Insulin stimulates sodium-dependent phosphate transport by osteoblast-like cells

The rat osteosarcoma cell line UMR-106-01 has an osteoblast-like phenotype. When grown in monolayer culture, these cells transport Pi via a Na(+)-dependent carrier. The Na(+)-Pi cotransport system is stimulated by parathyroid hormone (PTH). Because there are insulin receptors on osteoblast-like cells, we determined possible effects of insulin on Na(+)-Pi cotransport in UMR-106-01 cells. Incubation of cells with 10(-8) M insulin for 3 h produced a 73% increase (P less than 0.025) in Na(+)-Pi cotransport. There was no significant change in Na(+)-L-alanine cotransport or in Na(+)-independent uptake of Pi and alanine. The stimulatory action of insulin on Na(+)-Pi cotransport was present within 2 h and was dose dependent in the range 10(-10) to 10(-7) M. The increase in Na(+)-Pi cotransport was accompanied by an increase in apparent maximal velocity with no change in apparent Michaelis constant for Pi. Use of cycloheximide to block de novo protein synthesis did not interfere with this action of insulin. We conclude that insulin, like PTH, directly stimulates the Na(+)-Pi cotransport system in osteoblast-like cells. The mechanism remains to be determined.