Effects of carbachol and catecholamines on ultrastructure and intracellular calcium-ion dynamics of acinar and myoepithelial cells of lacrimal glands

Abstract.

The current study was carried out to investigate autonomic nervous control of secretory functions in the lacrimal gland. To distinguish the difference between the responses to cholinergic and adrenergic agonists in acinar and myoepithelial cells in the lacrimal gland of guinea pigs, the morphological and functional responses to the agonists were examined by electron microscopy and by digital-imaging analysis of the intracellular concentration of Ca²⁺ ([Ca²⁺]_i) using fluorescent Ca²⁺-indicators (Fura-2/AM and Indo-1/AM). In the resting state, exocytosis was rare, and the [Ca²⁺]_i in acinar and myoepithelial cells was low (less than 300 nM). Stimulation with carbachol (CCh) induced a rapid rise in [Ca²⁺]_i reaching a peak level followed by gradual decay and an appearance of many exocytotic figures. Approximately 4–8 s after an initial increase of [Ca²⁺]_i, myoepithelial cells commenced contraction. Noradrenaline or adrenaline induced an increase in [Ca²⁺]_i and exocytosis in acinar cells, but caused no [Ca²⁺]_i increase in myoepithelial cells. In a Ca²⁺-deficient environment, the responses to CCh in myoepithelial cells and those to noradrenaline in acinar cells were inhibited, whereas the responses to CCh in acinar cells remained unchanged. Isoproterenol caused no effect on [Ca²⁺]_i dynamics, although it occasionally induced exocytosis. Different cellular signaling pathways may be involved in the responses in acinar and in myoepithelial cells to different agonists. Lacrimation mechanisms are redundant.