WATER AND UREA PERMEABILITY PROPERTIES OF XENOPUS OOCYTES - EXPRESSION OF MESSENGER-RNA FROM TOAD URINARY-BLADDER

The Xenopus oocyte was evaluated as an mRNA expression system for water and urea transporters. Osmotic water permeability (P(f)) was measured from the time course of oocyte volume in response to osmotic gradients using a real-time imaging method. Diffusional water permeability (P(d)) was measured by (H2O)-H-3 efflux. In mature oocytes treated with collagenase to remove the follicular cell layer, P(f) was 8.6 +/- 0.6 x 10-4 (SD) cm/s (n = 32) at 25-degrees-C and independent of the time after oocyte removal (0-8 days). The activation energy (E(a)) for P(f) was 10.2 kcal/mol (10-32-degrees-C). P(f) was independent of osmotic gradient size (50-200 mosmol) in swelling experiments but decreased in an unpredictable manner in shrinking experiments. P(f) was not altered by removal of the vitelline membrane but was decreased by 75% when the follicular cell layer was intact. In collagenase-treated oocytes, amphotericin (0-500)-mu-g/ml) increased P(f) from 8 x 10-4 to 84 x 10-4 cm/s in a dose-dependent manner. P(d) was 3.4 +/- 0.2 x 10-4 (SE) cm/s at 25-degrees-C, 1.5 +/- 0.2 x 10-4 cm/s at 4-degrees-C, and 5.1 +/- 0.5 x 10-4 cm/s at 25-degrees-C in the presence of 500-mu-g/ml amphotericin; E(a) was 6.5 kcal/mol. Thus P(d), but not P(f), is unstirred layer limited. The urea permeability (P(urea)), measured from [C-14]urea efflux, was 9.5 +/- 0.7 x 10-7 cm/s at 25-degrees-C and 2.6 +/- 0.4 x 10-7 cm/s at 4-degrees-C. P(f) and P(urea) were not altered by HgCl2, phloretin, and cyclic AMP analogues. In oocytes microinjected with mRNA from toad urinary bladder, P(f) increased 4.3- to 4.6-fold (n = 23, 10-degrees-C) and P(urea) increased 3.6- to 3.9-fold (n = 10, 25-degrees-C) compared with oocytes injected with water or brain mRNA. These results indicate absence of endogenous water and urea transporters in Xenopus oocytes and demonstrate functional expression of mRNA from toad bladder.