Comparison of exogenous energy sources for in vitro maintenance of follicle cell-freeXenopus laevis oocytes

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Abstract
The purpose of these experiments was to determine which exogenous energy sources are suitable for isolated follicle cell-free oocytes from the frog,Xenopus laevis. In order to compare prospective energy sources, follicle cell-free oocytes from 0.4 to 1.3 mm in diameter were incubated in a 1 mM concentration of each of a variety of energy sources and scored daily for the maintenance of morphological characteristics. Vitellogenic oocytes placed in succinate or fumarate deteriorated at the same time as those in saline alone. Oocytes incubated in oxaloacetate (OAA) appeared to remain in the best morphological condition, followed by oocytes maintained in pyruvate or glucose. Fully grown oocytes were tested at various times of incubation for their ability to respond to progesterone by undergoing germinal vesicle breakdown. These experiments showed that oocytes placed in OAA or pyruvate retained the ability to respond to progesterone longer than those in the other energy sources. Increased respiratory rates were stimulated in isolated oocyte mitochondria by succinate as well as pyruvate and OAA. However, oocytes incubated in labelled pyruvate evolved 80 to 140 times as much labelled CO2 as oocytes incubated in labelled glucose or succinate. In addition, it was found that the rate of uptake of pyruvate is 20 to 25 times greater than the rate of uptake of glucose or succinate. It is concluded from these experiments that OAA and pyruvate are the most effective exogenous energy sources for the in vitro maintenance ofXenopus oocytes. One possible explanation for the ineffectiveness of glucose or succinate as exogenous energy sources is a restriction in their uptake into the oocytes.