MOBILIZATION OF CA-45 FROM INSULIN-PRODUCING RINM5F CELLS ATTACHED TO MICROCARRIERS

A technique was designed allowing studies of 45Ca efflux from insulin-releasing cells of a clonal cell line (RINm5F) ttached to fibronectin-coated plastic beads. Supporting the existence of Ca2+-Ca2+ and Na+-Ca2+ exchange mechanisms, the efflux of 45Ca in a Ca2+-deficient deficient perfusion medium was increased after introduction of Ca2+ and decreased after removal of Na+. The washout of radioacitivity was similar to that from pancreatic islets in also being inhibited by glucose in the presence of low concentrations of extracellular Ca2+. During perfusion with a medium containing 2.56 mM Ca2+, increasing the K+ concentration resulted in an increased 45Ca efflux, a process counteracted by D-600 [methoxyverapamil]. Exposure to tolbutamide also resulted in a distinct stimulatory peak. Glucose was a poor stimulator of 45Ca efflux, suggesting that this sugar lacks the ability to depolarize the RINm5F cells sufficiently to induce normal opening of the voltage-dependent channels in the plasma membrane. Such a defect might well explain why the RINm5F cells do not respond to glucose with stimulation of insulin release.