Inositol transport in preimplantation rabbit embryos: effects of embryo stage, sodium, osmolality and metabolic inhibitors.
The preimplantation period in the rabbit consists of a 3 day cleavage stage during which the number of cells increases with little change in embryo size, followed by a 3-4 day blastocyst stage during which the inner cell mass, the blastocoel and the trophectodermal layer are formed and the embryo grows rapidly in size and protein content. This study used [3H]inositol to investigate the transport of inositol, an essential component of the phosphatidylinositol signal transduction system, over the 6 days of preimplantation development by rabbit embryos. In the presence of 15 micromol inositol-1 in the incubation medium, there was a small linear increase in inositol uptake from 0.07 pmol per embryo per h at the one-cell stage (day 1) to 0.135 pmol at the late morula (day 3) stage. Inositol uptake increased to 0.58 pmol per embryo per h for early blastocysts (day 4) and 23.7 pmol for late blastocysts (day 6). There was a significant linear relationship between inositol uptake and blastocyst diameter and surface area. Efflux of inositol from early morulae was minimal (about 1.25% of embryo content per h), whereas efflux from mid-blastocysts (day 5) was much greater (about 15.6% of embryo content per h). Efflux of inositol from both early morulae and mid-blastocysts was increased by decreasing the osmolality of the incubation medium. Varying the osmolality had no effect on inositol uptake up to 2 h. Inositol uptake was dependent on sodium in cleavage-stage embryos but independent of sodium in blastocyst stages. In early morulae, inositol uptake was inhibited by glucose and the sodium-dependent hexose transport inhibitor, phloridzin, but not by the facilitated transport inhibitor, phloretin. Inositol uptake in early morulae was saturable; estimates of 0.227 and 0.288 pmol per morula per h for V(max) and 0.045 and 0.038 mmol-1 [corrected] for Km were obtained for sodium-dependent transport in two separate experiments. All of these results are consistent with the hypothesis that transport in cleavage stages occurs via a sodium myo-inositol transporter (SMIT) protein. Uptake in blastocysts was non-saturable. Uptake into blastocysts appeared to take place by a transcellular rather than a paracellular route.
Warner
S M
SM
Department of Physiology, National University of Ireland, Galway, University Road, Ireland.
Conlon
F V
FV
Kane
M T
MT
D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D001755:Blastocyst / Q000187:drug effects / Q000378:metabolism*; D002478:Cells, Cultured; D002970:Cleavage Stage, Ovum / Q000378:metabolism; D005260:Female; D005947:Glucose / Q000494:pharmacology; D007294:Inositol / Q000378:metabolism / Q000494:pharmacology*; D009004:Monosaccharide Transport Proteins / Q000037:antagonists & inhibitors; D009028:Morula / Q000378:metabolism; D009994:Osmolar Concentration; D010693:Phloretin / Q000494:pharmacology; D010695:Phlorhizin / Q000494:pharmacology; D011817:Rabbits; D012964:Sodium / Q000378:metabolism / Q000494:pharmacology; D013696:Temperature; D013997:Time Factors
D009004:Monosaccharide Transport Proteins; D007294:Inositol; D012964:Sodium; D010695:Phlorhizin; D005947:Glucose; D010693:Phloretin