Junctional communication between alpha-cells

We have noted that, when measured as a percentage of content, the amount of glucagon released from fluorescence-activated cell sorted (FACS) -cells is much greater than that from intact islets (7.6 ± 0.9%, n = 9, 30-min incubation; vs. 0.53 ± 0.1%, n = 7, 1-h incubation, respectively, in the presence of 2.5 mM glucose). A high rate of basal glucagon secretion was also observed from dispersed (unsorted) islet cells (6.1 ± 1.1% of content, n = 4, 30-min incubation), indicating that the loss of intercellular contacts underlies the increased rate of basal secretion, rather than the absence of or reduction in exposure to paracrine inhibitory factors (I. Franklin and C. B. Wollheim, unpublished data). In isolated -cells, the increased basal secretion rate was not reduced by the removal of extracellular Ca2+ (14). To investigate whether junctional communication exists between neighboring -cells and whether these contacts are indeed required for normal rates of basal glucagon release, as previously observed for insulin secretion in ß-cells (37, 38), we attempted to reform contacts between cells by reaggregating the isolated -cell fraction. Reaggregating the cells caused a 10-fold reduction in the rate of glucagon release in basal glucose conditions, without altering the response to the secretagogue pyruvate, indicating that intercellular contacts may be necessary for normal basal secretion. Doubling the density of cells per well from 20,000 to 40,000 did not alter the rate of basal glucagon release, demonstrating that the altered rate of glucagon release from the reaggregated cells was not due to an increase in the local concentration of secreted autocrine factors. Immunocytochemical analysis of the reaggregated cell fraction did not reveal positive staining for either of the gap junction proteins connexin 36 or connexin 43 (I. Franklin and C. B. Wollheim, unpublished data), although the involvement of other members of the connexin family in the formation of gap junctions between rat -cells cannot be excluded. Connexin expression has been detected in FACS-isolated non-ß-cells from rats and mice (39, 40). Gap junctions, composed of connexin 36, are required for normal glucose-induced insulin secretion by rat ß-cells (38, 41). An understanding of the mechanisms regulating basal rates of glucagon secretion may lead to the identification of novel drug targets for controlling hyperglucagonemia in diabetes.



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