Calcium and Pancreatic Secretion‐dynamics of Subcellular Calcium Pools in Resting and Stimulated Acinar Cells

Abstract

I Pulse-chase experiments were carried out on pancreatic tissue lobules incubated in vitro, with 45Ca as the tracer, in order to shed some light on the functional significance of the calcium pools associated with the various cell organelles of the acinar cell, especially in relation to stimulus-secretion coupling. 2 The kinetics of tracer uptake and release which were observed in the intact lobules suggest the existence of a number of intracellular pools, whose rate of exchange is slower than that across the plasmalemma. 3 The various subcellular fractions accumulate the tracer in different amounts: some (rough microsomes and postmicrosomal supernatant) showed little radioactivity and some (smooth microsomes and zymogen granule membranes) were heavily labelled; mitochondria and zymogen granules showed intermediate values. 4 The fractions are heterogeneous also in relation to the time course of uptake and release of the tracer: in rough and smooth microsomes and, especially, in the postmicrosomal supernatant both rates were fast; zymogen granules and zymogen granule membranes showed slow rates of uptake and little release during chase; intermediate rates were found in mitochondria.5 In agreement with previous findings we observed that in 45Ca preloaded lobules, stimulation of secretion (brought about by the secretagogue polypeptide caerulein) results in an increase of the tracer release which seems to be due primarily to the rise of the intracellular concentration of free Ca2+ and to the consequent increase of the transmembrane Ca2+ efflux. Among the cell fractions isolated from stimulated lobules only the mitochondria exhibited a significantly lower 45Ca level relative to the unstimulated controls. 6 It is concluded that, of the organelle-bound calcium pools, that associated with the mitochondria might be involved in the regulation of the calcium-dependent functions, including stimulus-secretion coupling; the calcium associated with the zymogen granule content probably has a role in the architecture of the organelle and in the functionality of the pancreatic juice, while the calcium bound to the membrane of the granules might be concerned with the regulation of its permeability properties.

Action of cholecystokinin and cholinergic agents on membrane-bound calcium in dispersed pancreatic acinar cells.

“…Clemente a(I Nleldolesi (28) have found that in homiiogeniates of intact guinlea pig pancreas, the distribution of tisstse calciumiii is: zym ogen granutiles, 35%; zyvimogenl granule membranes, 6%; plasmia meml)ranes, 5%; iuiitochondclria, 6%; imiicrosomiies, 19%. In a subsequent studx (22) 45 Ca during lvsis aind filtration wvill result not onlv in underestimation of the fraction of total cellular 45Ca that is membrane-bound but also in overestimation of the fraction that is free. 4 Wlhat ve have calculated as "free cellular 45Ca" should not be interpreted to mean "cytoplasmic 45Ca", siince our techni(ue does not distinguish between radlioactivity free in the cytoplasm and free but localized to anatomiically defined cellular comppartments sutcl as mitochondria, nuclei, or zymlogen granules.…”

Calcium dynamics in catecholamine-containing secretory vesicles

“…It has been known for more than 20 years that they contain large amounts of total calcium (20-60 mM), in particular chromaffin granules of the adrenal medulla and zymogen granules of the exocrine pancreas [1][2][3]. In addition, chromaffin granules contain also large amounts of proteins (100 mg/ml, of which 40% is chromo- granin A), catecholamines (0.5-0.6 M) and ATP (130 mM) [2,4].…”

Calcium dynamics in the secretory granules of neuroendocrine cells

“…This high calcium content is not exclusive of chromaffin granules but instead it is also found at similar or even higher values in most types of secretory granules, e.g. zymogen granules of pancreatic acinar cells [15], small synaptic vesicles of the neuromuscular junction [13], insulin granules [16] and many others (see [17] for a review).…”

“…In the case of the secretory granules, some reports suggest that at least part of the stored calcium is relatively immobile or exchanges very slowly with the free Ca 2+ pool. Clemente and Meldolesi [15] found that zymogen granules have a very slow rate of 45 Ca accumulation and release compared with other organelles, suggesting that the stored Ca 2+ could play mainly a structural role in the architecture of the organelle. Similarly, 45 Ca flux experiments performed in PC12 cells showed that the granule Ca 2+ pool was extraordinarily stable in periods of hours in EGTA-containing medium, and very slow to label [14], concluding that the stored Ca 2+ would only serve to keep granule components together and to be discharged to the extracellular space by exocytosis.…”