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The Xenopus oocyte was evaluated as an mRNA expression system for water and urea transporters. Osmotic water permeability (Pf) was measured from the time course of oocyte volume in response to osmotic gradients using a real-time imaging method. Diffusional water permeability (Pd) was measured by 3H2O efflux. In mature oocytes treated with collagenase to remove the follicular cell layer, Pf 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 (Ea) for Pf was 10.2 kcal/mol (10-32 degrees C). Pf was independent of osmotic gradient size (50-200 mosmol) in swelling experiments but decreased in an unpredictable manner in shrinking experiments. Pf 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 micrograms/ml) increased Pf from 8 x 10(-4) to 84 x 10(-4) cm/s in a dose-dependent manner. Pd 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 micrograms/ml amphotericin; Ea was 6.5 kcal/mol. Thus Pd, but not Pf, is unstirred layer limited.(ABSTRACT TRUNCATED AT 250 WORDS)
Zhang et al. (Tue,) studied this question.