Synchronization of Ca super(2+)-signals within insulin-secreting pseudoislets: effects of gap-junctional uncouplers

• Published in: Cell calcium (Edinburgh) Vol. 27; no. 5; pp. 287 - 296
• Main Authors: Squires, P E, Hauge-Evans, A C, Persaud, S J, Jones, P M
• Format: Journal Article
• Language: English
• Published: 01.05.2000
• ISSN: 0143-4160

The secretory response of the intact islet is greater than the response of individual beta -cells in isolation, and functional coupling between cells is critical in insulin release. The changes in intracellular Ca super(2+) ([Ca super(2+)] sub(i)) which initiate insulin secretory responses are synchronized between groups of cells within the islet, and gap-junctions are thought to play a central role in coordinating signalling events. We have used the MIN6 insulin-secreting cell line, to examine whether uncoupling gap-junctions alters the synchronicity of nutrient- and non-nutrient-evoked Ca super(2+) oscillations, or affects insulin secretion. MIN6 cells express mRNA species that can be amplified using PCR primers for connexin 36. A commonly used gap-junctional inhibitor, heptanol, inhibited glucose- and tolbutamide-induced Ca super(2+)-oscillations to basal levels in MIN6 cell clusters at concentrations of 0.5 mM and greater, and it had similar effects in pseudoislets when used at 2.5 mM. Lower heptanol concentrations altered the frequency of Ca super(2+) transients without affecting their synchronicity, in both monolayers and pseudoislets. Heptanol also had effects on insulin secretion from MIN6 pseudoislets such that 1 mM enhanced secretion while 2.5 mM was inhibitory. These data suggest that heptanol has multiple effects in pancreatic beta -cells, none of which appears to be related to uncoupling of synchronicity of Ca super(2+) signalling between cells. A second gap-junction uncoupler, 18 ( alpha -glycyrrhetinic acid, also failed to uncouple synchronized Ca super(2+)-oscillations, and it had no effect on insulin secretion. These data provide evidence that Ca super(2+) signalling events occur simultaneously across the bulk mass of the pseudoislet, and suggest that gap-junctions are not required to coordinate the synchronicity of these events, nor is communication via gap junctions essential for integrated insulin secretory responses.