No abstract

sn-1,2-Diacylglycerol (DAG) mass and translocation of protein kinase C a and ,B to a membrane fraction increased -7 min after insemination of Xenopus laevis eggs. The DAG mass increase of 48 pmol (from 62 to 110 pmol/cell) was greater than that for inositol 1,4,5-trisphosphate (1P3; an increase of -170 fmol or -280-fold smaller than the DAG increase), and DAG peaks -5 min after IP3. Choline mass (a measure of phosphatidylcholine-specific phospholipase D) also peaked before DAG and the choline increase (134 pmol/cell) was greater than that of DAG. There was no detectable change in phosphocholine mass (a measure of phosphatidylcholine-specific phospholipase C). During first cleavage, DAG decreased, PKC translocation was low, and choline increased and peaked (whereas published work shows an increase in IP3 mass). Artificial elevation of intracellular calcium ([Ca2+]

Although metformin is an important antidiabetic, its mechanism of action is still unknown. To study its mechanism, we examined metformin stimulation of insulin action on the Xenopus oocyte. Similar to therapeutic concentrations, maximal stimulation of insulin-induced meiotic cell division was achieved at about 1-10 microg/ml (or 7.7-77 /microM) metformin. An equivalent concentration of metformin was required to elevate receptor tyrosine kinase activity (in whole cells or a membrane-cortex preparation) and, through this tyrosine kinase activation, inositol 1,4,5-trisphosphate (IP3) production. With whole cells, the preincubation time for metformin stimulation of insulin action (approximately 1 h) was equivalent to the time required for metformin to maximize tyrosine phosphorylation and raise IP3, levels. With the membrane-cortex preparation, metformin was active within minutes; thus, metformin may act at an intracellular site. Since metformin can increase IP3, mass, we prevented elevation of calcium by prior microinjection of a calcium chelator or heparin (a drug that inhibits IP3 binding to the IP3 receptor). Both the chelator and heparin blocked metformin stimulation of insulin action on whole cells. Since microinjection of IP3, also stimulates insulin action, metformin may stimulate insulin action by elevation of intracellular calcium in addition to activation of the receptor tyrosine kinase.