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The Journal of Cell Biology, Vol 56, 389-398, Copyright © 1973 by Rockefeller University Press

ARTICLE

MEMBRANE CHARACTERISTICS AND OSMOTIC BEHAVIOR OF ISOLATED ROD OUTER SEGMENTS

Juan I. Korenbrot 1, Dennis T. Brown 1, and Richard A. Cone 1

1 From the Thomas C. Jenkins Department of Biophysics, Johns Hopkins University, Baltimore, Maryland 21218 and the Department of Cell Biology and Pharmacology, University of Maryland Medical School, Baltimore, Maryland 21201.

Dr. Korenbrot's present address is the Department of Physiology, University of California at Los Angeles School of Medicine, Los Angeles, California 90024.

Freshly isolated frog rod outer segments are sensitive osmometers which retain their photosensitivity; their osmotic behavior reveals essentially the same light-sensitive Na+ influx observed electrophysiologically in the intact receptor cell. Using appropriate osmotic conditions we have examined freeze-etch replicas of freshly isolated outer segments to identify the membrane which regulates the flow of water and ions. Under isosmotic conditions we find that the disc to disc repeat distance is almost exactly twice the thickness of a disc. This ratio appears to be the same in a variety of vertebrate rod outer segments and can be reliably measured in freeze-etch images. Under all our osmotic conditions the discs appear nearly collapsed. However, when the length of the outer segment is reduced by hyperosmotic shocks the discs move closer together. This markedly reduces the ratio of repeat distance to disc thickness since disc thickness remains essentially constant. Thus, the length reduction of isolated outer segments after hyperosmotic shocks primarily results from reduction of the extradisc volume. Since the discs are free floating and since they undergo negligibly small changes in volume, the plasma membrane alone must be primarily responsible for regulating the water flux and the light-sensitive Na+ influx in freshly isolated outer segments. On this basis we calculate, from the osmotic behavior, that the plasma membrane of frog rod outer segment has a Na+ permeability constant of about 2.8 x 10-6 cm/s and an osmotic permeability coefficient of greater than 2 x 10-3 cm/s.

Submitted on June 29, 1972
Revised on August 16, 1972


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