Changes in water proton relaxation times and in nuclear to cytoplasmic element gradients during meiotic maturation of Xenopus oocytes.

Abstract

Fully grown oocytes 1.2 mm in diameter were removed from Xenopus laevis ovaries and were exposed to progesterone (2.5 micrograms/ml in Ringer's solution) to induce completion of the first maturation division or germinal vesicle breakdown (GVBD). This process required 5.5 +/- 0.5 hr. Neither oocyte volume nor water content was observed to change throughout maturation. At selected times, the oocytes were quick frozen in liquid propane and cryosectioned. The sections were freeze-dried, and analyzed for K, Na, Cl, P, S, and Mg in millimolar per kilogram dry weight content in the nucleus and the yolk-free cytoplasm using electron probe X-ray microanalysis. Unstimulated oocytes showed significant nuclear to yolk-free cytoplasmic content gradients (N/C ratio) for the following elements: K (1.84), P (0.65), and S (1.56), but significant N/C content gradients were not found for Na and Mg. By 10 min after progesterone stimulation, a significant change in the N/C ratio of the following elements had occurred due to a rapid increase in nuclear content: K (2.29), Cl (2.11). A significant N/C ratio for Mg (1.35) had developed by 10 min after progesterone stimulation and a significant N/C ratio for Na (2.07) had developed by 45 min. In addition the following elements showed significant content increases in both the nucleus and the yolk-free cytoplasm from the time prior to progesterone stimulation to the time just prior to GVBD at 240 min: K, Na, Cl, P, S, and Mg. Nuclear magnetic resonance measurements of the spin-lattice relaxation time (T1) of water proton in oocytes showed a significant increase in the T1 time after progesterone exposure. The changes in N/C ratios of specific elements and in the physical parameter of water proton relaxation time suggest that progesterone is responsible for inducing changes in the physicochemical interactions between various macromolecules, specific elements, and water.