Cа2+ transporting systems in secretory cells of the rat exorbital lacrimal gland. І. Ca2+ pumps of plasma membrane and endoplasmic reticulum

The mechanism of secretion by lacrimal glands, the innervation and orientation effects, and the mechanism of transduction of primary agonists are similar to the major salivary glands. However, unlike the major salivary glands, about organization of Ca2+ signalling and Ca2+-transporting systems contribution to Ca2+ homeostasis maintenance in secretory cells of the lacrimal glands is not totally clear yet. The purpose of the work was to investigate the role of plasma membrane (PMCA) and endoplasmic reticulum (SERCA) Ca2+ pumps in maintenance of Ca2+ homeostasis in the secretory cells of the lacrimal glands. The study was performed on intact and digitonin-permeabilized secretory cells of exorbital lacrimal glands of rats. The functioning of the Ca2+-transporting systems was estimated by changes of Ca2+ content in cells, after incubation with agonists or antagonists. Ca2+ content was determined using metalochromic dye Arsenazo III. The baseline Ca2+ content in intact and permeabilized cells decreases during incubation time. This decrease is caused by dysfunction of SERCA. This pump is high effectively inhibited by eosin Y (10 µmol/l) and thapsigargin (1 µmol/l). PMCA plays important role in maintaining of Ca2+ homeostasis in the lacrimal gland secretory cells. Thus, the content of Ca2+ in intact cells of rat lacrimal glands did not change under inhibition by eosin Y in wide concentration range (5–20 µmol/l) (unlike digitonin-permeabilized cells). In addition, eosin Y (5 µmol/l) prevented time-dependent reduction of Ca2+ content in intact cells. In both cases, inhibition of PMCA eliminates the effect of inhibition of SERCA. Results of two-factor analysis of variance confirmed that in the absence of eosin Y incubation time and permeabilization of plasma membrane significantly affect the content of Ca2+ in cells. At the same time, in the presence of eosin Y (5 µmol/l) only permeabilization of plasma membrane reliably affects of the changes of Ca2+ content. Because in permeabilized cells, as well as eosin Y influence in concentration 5 µmol/l, PMCA does not transport Ca2+ from the cell.