Sodium-dependent calcium extrusion and sensitivity regulation in retinal cones of the salamander.
1. Membrane current was recorded from an isolated, dark-adapted salamander cone by sucking its inner segment into a tight-fitting glass pipette containing Ringer solution. The outer segment of the cell was exposed to a bath solution that could be changed rapidly. 2. After removing Na+ from the bath Ringer solution for a short period of time in darkness (the 'loading period'), a transient inward current was observed upon restoring it in bright light. A similar but longer-lasting current was observed when Na+ was restored in the light after a large Ca2+ influx was induced through the light-sensitive conductance in darkness. 3. The above transient current was not observed if Li+ or guanidinium was substituted for Na+ in the light, or if Ba2+ was substituted for Ca2+ during the dark loading period. However, a current was observed if Sr2+ was the substituting ion for Ca2+ during loading. These observations suggested that the current was associated with an electrogenic Na+-dependent Ca2+ efflux at the cone outer segment. 4. The saturated amplitude of the exchange current was 12-25 pA with a mean around 16 pA. This is very comparable to that measured in the outer segment of a salamander rod under similar conditions. 5. By comparing a known Ca2+ load in a cone outer segment to the subsequent charge transfer through the exchange, we estimated that the stoichiometry of the exchange was near 3Na+:1Ca2+. 6. With a small Ca2+ load, or in the presence of Cs+ around the inner segment, the final temporal decline of the Na+-Ca2+ exchange current was roughly exponential, with a mean time constant of about 100 ms. This decline is about four times faster than that measured in rods. We interpret the shorter time constant in cones to reflect a faster rate of decline of intracellular free Ca2+ in their outer segments resulting from the exchange activity. 7. In the absence of external Na+, and hence any Na+-dependent Ca2+ efflux, the absolute sensitivity of a cone to a dim flash was several times higher than in normal Ringer solution. 8. A roughly similar increase in light sensitivity was observed for a rod under the same conditions. 9. We conclude that the Na+-dependent Ca2+ efflux, through lowering intracellular free Ca2+ in the light, has a role in regulating the absolute light sensitivity in cones as it does in rods.(ABSTRACT TRUNCATED AT 400 WORDS)
Nakatani
K
K
Howard Hughes Medical Institute, Johns Hopkins University School of Medicine, Baltimore, MD 21205.
Yau
K W
KW
D015056:1-Methyl-3-isobutylxanthine / Q000494:pharmacology; D000200:Action Potentials / Q000187:drug effects; D000557:Ambystoma; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002413:Cations, Divalent / Q000494:pharmacology; D002414:Cations, Monovalent / Q000494:pharmacology; D002794:Choline / Q000494:pharmacology; D066298:In Vitro Techniques; D007473:Ion Channels / Q000378:metabolism; D008027:Light; D010786:Photoreceptor Cells / Q000502:physiology*; D012160:Retina / Q000502:physiology*; D012964:Sodium / Q000494:pharmacology*
D002413:Cations, Divalent; D002414:Cations, Monovalent; D007473:Ion Channels; D012964:Sodium; D002794:Choline; D002118:Calcium; D015056:1-Methyl-3-isobutylxanthine