THE PHYSIOLOGICAL CONCENTRATION OF INOSITOL 1,4,5-TRISPHOSPHATE IN THE OOCYTES OF XENOPUS-LAEVIS

To measure the concentration of inositol 1,4,5-trisphosphate ([IP3]) i n small regions of single Xenopus oocytes, a biological detector cell was combined with capillary electrophoresis, This method is 10,000 tim es more sensitive than all existing assays enabling subcellular measur ement of [IP3] in Xenopus oocytes. Upon addition of lysophosphatidic a cid to an oocyte, [IP3] increased from 40 to 650 nM within 2 min. IP3 concentrations as high as 1.8 mu M were measured after activation with lysophosphatidic acid, suggesting that the physiologic concentration of IP3 ranges from the tens of nanomolar to a few micromolar in Xenopu s oocytes. Since the IP3 receptor in Xenopus oocytes is nearly identic al to the type I receptor of mammalian cells, the range of [IP3] in mo st mammalian cells is likely to be similar to that in the oocyte. By s electing or engineering the appropriate detector cell, this strategy s hould be applicable to cyclic adenosine diphosphate ribose and nicotin ic acid adenine dinucleotide phosphate, and to the discovery of new Ca 2+-releasing second messengers.