Elevated retinal cGMP is not associated with elevated circulating cGMP levels in a canine model of retinitis pigmentosa

PURPOSE: To investigate whether raised levels of retinal cyclic guanosine monophosphate (cGMP) was reflected in plasma levels in PDE6A(-/-) dogs. MATERIALS AND METHODS: Retina was collected from 2-month-old wildtype dogs (PDE6A(+/+), N = 6), heterozygous dogs (PDE6A(+/-), N = 4) and affected dogs (PDE6A(-/-), N = 3) and plasma was collected from 2-month-old wildtype dogs (PDE6A(+/+), N = 5), heterozygous dogs (PDE6A(+/-), N = 5) and affected dogs (PDE6A(-/-), N = 5). Retina and plasma samples were measured by ELISA. RESULTS: cGMP levels in retinal samples of PDE6A(-/-) dogs at 2 months of age were significantly elevated. There was no significant difference in plasma cGMP levels between wildtype and PDE6A(-/-) or PDE6A(+/-) puppies. However, the plasma cGMP levels of the PDE6A(-/-) puppies were significantly lower than that of PDE6A(+/-) puppies. CONCLUSION: cGMP levels in the plasma from PDE6A(-/-) was not elevated when compared to control dogs. At the 2-month timepoint, cGMP plasma levels would not be a useful biomarker for disease.