title: Cation fluxes and cation channels in outer segment membranes of bovine retinal rods: contamination by antibiotics applied to cattle?
abstract: 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.
authors.affiliation: Department of Medical Biochemistry, University of Calgary, Health Sciences Centre, Canada.
authors.forename: R
authors.initials: R
authors.lastname: Lopez-Escalera
authors.forename: R J
authors.initials: RJ
authors.lastname: French
authors.forename: P P
authors.initials: PP
authors.lastname: Schnetkamp
mesh_terms: D000818:Animals; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D002412:Cations / Q000378:metabolism*; D002417:Cattle; D006152:Cyclic GMP / Q000502:physiology; D006096:Gramicidin / Q000494:pharmacology; D066298:In Vitro Techniques; D015640:Ion Channel Gating / Q000502:physiology; D007473:Ion Channels / Q000378:metabolism*; D008566:Membranes / Q000378:metabolism; D018408:Patch-Clamp Techniques; D011522:Protons; D012160:Retina / Q000187:drug effects / Q000378:metabolism*; D012374:Rod Cell Outer Segment / Q000187:drug effects / Q000378:metabolism*; D012965:Sodium Chloride / Q000494:pharmacology
chemical_list: D000900:Anti-Bacterial Agents; D002412:Cations; D007473:Ion Channels; D011522:Protons; D006096:Gramicidin; D012965:Sodium Chloride; D006152:Cyclic GMP