DIFFERENTIAL-EFFECTS OF HEPARIN ON INOSITOL 1,4,5-TRISPHOSPHATE BINDING, METABOLISM, AND CALCIUM RELEASE ACTIVITY IN THE BOVINE ADRENAL-CORTEX

Google Scholar
  • 1 March 1989
    • journal article
    • research article
    • Vol. 35  (3) , 339-344
Abstract
In a wide variety of cells, inositol-1,4,5-trisphosphate is a second messenger that interacts with specific intracellular receptors and triggers the release of sequestered Ca2+ from an intracellular store. We have looked at the influence of heparin on the action and metabolism of inositol-1,4,5-trisphosphate in the bovine adrenal cortex. Heparin blocked inositol-1,4,5-trisphosphate binding with half-maximal efficiency around 10 .mu.g/ml. Scatchard analyses revealed that heparin did not change the affinity but decreased the number of available binding sites. The Ca2+-releasing activity of inositol-1,4,5-trisphosphate was monitored with the fluorescent indicator, fura-2. Heparin blocked this activity with half-maximal efficiency around 10 .mu.g/ml. The effect of heparin could be overcome by a supramaximal dose of inositol-1,4,5-trisphosphate (25 .mu.M). The activity of inositol-1,4,5-trisphosphate-3-kinase from bovine adrenal cortex cytosol was also studied. Heparin inhibited the activity of the kinase with a half-maximal efficiency around 0.4 .mu.g/ml. Lineweaver-Burk plots revealed that this potent effect was noncompetitive. Finally, we observed that heparin is without effect on inositol-1,4,5-trisphosphate-5-phosphatase (at concentrations as high as 2 mg/ml). These results are consistent with the suggestion that the binding sites for inositol-1,4,5-trisphosphate are the intracellular receptors responsible for the Ca2+-mobilizing effects of inositol-1,4,5-trisphosphate. These results also show that the kinase, the phosphatase, and the receptor are three different molecular entities, which are affected in a different manner by heparin.