Increased intracellular calcium is required for spreading of rat islet beta-cells on extracellular matrix

Rat islet beta -cells spread in response to glucose when attached on the ma trix produced by a rat bladder carcinoma cell, line (804G). Furthermore, in a mixed population of cells, it has been observed previously that spread c ells secrete more insulin acutely in response to glucose, compared with cel ls that remain rounded. These results suggest bi-directional signaling betw een the islet beta -cell and the extracellular matrix. In the present study , the role of increased intracellular free Ca2+ concentration [Ca2+](i) as an intracellular step linking glucose stimulation and beta -cell spreading (inside-out signaling) was investigated. purified rat beta -cells were atta ched to this matrix and incubated under various conditions known to affect [Ca2+](i). The effect of glucose on beta -cell spreading was mimicked by 25 mmol/l KCI (which induces calcium influx) and inhibited by diazoxide (whic h impairs depolarization and calcium entry) and by the L-type Ca2+ channel blocker SR-7037. When a 24-h incubation at 16.7 glucose was followed by 24 h at 2.8 mmol/l, beta -cells that had first spread regained a round phenoty pe. In the presence of thapsigargin, spreading progressed throughout the ex periment, suggesting that capture of calcium by the endoplasmic reticulum i s involved in the reversibility of spreading previously induced by glucose. Spreading was still observed in degranulated beta -cells and in botulinum neurotoxin E-expressing beta -cells when exocytosis was prevented. In summa ry, the results indicate that increased [Ca2+](i) is required for the gluco se-induced spreading of beta -cells on 804G matrix and that it is not a con sequence of exocytotic processes that follow elevation of [Ca2+](i).