Adrenergic stimulation of lacrimal protein secretion is mediated by G(q/11)alpha and G(s)alpha.

PURPOSE: The intent of this work was to continue the characterization of G protein coupling of receptors to lacrimal secretion by determination of whether the alpha subunits of the heterotrimeric G proteins G(q/11) and G(s) couple alpha and beta-adrenergic receptor activation to stimulation of protein secretion by isolated lacrimal acini. In addition, we assessed the possibility that the G beta gamma dimer influences stimulated lacrimal protein secretion.
METHODS: Primary cultures of rabbit lacrimal acini were permeabilized by streptolysin-O (SLO) to allow cellular insertion of polyclonal antibodies to G(q/ll)alpha, G(s)alpha or G beta or GDP beta S. Following this, secretion of exocytotic protein was measured in response to vehicle, the alpha(1)-adrenergic agonist phenylephrine, the beta-adrenergic agonist isoproterenol, the cholinergic agonist carbachol or vasoactive intestinal peptide (VIP).
RESULTS: Phenylepherine and isoproterenol resulted in a significant increase in protein release from cultured lacrimal acini. The increase in secretion elicited by the adrenergic agonists, however, was much less than that induced by either carbachol or VIP. Antibody to G(q/11)alpha blocked phenylephrine stimulated secretion 32% but had no effect on isoproterenol or VIP stimulation of secretion. In contrast, the same antibody blocked carbachol stimulated secretion by 72%. Antibody to G(s)alpha blocked isoproterenol stimulated secretion 20%, had no effect on phenylephrine stimulation, blocked carbachol stimulation by 27% and VIP stimulation by 69%. Antibody to G beta did not effect stimulation of secretion by any agonist. The degree of inhibition of secretion following exposure to GDPbetaS did not exceed that obtained with the G protein subunit antibodies.
CONCLUSIONS: The alpha subunit of G(q/11) couples activation of alpha(1)-adrenergic receptors to exocytotic release of protein in the lacrimal gland. Activation of beta-adrenergic receptors does not involve G(q/11) but is mediated by the alpha subunit of G(s). G protein coupling of the adrenergic receptors to secretion appears to be limited compared to cholinergic and VIP stimulation and might suggest the occurrence of the activation of intracellular signaling pathways independent of receptor-G protein-effector regulation of adrenergic stimulation of lacrimal secretion.