The relationship between secretion of retinol and protein by the lacrimal gland

Secretion of retinol and protein by the rabbit lacrimal gland appear to be closely related, suggesting that they are secreted by the same mechanism. Pilocarpine and vasoactive intestinal peptide (VIP) both stimulate protein and retinol secretion rate in a dose-dependent manner, but the concentrations of retinol and protein in the lacrimal gland fluid are independent of fluid flow at flow rates in excess of 1 microliter/min. Under all conditions of stimulation with pilocarpine and VIP that were studied, the retinol:protein ratio in lacrimal gland fluid of normal rabbits remained constant at 3.3 ng retinol/mg protein. This correlation between retinol and protein secretion by the lacrimal gland suggests that retinol is protein-bound in lacrimal gland fluid. To identify this protein, vitamin A-deficient rabbits were treated orally with 3H-retinyl acetate. Lacrimal gland fluid was collected and analyzed for 3H-retinol and protein. The 3H-retinol in lacrimal gland fluid was identified by reverse-phase HPLC and analysis of protein by gel filtration chromatography showed that this 3H-retinol was associated with protein which eluted from the columns in the 20 kD range.