A. J. Yool et al. studied membrane conductance in Xenopus laevis oocytes injected with aquaporin1 complementary RNA (AQP1 cRNA) and concluded that the conductance observed represented an intrinsic property of the protein (1). We have reevaluated our own studies of the AQP1 protein and have conducted new experiments; our results support our earlier conclusion that AQP1 transports water, but does not conduct ions (2).As measured in our laboratory (2), oocytes injected with water (as a control) or with AQP1 cRNA exhibited similar, low conductances, while their permeabilities to water differed greatly [coefficient of osmotic water permeability (P f )ϳ10 m/s and ϳ200 m/ s, respectively]. Because we had not previously investigated effects of forskolin on membrane-conductance, we used a two-electrode voltage clamp. Yool et al.(1) describe a rising scale of currents (measured in 100 mM NaCl, 2 mM KCl, 4.3 mM MgCl 2 , and 5 mM Hepes; pH 7.3): waterinjected oocytes rose 2.9 A/V; AQP1 oocytes, 8.6 A/V; and AQP1 oocytes with forskolin, 63 A/V. In contrast, using standard conditions (frog Ringers solution: 115 mM NaCl, 2 mM KCl, 1 mM CaCl 2 , 1 mM MgCl 2 , and 5 mM Hepes; pH 7.4), we did not see any significant difference in membrane conductance when water-injected oocytes (n ϭ 7) or AQP1 oocytes (n ϭ 20) were compared before or after forskolin treatment (Ͻ1 A/V for all measurements). We obtained similar results when CaCl 2 was omitted or when 0.5 mM 4.4Ј-diisothiocyanato-stilbene-2,2Ј-disulfonate (DIDS, Sigma, St. Louis, MO) was added to the solutions. Because AQP1 does not contain a classic cyclic AMP (cAMP)-dependent protein kinase A (PKA) phosphorylation consensus site, we performed experiments to evaluate oocytes that express AQP5 (which contains a classic PKA motif), but found that forskolin treatment also did not increase membrane currents of AQP5 oocytes (Ͻ1 A/V in all measurements, n ϭ 16).The differences in membrane behaviors observed in our laboratories are not a result of the AQP1 cDNA, because the construct used by Yool et al. came from our laboratory. Recently, we provided oocytes injected with AQP1 cRNA in our laboratory to Yool et al.; when they analyzed these oocytes in their laboratory using their technique, forskolin-induced ion currents were observed (3). The explanation for this discrepancy is not known. Science 273, 1216Science 273, (1996. 2. G. M. Preston et al., ibid. 256, 385 (1992). 3. J. W. Regan, personal communication.
