyclases are expressed in many regions of the mammalian brain and are thought to participate in functions as diverse as synaptogenesis and long‐term potentiation. In this study, we have characterized cytoplasmic guanylate cyclases in the nervous system of an invertebrate, the American lobster. Cytoplasmic cyclase specific activity is higher in lobster nerve cord than in any other lobster tissue tested, and considerably higher than in typical rat tissues (cerebellum, lung, and liver). However, nitric oxide donors have minimal effects on lobster nerve cord cyclic GMP production, when applied either to intact tissue or to cytoplasmic extracts. Parallel immunocytochemical studies, using an anti‐cyclic GMP antibody, reveal that only a small subset of lobster neurons responds to nitric oxide with a significant elevation of cyclic GMP levels. HPLC analysis of nerve cord cytoplasm reveals two chromatographically separable cyclases, a minor nitric oxide‐sensitive form whose retention time is identical to that of the conventional mammalian enzyme and a more abundant nitric oxide‐insensitive form that appears to be novel. The physiological function and phylogenetic distribution of this nitric oxide‐insensitive enzyme, and the signaling mechanisms that regulate its activity, are not known.