Not the cAMP/PKA pathway but the cGMP/PKG pathway promotes branchial ammonia excretion in the green shore crab, Carcinus maenas

Ammonotelic animals excrete their toxic waste ammonia directly into the surrounding environment. To deal with conditions causing elevated hemolymph ammonia levels, such as feeding, an efficient ammonia excretion process must be in place to keep hemolymph ammonia levels within a tolerable range. Although the actual ammonia excretion mechanisms in fish and crustaceans have been investigated extensively, the intracellular regulation of these processes is more or less unknown. To investigate the involvement of the cAMP/PKA and cGMP/PKG pathway in the branchial excretion process, gills from osmoregulating green shore crabs, Carcinus maenas, were isolated and perfused with solutions containing either 200 or 500 μmol L^-1 NH₄Cl, mimicking the hemolymph ammonia levels in the resting state and after feeding, respectively. Basolateral application of forskolin or 8-Bromo-cAMP caused a significant decrease in the ammonia excretion rate. Moreover, cAMP activation caused an increase in metabolically generated ammonia, of which the majority was now transported towards the hemolymph side and not, as seen under control conditions, into the environment. This suggests that cAMP/PKA pathway promotes a so far not described ammonia retention mechanism. In contrast, activating the cGMP/PKG pathway by 8-Bromo-cGMP resulted in an increase of the transbranchial ammonia excretion rate, which could be partially inhibited by the PKG blocker KT5823. Participation of branchial expressed nitric oxide synthase in cGMP synthesis via the soluble guanylyl cyclase is suggested due to the observed inhibitory effect after the application of L-NAME. This study opens a new venue of investigations regarding epithelial ammonia transport regulation in animal systems.