Intercellular Communication in Pancreatic Islet Monolayer Cultures: A Microfluorometric Study

Abstract

Single islet cells in monolayer cultures of neonatal rat pancreas were microinjected with fluorescein and scanned topographically by microfluorometry. Fluorescein spread from an injected islet cell directly into neighboring islet cells, and, in the presence of 16.7 millimolar glucose, significantly more islet cells communicated with the injected cell than in glucose-free medium. Islet cells were also microinjected with glycolytic substrates and activators that produced transient changes in cellular levels of reduced pyridine nucleotides-nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate [NAD(P)H]. Changes in NAD(P)H fluorescence were observed in islet cells incubated first for 18 hours in very low glucose concentrations and then in a glucose-free medium and injected with glycolytic substrates and activators; however, little change of fluorescence occurred in adjacent islet cells. In contrast, after adding 16.7 millimolar glucose to the medium, injection of glycolytic substrates and activators produced transient changes in NAD(P)H fluorescence in the injected cell and in neighboring cells.

Beta cells preferentially exchange cationic molecules via connexin 36 gap junction channels

“…This knowledge should facilitate the identification of these factors, in as far as most endogenous metabolites are negatively charged at cytosolic pH [4,11,23]. Second, our experiments indicate that the ionic and molecular coupling of beta cells may not be as different as previously thought [5,7,8,10,11,15,23,29]. The new data show that the extent of coupling provided by Cx36 channels is sufficient to involve sizable proportions of, though not all beta cells of an islet.…”

“…Thus, whereas electrophysiological data and Ca 2+ measurements have indicated that ionic coupling may extend throughout entire islets [5,7,21,22,29,44,45], experiments evaluating the intercellular exchange of endogenous metabolites or exogenous tracers have indicated a more restricted coupling [8, 11, 12, 15, 17-19, 21, 23, 29]. However, the comparison of these findings is complicated, since previous studies have evaluated islet cell coupling with different techniques [28,29], tested junctional conductance indirectly [5,7,10,22] and/or studied beta cells in a non-physiological environment [5,8,10,11,15,22,45]. Thus, the question of what is transferred between beta cells via the Cx36 channels of intact islets remains to be established.…”

“…Previous electrophysiological studies have shown that ionic coupling extends to large regions of the islets, as judged by both the electrical synchronisation of beta cells and the intercellular diffusion of electrotonic currents [5,7,10,22]. In contrast, experiments testing the cell exchange of radioactive precursors [11] and exogenous tracers [8,12,18] have suggested that metabolic coupling within pancreatic islets may be restricted to territories comprising only a few beta cells [8,11,15]. This apparent discrepancy may reflect the different sensitivity of the techniques used to evaluate coupling [28,29], the specific characteristics of the Cx36 channels [4,13,30,31] and/or an influence of the islet setting that could modulate these channels [5,10,13,23,32].…”

“…current-carrying ions and other small cytosolic molecules [7][8][9][10][11][12][13][14][15]. Recent evidence has shown that these exchanges ensure the synchronisation and recruitment of beta cells during insulin secretion [9,10,[15][16][17][18][19][20][21][22][23].…”

Direct communication of homologous and heterologous endocrine islet cells in culture.

“…The data indicate that the exchange of exogenous cytoplasmic molecules occurs between different types of endocrine islet cells. However, within a single cluster, all islet cells are not metabolically coupled to one another, at a given time.Ionic and metabolic cell-to-cell coupling (1) have been recently demonstrated between B-cells of the islets of Langerhans (2,5,8,11). In view of the heterogeneous endocrine cell composition of the islet (14) and of the possible functional interactions between various islet cells (for review, see reference 22), it was important to assess whether coupling occurred between different islet cell types .…”