WATER PERMEABILITY PROPERTIES OF THE OVARIAN OOCYTES FROM BUFO-ARENARUM AND XENOPUS-LAEVIS - A COMPARATIVE-STUDY

The water permeability properties of ovarian oocytes from Xenopus laevis and Bufo arenarum, a toad species found in the Buenos Aires region, were studied. We report that: (i) the water osmotic permeability (P-f, cm/sec x 10(-4)) was significantly higher in Bufo (6 degrees C = 12.3 +/- 2.4; 18 degrees C = 20.8 +/- 4.8) than in Xenopus oocytes (6 degrees C = 5.3 +/- 0.3; 18 degrees C = 6.2 +/- 1.6). The corresponding water diffusion permeability values (P-d, cm/sec x 10(-4)) were: Xenopus = 2.3 +/- 0.3 (6 degrees C) and 4.8 +/- 0.7 (18 degrees C); Bufo = 2.7 +/- 0.4 (6 degrees C) and 6.0 +/- 0.5 (18 degrees C). (ii) Amphotericin B increased the P-f and P-d values. The observed Delta P-f/Delta P-d ratio was not significantly different from the expected results (n = 3), after amphotericin B incorporation in both species. This means that the influence of unstirred layers and other potential artifactual compounds did not significantly affect our experimental results. (iii) Preincubation with gramicidin during 12 hr induced a clear increase in the oocyte volume. After that, a hypotonic shock only slightly increased the oocyte volume. Conversely, a hypertonic challenge induced a volume change significantly higher than the one observed in control conditions. (iv) Mercury ions did not affect the osmotic permeability in Xenopus oocytes but clearly inhibited, in a reversible way, the osmotic permeability in oocytes from B. arenarum. (v) Mercury ions did not reduce P-d values in either species. (vi) The Delta P-f/Delta P-d values calculated from the differences observed in these parameters between both species were 11.9 +/- 5.1 at 18 degrees C and 15.5 +/- 2.4 at 6 degrees C. These numbers are similar to those previously reported in the case of membranes having water channels. From these results, we propose that water channels are present in the ovarian oocyte from B. arenarum but not in the ovarian oocyte from X. laevis.