Regulatory role of fructose-2,6-bisP on glucose metabolism in frog oocytes: in vivo inhibition of glycogen synthesis

The beneficial effect of fructose and glucose on in vitro maturation and the fertilization of porcine oocytes

Purpose

This study was conducted to investigate the effects of supplementing fructose in the culture medium on in vitro maturation (IVM), in vitro fertilization (IVF), and metabolism of porcine oocytes.

Methods

Porcine oocytes were matured in vitro in modified North Carolina State University 37 medium (NCSU-37) and then supplemented with either glucose (5.5 mM), fructose (5.5 mM), or glucose (2.75 mM) plus fructose (2.75 mM). The maturation and fertilization of oocytes, and the incorporation and oxidation of ¹⁴C-glucose, ¹⁴C-fructose, and ¹⁴C-methionine in oocytes at different stages of development were examined.

Results

The supplementation of glucose plus fructose significantly promoted (P < 0.05) oocytes germinal vesicle break down (GVBD), maturation to metaphase II (MII), penetration by spermatozoa, and male pronuclear formation compared with glucose. The incorporation and oxidation of ¹⁴C-methionine into the oocyte significantly increased (P < 0.05) with glucose plus fructose supplementation than glucose. A significantly higher (P < 0.05) rate of incorporation and oxidation was achieved with ¹⁴C-fructose compared to ¹⁴C-glucose.

Conclusions

Glucose plus fructose supplementation improved maturation, penetration by spermatozoa, male pronuclear formation, and energy metabolism by porcine oocytes.

Glycogen synthesis by the direct or indirect pathways depends on glucose availability: In vivo studies in frog oocytes

Besides the classic direct route, frog oocytes incorporate glucosyl units into glycogen by the so-called indirect pathway. The operation of both pathways depends on glucose availability. Below 0.5 mM glucose (calculated intracellular concentration), the indirect route accounts for 90% of polysaccharide formation, while the direct pathway supports 70% of total glucose incorporation when administered glucose is above 1.5 mM. A sigmoidal curve was obtained for the direct pathway with n(H)=2.04, and half saturation was reached at 2.6 mM glucose. The curve for the indirect route presented an n(H) of 1.15 and an S(0.5) of 0.9 mM glucose.

In vivo operation of the pentose phosphate pathway in frog oocytes is limited by NADP+ availability

Evolution of CO2 from labelled glucose microinjected into frog oocytes in vivo may be ascribed to the pentose-P pathway, as measured by radioactive CO2 production from [1-(14)C] and [6-(14)C]glucose. Coinjection of NADP+ and [14C]glucose significantly stimulated 14CO2 production. The effect depends on the amount of NADP+ injected, half maximal stimulation being obtained at 0.13 mM. The increase in CO2 production was also observed with microinjected glucose-1-P, glucose-6-P or fructose-6-P used as substrates. Phenazine methosulfate, mimicked the effects of NADP+. A high NADPH/NADP+ ratio of 4.3 was found in the cells, the intracellular concentration of NADP+ being 19 microM.