Calcium release induced by inositol 1,4,5-trisphosphate in thymocyte microsomes. Inhibition by barium and strontium

The properties of calcium transport in microsomes and the effect of inositol 1,4,5-trisphosphate (IP3) on accumulated calcium were studied in rat thymocytes. Active calcium transport shows an apparent affinity constant for calcium of 0.2 +/- 0.01 microM and a maximal velocity of 2.3 +/- 0.6 nmol/mg/30 min (mean +/- SD). IP3 was able to induce release of calcium only in the absence of oxalate. At 6 microM ambient free calcium, half-maximal effect of IP3 was attained at 2 microM and maximal calcium release was produced by IP3 concentrations over 5 microM. Barium and strontium did not modify calcium uptake by microsomes but markedly inhibited the action of IP3.