Cyclic nucleotide-dependent enzyme secretion in the rat lacrimal gland

To characterize the role of cyclic nucleotides in secretion of enzymes by the lacrimal gland, pieces of rat exorbital glands were perfused with (1) 8-bromoadenosine-3',5'-cyclic monophosphate (8 Br cyclic AMP), (2) 8-bromoguanosine-3',5'-cyclic monophosphate (8 Br cyclic GMP), (3) forskolin, a stimulator of adenylate cyclase activity, (4) 3-isobutyl-1-methylxanthine (IBMX), an inhibitor of phosphodiesterase activity, or (5) carbachol, a cholinergic agonist. As a measure of enzyme secretion, timed collections of the perifusate effluent were analysed for peroxidase, an enzyme secreted by the lacrimal gland. Control peroxidase secretion was 0.3-0.9 (u./min per milligram protein). Peroxidase secretion was stimulated by 8 Br cyclic AMP (1 mM), but not by 8 Br cyclic GMP (1 mM). A 2-fold increase was detected. Peroxidase secretion was also stimulated by forskolin (60 microM), IBMX (1 mM), and the cholinergic agonist carbachol, which all stimulated peroxidase secretion 2-or 3-fold. The effect of maximally effective concentrations of IBMX (1 mM) and carbachol (0.1 mM) on secretion was additive. Finally, Ca2+ depletion in the presence of EGTA (1 mM) inhibited both IBMX-and carbachol-induced secretion by 45% and 60% respectively. We conclude that cyclic AMP, but not cyclic GMP, can stimulate lacrimal gland enzyme secretion. Cyclic AMP appears to utilize a pathway separate from but convergent with cholinergic agonists.