MODULATORY EFFECT OF CYCLIC-AMP ON CALCIUM FLUXES IN FRTL-5 CELLS

The aim of the present study was to investigate the effect of cAMP on calcium fluxes in Fura 2 loaded thyroid FRTL-5 cells. Preincubating th e cells with the phosphodiesterase inhibitor Ro-201 724 decreased the ATP-stimulated entry of calcium, while having no effect on the release of sequestered calcium. Pretreatment with forskolin decreased both th e release of sequestered calcium and the entry of calcium in response to ATP. We then incubated the cells with phenylisopropyl adenosine (PI A), a P2i-receptor agonist earlier shown to decrease cAMP in FRTL-5 ce lls. Although we did not observe a decrease in cellular cAMP after PIA , the ATP-evoked calcium response was enhanced. Forskolin decreased ca lcium entry induced by thapsigargin, a Ca2+-ATPase inhibitor, but fors kolin had no effect on the thapsigargin-evoked release of sequestered calcium. Addition of calcium to cells stimulated with ATP in a calcium -free buffered resulted in a rapid influx of calcium. This response in [Ca2+]i was decreased in cells pretreated with forskolin. In cells st imulated with thapsigargin, the increase in [Ca2+]i after addition of calcium was inhibited in part by forskolin and enhanced by PIA. The re sults suggest that cAMP may regulate calcium fluxes in FRTL-5 cells Fu rthermore, PIA increased agonist-induced calcium entry through a prese ntly unknown mechanism. (C) 1993 Wiley-Liss, Inc.