Role of calcium in cholinergic stimulation of lacrimal Gland protein secretion

To characterize the role of Ca²⁺ in cholinergic stimulation of lacrimal gland protein secretion, the effects of inhibitors of cellular Ca²⁺ handling on protein secretion were investigated. Protein secretion was measured from rat exorbital glands using either pieces of gland in perifusion or acini isolated by collagenase digestion. Peroxidase was used as a measure of protein secretion. An inhibitor of Ca²⁺ influx via voltage sensitive Ca²⁺ channels (verapamil) at 10^-5 and 5 × 10^-5 M did not alter protein secretion stimulated by the cholinergic agonist carbachol at. 10^-5 M. Inhibition of Ca²⁺ efflux via Na⁺/Ca²⁺ exchange by removal of extracellular Na⁺ or by inhibition of Na⁺-K⁺ -ATPase activity using ouabain (10^-3 M) or extracellular K⁺ removal did not stimulate protein secretion. In contrast, inhibition of Ca²⁺ release from intracellular stores with TMB-8 at 100 μm completely blocked protein secretion stimulated by carbachol at 10^-5 M. Similarly, the Ca²⁺/calmodulin (CaM) antagonists W-13 and W-12 decreased carbachol-induced protein secretion with potencies similar to those which inhibit Ca²⁺ /CaM dependent processes. We conclude that cholinergic agonists stimulate lacrimal gland protein secretion primarily by mobilizing Ca²⁺ from intracellular stores and that one mechanism by which this Ca²⁺ could activate secretion is in conjunction with calmodulin.