[ABSTRACT]
1. Membrane current was recorded from an isolated, dark-adapted toad
rod by sucking either its inner segment or outer segment into a tight-fitting
glass pipette containing Ringer solution. The remainder of the cell was
exposed to bath solution which could be changed rapidly.
2. In normal Ringer solution the current response of a cell to a saturating
flash or step of light showed a small secondary rise at its initial peak.
The profile of this secondary rise (i.e. amplitude and time course) was
independent of both the intensity and the duration of illumination once
the light response had reached a plateau level.
3. This secondary rise disappeared when external Na+ around the outer
segment was replaced by Li+ or guanidinium, suggesting that it represented
an electrogenic Na+-dependent Ca2+ efflux which was declining after the
onset of light.
4. This Na+-Ca2+ exchange activity showed a roughly exponential decline,
with a time constant of about 0.5 s. Exponential extrapolation of the exchange
current to the time at half-height of the light response gave an initial
amplitude of about 2 pA. Using La3+ as a blocker, we did not detect any
steady exchange current after the initial exponential decline.
5. An intense flash superposed on a just-saturating steady background
light failed to produce any incremental exchange current transient.
6. Our interpretation of the above results is that in darkness there
are counterbalancing levels of Ca2+ influx (through the light-sensitive
conductance) and efflux (through the Na+-Ca2+ exchange) across the plasma
membrane of the rod outer segment. The exchange current transient at the
onset of light merely represents the unidirectional Ca2+ efflux which becomes
revealed as a result of the stoppage of the Ca2+ influx, rather than a
de novo Ca2+ efflux triggered by light.
7. Consistent with this interpretation, a test light delivered soon
after a saturating, conditioning light elicited little exchange current,
which then gradually recovered to control value with a time course parallel
to the restoration of the dark current. Conversely, when the dark current
was increased above its physiological level by IBMX (isobutylmethylxanthine)
the exchange current transient became larger than control.(ABSTRACT TRUNCATED
AT 400 WORDS)