Electrical properties of the light-sensitive conductance of rods of the salamander Ambystoma tigrinum

Guanosine 3',5'-cyclic monophosphate-activated conductance studied in a truncated rod outer segment of the toad

1. In darkness, a single rod outer segment isolated from the toad retina was sucked partially, tip first, into a tight-fitting, Ringer solution-filled glass pipette for recording membrane current. The basal end of the outer segment outside the pipette was sheared off with a probe to allow internal dialysis. The potential between the inside and the outside of the pipette was held at 0 mV. 2. With cyclic GMP and IBMX (isobutylmethylxanthine) in the dialysis solution, a large inward current appeared across the plasma membrane of the outer segment; this current saturated at around 1 mM-cyclic GMP. IBMX by itself was ineffective. 3. The saturated cyclic GMP-induced current recorded varied in size with the length of outer segment (L) within the suction pipette. For L less than 25 micron, the relation was linear, with a current density of 4-20 pA micron-1. 4. At short L (less than 25 micron), the dose-response relation between current magnitude and cyclic GMP concentration was sigmoidal, with a Hill coefficient (n) of 1.8-3.1 and a half-saturating cyclic GMP concentration (K1/2) of 30-85 microM. 5. In the presence of IBMX and the absence of GTP, the dose-response relation was the same in continuous bleaching light as in darkness. This indicates that both the characteristics of cyclic GMP binding and the intrinsic conduction properties of the open conductance are not affected by light. 6. Removing IBMX from the dialysing solution had little effect on the saturated current, but substantially reduced the current induced at low concentrations of cyclic GMP. When the analogue 8-bromo cyclic GMP was used instead, however, the presence of IBMX was relatively unimportant even at low agonist concentrations. These observations indicated that significant phosphodiesterase activity was present within the truncated outer segment. 7. In the absence of IBMX and the presence of GTP, the cyclic GMP-induced current could be suppressed by light. When ATP was also present in the dialysing solution, the effect of light was significantly reduced and the suppression also became more transient. 8. We conclude from the above results that the cyclic GMP-gated conductance is indeed present in the plasma membrane of the rod outer segment, and that this conductance and the light-sensitive conductance are one and the same entity. 9. From the results, we estimate that only about 1% of the conductance is normally open in darkness. This fraction of open conductance corresponds to a free cyclic GMP concentration of a few micromolar.

Kinetics and components of the flash photocurrent of isolated retinal rods of the larval salamander, Ambystoma tigrinum

1. Membrane currents initiated by intense, 20 microseconds flashes (photocurrents) were recorded from isolated salamander rods by combined extracellular suction electrodes and intracellular tight-seal electrodes either in current or voltage clamp mode. The magnitudes (mean +/- 2 S.E.M.) of the maximal photoresponses recorded by the suction and by the intracellular electrode respectively were 40 +/- 5 pA (n = 18) and 35 +/- 7 mV (n = 8) for current clamp at zero current; 43 +/- 9 pA and 66 +/- 13 (n = 11) pA for voltage clamp at the zero-current holding potential, -24 +/- 3 mV. 2. Photocurrents initiated by flashes isomerizing 0.1% or more of the outer segment's rhodopsin achieved a saturated velocity and were 95% complete in less than 50 ms. The effect of incrementing flash intensity above 0.1% isomerization can be described as a translation of the photocurrent along the time axis towards the origin. Within the interval 0-50 ms the latter two-thirds of the velocity-saturated photocurrent is well described as a single-exponential decay. The decay was much faster in voltage clamp (2.8 +/- 1.2 ms, n = 11) than in current clamp mode (17 +/- 5 ms, n = 17). 3. The initial third of the velocity-saturated photocurrent, occurring over the interval from the flash to the onset of exponential decay, followed about the same time course in current and voltage clamp. The time interval occupied by this initial 'latent' phase decreased with increasing flash intensity and attained an apparent minimum of about 7 ms in response to flashes isomerizing 10% or more of the rhodopsin at ca. 22 degrees C. 4. The hypothesis that the decay of outer segment light-sensitive membrane current is the same in current and voltage clamp was supported by an analysis of the difference between outer segment currents measured successively in the two recording modes. First, the tail of the difference current decayed exponentially with a time constant approximately equal to R x C, where R and C are independently estimated slope resistance and capacitance of the rod. Secondly, the integral of the difference current, when divided by outer segment capacitance, closely approximated the hyperpolarizing light response measured under current clamp. Thus, displacement current accounted for the difference between photocurrents measured in current and voltage clamp.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of ions on sodium-calcium exchange in salamander rods

1. The influence of external cations on the rate at which a Ca2+ load was eliminated in exchange for external Na+ was studied by measuring the inward current associated with Na+-Ca2+ exchange in salamander rods. 2. In Ringer solution the exchange current saturated at a well-defined level of about 20 pA at 20 degrees C. 3. The saturation level of exchange current, j(sat), was increased by lowering the external concentrations of H+, Ca2+, Mg2+ and K+; it was decreased by raising the external concentration of these ions or by lowering [Na+]O. 4. J(sat) varied approximately as [Na+]O2.4 between 35 and 110 mM-Na+. 5. The inhibitory constants for external Ca2+ and Mg2+ were about 1 and 4 mM, respectively. 6. An acid pH decreased j(sat) and an alkaline one increased it; the shape of the relation between current and pH suggests that one inhibitory proton combines between pH 8 and 10 and a pair combine between pH 6 and 7. 7. Removing K+, Mg2+, and Ca2+, and increasing the pH from 7.5 to 10 increased the measured exchange current from 20 to ca. 100 pA. 8. The integral of the Na+-Ca2+ exchange current varied with the Ca2+ load but was largely independent of external ionic changes in spite of large changes in j(sat). The apparent Na+-Ca2+ exchange ratio remained at a little under 3 over a wide range of conditions. 9. The constancy of the integral of the exchange current was brought about by reciprocal variations of the amplitude and duration of the current transient. Records in different solutions could usually be matched by scaling amplitude and time by reciprocal factors. 10. Increasing Nai+ by allowing large light-sensitive currents to flow in low-Ca2+ solutions affected the Na+-Ca2+ exchange transient in a different way from lowering [Na+]o or raising [Ca2+]o, etc. In an Na+-rich rod there was little reduction in j(sat) but the response was prolonged and larger Ca2+ loads were needed to reach saturation. Analysis in terms of a simple model indicated that a substantial Na+ load might reduce the apparent affinity of the internal pumping sites for Ca2+ by a factor of 10. 11. An attempt is made to relate these findings to a model of Na+-Ca2+ exchange.

Measurement of sodium-calcium exchange in salamander rods

1. Methods employing suction electrodes to measure the small inward currents associated with the exchange of internal Ca2+ for external Na+ in salamander rod outer segments are described. 2. The ratio of the integral of the exchange current to the integral of the Ca2+ current during the loading period averaged 0.37, which is consistent with 1 Ca2+ ion exchanging with 2.7 Na+ ions, in approximate agreement with Yau & Nakatani (1984b). 3. The transient pumping current observed when external Na+ was restored after a few seconds in isotonic Ca2+ with IBMX (3-isobutyl-1-methylxanthine) consisted of a phase with current at a constant saturated level followed by a phase in which current declined along a characteristic S-shaped curve that was much steeper than expected from the Michaelis equation. 4. The relation between Ca2+ load and pumping current was also steeper than a Michaelis relation. 5. In Ringer solution at 20 degrees C the saturated exchange current was about 20 pA and the value of charge at which the current was half-saturated was 1-5 pC corresponding to 6-30 X 10(6) Ca2+ ions per rod outer segment. 6. The Ca2+ exchange current after small loads declined along the same curve as that determined with medium loads but fell more slowly after large loads. 7. The exchange current at the beginning of the plateau of a strong flash response usually declined along the curve determined with small or medium Ca2+ loads. 8. There was evidence that the exchange current at the tip of the outer segment remained saturated for longer than at the base. 9. The time to pump out Ca2+ through the Na+-Ca2+ exchange system is largely responsible for the delay in the recovery of the light-sensitive current after a Ca2+ load. 10. A theoretical analysis of some of the observations in this and the succeeding paper is based on assumptions about the binding of Ca2+ by exchange sites and by cytoplasmic Ca2+ buffers.

The properties and function of inward rectification in rod photoreceptors of the tiger salamander

1. Rod photoreceptors were isolated from the retinae of tiger salamanders and voltage clamped using the whole-cell patch-clamp technique. 2. Hyperpolarizing the cell to potentials more negative than -50 mV evoked an inward current termed Ih. 3. Ih did not turn on immediately following a hyperpolarizing step but showed a marked delay. The activation time course of Ih could be described by the sum of two exponential components of opposite polarity. 4. The steady-state chord-conductance was half activated at -67 mV. 5. The reversal potential of Ih was close to -30 mV in normal standard salt solution. Increasing the external potassium concentration tenfold shifted the reversal potential by +17 mV. 6. The conductance-voltage relation and the kinetic parameters were not affected by changes in the external potassium concentration. 7. When fully activated, the zero-current conductance underlying Ih depended on the square root of the concentration of external potassium. 8. The permeability ratio PNa/PK depended on the external potassium concentration. It was 0.2 at an external potassium concentration of 2.0 mM and 0.3 at an external potassium concentration of 10.0 mM. The interaction of potassium with Ih suggests that Ih is a multi-ion pore. 9. It is concluded that Ih differs from the inward rectifier that is found in egg cells, frog muscle and heart muscle. 10. The kinetics and voltage sensitivity of Ih suggest that it does not play a role in the dark resting state or in the response to dim flashes of light. Its properties indicate that it may have a major role in the response to bright flashes.

Properties of ion channels closed by light and opened by guanosine 3',5'-cyclic monophosphate in toad retinal rods

1. In patch-clamp recordings from outer segments of dark-adapted rod photoreceptors, single-channel recordings were obtained from the light-sensitive conductance when divalent cations were omitted from the pipette solution bathing the extracellular face of the recorded patch of membrane. 2. Activity of the light-sensitive channel was suppressed by light within the normal response range of the dark-adapted rod. During dim, steady illumination, the rate of opening of the channel fluctuated dramatically, as expected qualitatively from statistical fluctuations in the number of photoisomerizations occurring within the effective collecting area of the recorded patch. 3. The light-sensitive channel flickered rapidly in the open state, so that individual events appeared as a burst of openings and closings. The average duration of a burst was 0.78 +/- 0.03 ms (mean +/- S.E.). The average duration of an individual opening was 0.18 +/- 0.008 ms. The average closed duration within a burst was 0.37 +/- 0.02 ms. 4. Hyperpolarization of the recorded patch had no effect on average burst or open duration, although opening frequency increased slightly (+18.6 +/- 4.9%; n = 13; mean +/- S.E.). Average single-channel current increased linearly with hyperpolarization, giving an estimated single-channel conductance of 20.5 +/- 1.1 pS. By extrapolation of the relation between channel current and hyperpolarization, the dark driving force was estimated to be about 48 mV. 5. In addition to reducing the rate of channel events, dim non-saturating light also reduced the average duration of a burst of openings and the average duration of openings within a burst. 6. About 50% of cell-attached patches showed no channel activity in darkness. Light-suppressable channel activity could be induced in these silent patches by perfusing the outer segment with low-Ca2+ Ringer solution. Similarly, activity could be increased dramatically by low-Ca2+ Ringer solution in patches that did show channel activity in the dark. From the maximal channel activity observed during low-Ca2+ perfusion, the lower limit for the number of channels per patch was 20-70, corresponding to an estimated channel density of 100-350 channels micron-2. 7. After recording light-sensitive channel activity in the intact rod, the patch of membrane was excised, exposing the intracellular membrane face. Application of guanosine 3',5'-cyclic monophosphate (cyclic GMP) to the intracellular face activated channels (Haynes, Kay & Yau, 1986; Zimmerman & Baylor, 1986; Matthews, 1986d, 1987) whose properties could then be compared directly with the light-sensitive channels recorded earlier in the same patch of membrane.(ABSTRACT TRUNCATED AT 400 WORDS)

Role of calcium in regulating the cyclic GMP cascade of phototransduction in retinal rods

Both cGMP and Ca2+ appear to be involved in the process of phototransduction in vertebrate rods, but their precise roles have been the subject of debate. To investigate the role of Ca2+ we have artificially increased the calcium buffering capacity of the rod by using a patch pipet to incorporate the calcium buffer 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) into the rod cytoplasm. In the presence of buffer the Na+-Ca2+ exchange current became greatly slowed, suggesting that the cytoplasmic calcium concentration (Cai) had indeed been buffered substantially. Although the presence of buffer had negligible effect on the rising phase of the light response, it profoundly altered the later behavior. Responses to brief flashes became prolonged and exhibited an overshoot, apparently because the shut-off process was modified. The normal acceleration of time-to-peak with brighter flashes (an early sign of light adaptation) disappeared. Responses to steady adapting illumination took much longer than normal to settle to a steady level, although the final level represented a similar fractional suppression of current. With superimposed test flashes the presence of such adapting illumination caused a more rapid recovery, whereas the presence of calcium buffer slowed the recovery. The results are consistent with the idea that the rapid drop in Cai, which has recently been shown to accompany the light response, is involved in terminating the light response, and that Cai is thereby involved in setting the operating point and sensitivity of phototransduction. From comparison with other work we infer that Cai appears to act, at least in part, by means of control of cGMP phosphodiesterase activity.

Incorporation of calcium buffers into salamander retinal rods: a rejection of the calcium hypothesis of phototransduction

The suction-electrode technique was used to monitor the photocurrent of isolated retinal rods from the tiger salamander, by drawing in the light-sensitive outer segment, or sometimes the inner segment. Calcium buffers or other agents were then introduced into the rod cytoplasm by the 'whole-cell patch-clamp' technique. A patch pipette was sealed against the region of the rod protruding from the suction pipette (usually the inner segment), and the membrane patch was ruptured to obtain a whole-cell recording. Several lines of evidence indicated that the pipette contents diffused into the outer segment, and showed that the cell could be adequately voltage clamped. With only trace quantities of chelator in the patch pipette (to bind stray calcium), a gradual decline of the dark current and slowing of responses was usually observed over a period of 10-20 min after rupture of the patch. When the patch pipette contained no added calcium and 10 mM of the calcium chelator BAPTA (1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) (free Ca2+ ca. 10(-9) M) rupture of the patch led, over a period of a few minutes, to an increase in mean dark current, an increased duration of responses, a substantial increase in flash sensitivity, and a pronounced overshoot in the recovery phase, but with virtually no change in the rising phase of the response to bright flashes. Similar results were obtained when EGTA was used in place of BAPTA, and also in the few cases when successful rupture of the outer segment membrane was obtained. With the free calcium concentration in the patch pipette buffered to the higher level of 1 microM (with 10 mM-Ca2+/11 mM-BAPTA) the results were qualitatively similar to those obtained with BAPTA alone, except that the mean dark current did not increase. This is consistent with a resting free calcium concentration in darkness in the region of 1 microM. In the presence of bright steady illumination with BAPTA in the cell the suppression of outer segment current could be maintained for at least 15 min. Upon extinction of the light a very large current transient developed (similar to the overshoot with flashes) which was light suppressible. With backgrounds of moderate intensity the incorporation of buffer led to a gradual reduction of the residual current.(ABSTRACT TRUNCATED AT 400 WORDS)