ION, WATER AND NEUTRAL SOLUTE TRANSPORT IN XENOPUS OOCYTES EXPRESSINGFROG LENS MIP

We have expressed frog (Rana pipiens) lens major intrinsic protein (MI P) in Xenopus oocytes and observed its effect on ion conductance, wate r permeability and neutral solute transport. SDS-PAGE and immunoblotti ng demonstrated oocytes injected with MIP mRNA expressed the protein a t high levels. Immunolocalization indicated the expressed MIP migrated to the plasma membrane. MIP had no effect on the slope of oocyte I-V relations in the range - 50 to + 10 mV, although the averaged I-V curv e was shifted 10 mV positive to control. MIP increased oocyte water pe rmeability by a factor of 1.9 +/- 0.2 whereas the permeability to sucr ose, 2-deoxyglucose, inositol, sorbitol, reduced glutathione or urea w as unchanged. Glycerol permeability was enhanced in oocytes expressing MIP. In contrast to control oocytes, H-3-glycerol radioactivity accum ulation did not follow first order kinetics. Radioactivity continued t o accumulate even after 19 h of uptake and went beyond equilibrium wit h the bath. The time course of MIP-mediated glycerol uptake was modele d assuming metabolic trapping with good results. Based on this model, MIP increased oocyte glycerol permeability by a factor of 2.7. (C) 199 5 Academic Press Limited.