title: Dark ionic flux and the effects of light in isolated rod outer segments.
abstract: We have determined the permeability properties of freshly isolated frog rod outer segments by observing their osmotic behavior in a simple continuous flow apparatus. Outer segments obtained by gently shaking a retina are sensitive but nonideal osmometers; a small restoring force prevents them from shrinking or swelling quite as much as expected for ideal behavior. We find that Na(+), Cl(-), No(3) (-), glycerol, acetate, and ammonium rapidly enter the outer segment, but K(+), SO(4) (=), and melezitose appear impermeable. The Na flux is rectified; for concentration gradients in the physiological range, 2 x 10(9) Na(+) ions/sec enter the outer segment, but we detect no efflux of Na(+), under our conditions, when the gradient is reversed. Illumination of the outer segment produces a specific increase in the resistance to Na(+) influx, but has no effect on the flux of other solutes. This light-dependent Na(+) resistance increases linearly with the number of rhodopsin molecules bleached. We find that excitation of a single rhodopsin molecule produces a transient ( approximately 1 sec) "photoresistance" which reduces the Na(+) influx by about 1%, thus preventing the entry of about 10(7) Na(+) ions. At considerably higher light levels, a stable afterimage resistance appears which reduces the Na influx by one-half when 10(6) rhodopsin molecules are bleached per rod. We have incorporated these findings into a model for the electrophysiological characteristics of the receptor.
mesh_terms: D000085:Acetates / Q000378:metabolism; D000641:Ammonia / Q000378:metabolism; D000818:Animals; D001001:Anura; D002118:Calcium / Q000378:metabolism; D002712:Chlorides; D004594:Electrophysiology; D005990:Glycerol / Q000378:metabolism; D007700:Kinetics; D008027:Light; D008433:Mathematics; D009566:Nitrates / Q000378:metabolism; D010084:Oxidation-Reduction; D010539:Permeability; D010786:Photoreceptor Cells / Q000378:metabolism / Q000502:physiology*; D011188:Potassium / Q000378:metabolism; D011189:Potassium Chloride / Q000378:metabolism; D011894:Rana pipiens; D051381:Rats; D012168:Retinal Pigments / Q000502:physiology; D012964:Sodium / Q000378:metabolism; D012965:Sodium Chloride / Q000378:metabolism; D013431:Sulfates / Q000378:metabolism
chemical_list: D000085:Acetates; D002712:Chlorides; D009566:Nitrates; D012168:Retinal Pigments; D013431:Sulfates; D012965:Sodium Chloride; D011189:Potassium Chloride; D000641:Ammonia; D012964:Sodium; D005990:Glycerol; D011188:Potassium; D002118:Calcium