Protein secretion induced by isoproterenol or pentoxifylline in lacrimal gland: Ca2+ effects

In exorbital lacrimal glands, pentoxifylline (a methylxanthine) induces labeled protein secretion in a dose-related manner: the half-maximal and maximal stimulations are at 4 and 10 mM, respectively. In the presence of papaverine (10(-5) M), a phosphodiesterase inhibitor, labeled protein discharge is strongly stimulated by isoproterenol, via beta-adrenergic receptors: the maximal response is at 10(-6) M. l-Propranolol specifically inhibits the secretory stimulation to isoproterenol in a dose-related manner: for 5 X 10(-6) M isoproterenol in the presence of 10(-5) M papaverine, the half-maximal and maximal inhibitions are at 3 X 10(-7) and 10(-5) M, respectively. The beta-adrenergic response is mimicked by the adenosine 3',5'-cyclic monophosphate (cAMP) analogue dibutyryl cAMP (DBcAMP) at a 10(-3) M concentration. The time course of labeled protein secretion induced by pentoxifylline, DBcAMP, and isoproterenol shows a latency. In the presence or absence of extracellular calcium, pentoxifylline and isoproterenol immediately increase the cAMP intracellular level. Extracellular calcium omission increases the observed latency and also affects the maximal rate of protein secretion. As opposed to the cholinergic agonist, pentoxifylline has only a slight but sustained effect on 45Ca efflux, whereas isoproterenol has none. These data suggest that labeled protein secretion, such as that of peroxidase, can also be stimulated in rat exorbital lacrimal gland, through beta-adrenergic receptors; in the stimulation evoked by a beta-adrenergic agonist, DBcAMP, or methylxanthine, calcium could play a key role.