Calcium and magnesium fluxes across the plasma membrane of the toad rod outer segment.

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)

Toward a unified model of vertebrate rod phototransduction

“…This feature has been observed at step-offset in other recordings (e.g. salamander: Nakatani & Yau, 1988;Koutalos et al, 1995;primate: Tamura et al, 1991).…”

“…7, the model did not have an explicit mechanism to reproduce the pronounced slow phase of rod photocurrent recovery frequently seen at the offset of steps of light (e.g. Nakatani & Yau, 1988;Forti et al, 1989;Tamura et al, 1991;Koutalos et al, 1995) and late in the recovery to bright flashes (e.g. Baylor et al, 1974;Torre et al, 1986;Pepperberg et al, 1992Pepperberg et al, ,1994.…”

Molecular cloning, functional expression and chromosomal localization of a human homolog of the cyclic nucleotide‐gated ion channel of retinal cone photoreceptors

“…A solenoid-controlled rotary valve system [40] was used to change solutions; the change was usually complete within 2 s. All experiments were performed at room temperature (20-25°C).…”

Interactions between divalent cations and the gating machinery of cyclic GMP-activated channels in salamander retinal rods.

“…Although internal Ni 2+ competes with internal Mg 2+ (Fig. 2), and Mg 2+ passes through the channel (Nakatani and Yau, 1988a;Baylor, 1988, 1992;Colamartino et al, 1991), we do not know whether Ni r+ can pass through the channel. This, as well as the fractional electrical distance of the blocking sites from both sides of the membrane, are subjects for future study.…”

“…The cGMP-activated channel has a very low effective unit conductance under physiological conditions (Bodoia and Detwiler, 1985;Gray and Attwell, 1985) due to block of Na ÷ entry by external Ca 2+ and Mg 2+ ions (Haynes, Kay, and Yau, 1986;Zimmerman and Baylor, 1986), which themselves carry a small fraction of the inward current (Nakatani and Yau, 1988a). Ca ~+ entry in the dark provides negative feedback control of cGMP synthesis (LoUey and Racz, 1982;Pepe, Panfoli, and Cugnoli, 1986;Hodgkin and Nunn, 1988;Koch and Stryer, 1988;Dizhoor, Ray, Kumar, Niemi, Spencer, Brolley, Walsh, Philipov, Hurley, and Stryer, 1991).…”