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, glycerol, acetate, and ammonium rapidly enter the outer segment, but K+, SO, and melezitose appear impermeable. The Na flux is rectified; for concentration gradients in the physiological range, 2 X 109 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 (1I sec) "photoresistance" which reduces the Na+ influx by about 1 %, thus preventing the entry of about 107 Na + ions. At considerably higher light levels, a stable afterimage resistance appears which reduces the Na influx by one-half when 10°rhodopsin molecules are bleached per rod. We have incorporated these findings into a model for the electrophysiological characteristics of the receptor.

Transmission along and between rods in the tiger salamander retina.

“…The studies presented above, which are generally consistent with the earlier papers (Werblin 1975;Cervetto et al 1977), generate two important questions: (1) how can the lack of a reversal potential (Werblin, 1975, and here), or a reversal potential more negative than the dark level (Lasansky & Marchiafava, 1973;Cervetto et al 1977), be reconciled with evidence for a light-elicited sodium conductance change (Hagins, Penn & Yoshikami, 1970;Yoshikami & Hagins, 1971;Korenbrot & Cone, 1972) and, (2) is it legitimate to look for a conductance change when recording from a single rod when the rods may be strongly coupled (Schwartz, 1976)? Some insight into the resolution of these questions can be gained by studying the coupling of the components of an individual rod, and the coupling between rods, as outlined below.…”

“…Although there is good evidence that the light response is initiated by a conductance decrease at the rod outer segment (Brown & Pinto, 1974;Hagins, Penn & Yoshikami, 1970;Korenbrot & Cone, 1972; Sillman, Ito & Tomita, 1969), it has been suggested that the distributed nature of light-elicited current through the rod network might obscure the intracellular measurement of this conductance decrease (Schwartz, 1976). Although a light-elicited conductance decrease has been measured in some preparations (Toyoda, Nosaki & Tomita, 1969;Toyoda, Hashimoto & Anno, 1970), no conductance change was measured in mudpuppy (Werblin, 1975) and a conductance increase has been measured in tiger salamander (Lasansky & Marchiafava, 1974).…”

In vivo optophysiology reveals that G-protein activation triggers osmotic swelling and increased light scattering of rod photoreceptors

“…In constructing a model for phototransduction-induced osmotic swelling of the rod OS, we first considered the consequences of water influx. As moderate osmotic perturbation alters only rod OS length and not width (40,41), length increases can be readily converted into volume changes and influx rates. The swelling response showed a light dependence consistent with maximal activation of Gα t , reaching a saturated amplitude in response to light that isomerizes ∼1 R* per Gα t (Figs.…”

“…Because most proteins can act as elastic elements (50), it is reasonable to hypothesize that such disc-to-disc "linkers" serve as springs that resist swelling, affording some protection of the rod OS from stimulus-induced osmotic overload. As rod OSs primarily swell axially in response to osmotic challenge (40,41), if the OS has an elastic response, the steadystate elongation should obey Hookes' law…”

Effects of calcium and guanosine‐3',5'‐cyclic‐monophosphoric acid on receptor potentials of toad rods.

“…The absorption of light causes photoisomerization of rhodopsin in the rod disk membrane and a subsequent decrease in the Na conductance ofthe plasma membrane and in the Na conductance of the plasma membrane of the rod outer segment (Yoshikami & Hagins, 1971; Korenbrot & Cone, 1972;Brown & Pinto, 1974). Little is known about the events that intervene between the absorption of photons in rod disks and changes in plasma membrane Na conductance, but it is thought that light induces a change in the concentration of an intracellular messenger (Baylor & Fuortes, 1970) which diffuses from the disk membrane to the plasma membrane and reduces the Na conductance.…”