Membrane potential and conductance changes evoked by hormones and neurotransmitters in mammalian exocrine gland cells

Polarized calcium and calmodulin signaling in secretory epithelia

This review examines polarized calcium and calmodulin signaling in exocrine epithelial cells. The calcium ion is a simple, evolutionarily ancient, and universal second messenger. In exocrine epithelial cells, it regulates essential functions such as exocytosis, fluid secretion, and gene expression. Exocrine cells are structurally polarized, with the apical region usually dedicated to secretion. Recent advances in technology, in particular the development of videoimaging and confocal microscopy, have led to the discovery of polarized, subcellular calcium signals in these cell types. The properties of a rich variety of local and global calcium signals have now been described in secretory epithelial cells. Secretagogues stimulate apical-to-basal waves of calcium in many exocrine cell types, but there are some interesting exceptions to this rule. The shapes of intracellular calcium signals are determined by the distribution of calcium-releasing channels and mechanisms that limit calcium elevation. Polarized distribution of calcium-handling mechanisms also leads to transcellular calcium transport in exocrine epithelial cells. This transport can deliver considerable amounts of calcium into secreted fluids. Multicellular polarized calcium signals can coordinate the activity of many individual cells in epithelial secretory tissue. Certain particularly sensitive cells serve as pacemakers for initiation of intercellular calcium waves. Many calcium signaling pathways involve activation of calmodulin. This ubiquitous protein regulates secretion in exocrine cells and also activates interesting feedback interactions with calcium channels and transporters. Very recently it became possible to directly study polarized calcium-calmodulin reactions and to visualize the process of hormone-induced redistribution of calmodulin in live cells. The structural and functional polarity of secretory epithelia alongside the polarity of its calcium and calmodulin signaling present an interesting lesson in tissue organization.

Growth of exocrine acinar cells on a reconstituted basement membrane gel

Methods have been developed for culturing a dividing population of morphologically differentiated rat parotid, lacrimal, and pancreatic acinar cells in vitro. Isolated acinar cells were plated onto tissue culture dishes coated with a three-dimensional, reconstituted basement membrane gel. After attachment in Ham's nutrient mixture F12, the cells were cultured at 35 degrees C in F12 supplemented with 10% heat inactivated rat serum, epidermal growth factor, dexamethasone, insulin, transferrin, selenium, putrescine, reduced glutathione, ascorbate, penicillin, streptomycin, and the appropriate secretagogue. Under these conditions, the cells attached rapidly and DNA synthesis was initiated within 2 to 3 d. Although the cells flattened on the substratum, they continued to maintain their differentiated morphology. The cells contained secretory granules, and the secretory enzymes peroxidase and amylase could be detected. The use of a reconstituted basement membrane gel proved critical for the attachment and growth of exocrine acinar cells.

Stimulation of parotid cell glucose oxidation. Role of alpha 1-adrenergic receptors and calcium mobilization

Epinephrine stimulated glucose oxidation in isolated rat parotid cell aggregates through alpha- and beta-adrenergic mechanisms. The alpha-adrenergic component appeared to be of the alpha 1-receptor subtype as evidenced by inhibition studies with selective antagonists. Calcium mobilization in the presence of the ionophore A23187 was as capable as epinephrine in eliciting this response. Epinephrine stimulation of glucose oxidation was, however, only partially dependent on extracellular calcium. Only the upper 50% of the maximal (10(-5) M) epinephrine response required external calcium, since stimulation by 10(-6) M epinephrine, which elicits only about 40% of the maximal (10(-5) M epinephrine) response, was independent of external calcium. Furthermore, no external calcium dependence was observed for pure alpha- or beta-adrenergic stimulation alone, which comprised about 60 and 40%, respectively, of the maximal epinephrine response. Thus, adrenergic mediated parotid cell glucose oxidation may proceed by different mechanisms depending on the extent and nature of cellular stimulation.