[ABSTRACT]
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)