Ca2+ - and cAMP-induced protein phosphorylation in lacrimal gland basolateral membranes

Abstract

Basolateral plasma membranes play an integral role in coupling of stimulus to secretion of fluid and protein from the lacrimal gland. To determine if basolateral plasma membranes contain Ca2+- or adenosine 3', 5'-cyclic monophosphate (cAMP)-dependent protein kinases, which could phosphorylate specific proteins important for secretion, a purified preparation of basolateral plasma membranes was prepared from rat exorbital lacrimal glands by differential and density gradient centrifugation. Phosphorylation of basolateral plasma membrane proteins was studied in the presence of [gamma-32P]ATP and was analyzed by sodium dodecyl sulfate-poly-acrylamide gel electrophoresis. Increasing the Ca2+ concentration in the presence of calmodulin stimulated phosphorylation of a 52,000-Mr peptide with a maximal increase in phosphorylation obtained at 3 and 66 microM free Ca2+. The phenothiazines trifluoperazine and promethazine inhibited phosphorylation of this 52,000-Mr peptide; 50% inhibition was obtained at 15 and 95 microM, respectively. Increasing the cAMP level from 0 to 10 microM stimulated phosphorylation of another peptide of 91,000 Mr. This effect could be reproduced by guanosine 3', 5'-cyclic monophosphate, but only at 100 microM. The cAMP concentration causing 50% of maximal phosphorylation was 0.3 microM. We conclude that lacrimal gland basolateral plasma membranes contain Ca2+/calmodulin- and cAMP-dependent protein kinases and protein substrates.