Amoebic gill disease and natriuretic peptide receptors in the gills of Atlantic salmon

Amoebic gill disease (AGD) is a problem in the farming of Atlantic salmon, and may compromise osmoregulatory, cardiovascular and respiratory functions. We examined the effects of AGD on atrial and C-type natriuretic peptide (ANP and CNP) stimulated branchial cyclic GMP formation, since natriuretic peptides (NPs) are involved in cardiovascular function and osmoregulation. NPs act via guanylyl cyclase receptors (NPR), which stimulate cGMP formation. NPR activity was measured by ANP and CNP stimulation of branchial cGMP formation, and compared between diseased and healthy salmon over an 11 day AGD infection. We also measured plasma osmolality. Osmolality increased in AGD infected salmon from an initial 355 mmol.kg-1 to 411 mmol.kg-1 at 11 days. There was no evidence that branchial cGMP formation changed in response to AGD. In all groups, CNP stimulation of guanylyl cyclase was 190% of basal rate, whereas ANP was 150% of basal. After 11 days, all groups were given a 4 h freshwater bath, the usual treatment for AGD. Another group was given a seawater to seawater transfer, to control for handling. In this group, plasma osmolality at 11 days was the same as in AGD fish. This elevation may be due to these fish experiencing disturbance for the first time in 11 days. ANP and CNP branchial NPR activity at the conclusion of the 4 h transfers was elevated in all groups compared to that at 11 days. The increased cGMP formation in the handling control suggests a NPR response to the transfer/handling stress. AGD fish demonstrated the greatest elevation in ANP and CNP guanylyl cyclase activity immediately following the bath; these values were greater than in the control groups. The AGD infected salmon, therefore, responded more emphatically to the freshwater treatment, suggesting that the NP system is involved in some aspects of AGD.