GLUCOSE AND PYRUVATE METABOLISM IN PREIMPLANTATION BLASTOCYSTS FROM NORMAL AND DIABETIC RATS

Glucose metabolism was analysed in day-5 rat blastocysts incubated in the presence of [5-H-3]-, [6-C-14]- or [U-C-14]glucose. Glycolysis, qu antified by (H2O)-H-3 recovery rate, was the main pathway of glucose u tilization by fresh (11.5 +/- 0.36 pmol per embryo h-1) or cultured (2 4 h) blastocysts (20.4 +/- 0.6 pmol per embryo h-1). Glucose consumpti on rate was almost saturated at a medium glucose concentration of 0.28 mmol l-1 (K(m): 0.17 mmol l-1; V(max): 23 pmol per embryo h-1). A fur ther 10% increase in glucose utilization was obtained with a tenfold h igher glucose concentration (3 mmol l-1). Phloretin completely abolish ed the rapid component of glucose utilization kinetics, suggesting the existence of a Na+-independent glucose transport system. Less than 1% of [6-C-14]glucose consumed by cultured blastocysts was oxidized thro ugh the Krebs cycle. [1-C-14]pyruvate, however, was oxidized at a rate of 2 pmol per embryo h-1 by fresh blastocysts. The pentose-phosphate pathway accounted for about 2% of glucose utilization. One to two per cent of the total glucose metabolized in 24 h was retained in macromol ecules. Insulin had no effect on glucose uptake, utilization, incorpor ation and turnover, or on pyruvate oxidation. Blastocysts from diabeti c mothers utilized glucose at a rate similar to that of normal blastoc ysts. These results show that glucose is actively taken up by rat blas tocysts and utilized mainly through the Embden-Meyerhof pathway, which is rapidly saturated at low glucose concentrations. Retention of gluc ose-derived products in macromolecules, although relatively small, may modulate the effect of high glucose concentrations on embryo growth. The lack of effect of insulin on the main metabolic pathways of glucos e metabolism does not rule out a more discrete insulin effect at some molecular levels.