Oscillation of cytoplasmic free calcium concentration induced by insulin-like growth factor I

The effect of insulin-like growth factor I (IGF-I) on cytoplasmic free calcium concentration ([Ca2+]c) was studied in single BALB/c 3T3 cells by monitoring fura-2 fluorescence. In primed competent cells, IGF-I (1 nM) increased [Ca2+]c in approximately 60% of the cells tested. IGF-I-mediated elevation of [Ca2+]c was observed after a 4- to 17-min lag period. Elevation of [Ca2+]c was only transient but was followed by repetitive increases in [Ca2+]c. The interval between each peak was quite constant in each cell at a given concentration of IGF-I. When the concentration of IGF-I was reduced, both the latent period and interval of each peak were prolonged, whereas amplitude of the increase in [Ca2+]c was not altered. In medium containing 10 microM extracellular calcium, IGF-I did not cause any increase in [Ca2+]c. Likewise, blockade of IGF-induced calcium entry by either cobalt or tetramethrin abolished IGF-I action on [Ca2+]c. IGF-I-mediated oscillation was not affected by either ryanodine or caffeine, compounds that affect calcium-induced calcium release. In addition, pretreatment of the cell with neomycin did not affect IGF-I-mediated oscillation. In agreement with this, IGF-I did not augment production of [3H]inositol trisphosphate in cells prelabeled with [3H]inositol. These results indicate that IGF-I induces oscillation of [Ca2+]c in a single primed competent cell and that the oscillation is totally dependent on IGF-I-mediated calcium entry.