Glucose transport across the plasma membrane of isolated bovine rod outer segments (ROS) was measured by uptake of 14C-labeled 3-O-methylglucose and 2-deoxyglucose and was inferred from deenergization of ROS with 2-deoxyglucose. Glucose transport was mediated by a facilitated diffusion glucose transporter that equilibrated external and internal free hexose concentrations. Glucose transport in ROS displayed two components as judged from kinetic analysis of hexose equilibration and as judged from inhibition by cytochalasin B and phloretin. Transport under exchange conditions was considerably faster as compared with net hexose uptake, similar to that observed for the erythrocyte glucose transporter. Sensitivity to cytochalasin B and affinity to 3-O-methylglucose were similar to those observed for the hepatocyte glucose transporter. The cytochalasin-insensitive component appears unique to ROS and did not reflect leakage transport as judged from a comparison with L-glucose uptake. Glucose transport feeds glycolysis localized to ROS. We suggest that a major role for glycolysis in ROS is phosphorylation of GDP to GTP via pyruvate kinase and PEP, while phosphorylation of ADP to ATP can use the creatine kinase/phosphocreatine pathway as well.

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A protective effect of taurine on morphological alterations induced in isolated ROS by remotion of divalent cations is described. ROS damage is not observed when sodium or chloride is replaced by impermeant ions or in the presence of nifedipine (10 microM). An increased accumulation of 22Na but not of 36Cl occurs in retinas incubated in divalent cation free medium. Taurine (5-25 mM) provides a concentration-dependent protection to ROS structure. Taurine is also able to reduce the increased 22Na influx to control values. These results suggest a protective action of taurine probably involving an interaction with calcium sites on ROS membranes which regulate sodium permeability.

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Membrane vesicles were prepared from intact rod outer segments (ROSs) isolated from bovine retinas and were examined for the presence of cation-selective conductances. We performed macroscopic flux measurements in an ensemble of ROS membrane vesicles and single-channel measurements after fusion of ROS membrane vesicles with planar bilayer membranes. Two K(+)-permeable conductances were observed, the well-established cyclic GMP (cGMP) gated channel and an apparently new K+ channel with some unusual properties. Flux and single-channel data showed that the new conductance passed K+, Rb+, and Cs+ equally well but was much less permeable to Na+, Li+ and protons. Single-channel measurements revealed a linear current-voltage relationship and three unitary conductance states of 15, 11, and 8 pS, using symmetric 150 mM KCl solutions. Measured macroscopic K+ fluxes varied considerably among different preparations, suggesting some unknown regulation of the channel; the variability appeared to arise from variation in the channel's open probability, not the unit conductance or the number of channels present. The recorded single-channel events and the selectivity data are remarkably similar to those reported for antibiotic channel-forming ionophore gramicidin. We believe that the variability in both macroscopic permeability experiments and single-channel experiments may reflect a variable contamination with gramicidin applied to the animals as the topical antibiotic V-Sporin.