Extrusion of calcium from rod outer segments is driven by both sodium and potassium gradients

Response properties of cones from the retina of the tiger salamander

1. Spectral sensitivity measurements using the suction electrode technique reveal three types of cone in the retina of the tiger salamander, showing maximum sensitivity at wavelengths 610 nm (red-sensitive cone), 444 nm (blue-sensitive cone) and below 400 nm (UV-sensitive cone). 2. The absolute sensitivities of red- and blue-sensitive cones to flashes of optimal wavelength are 0.022 and 0.33 pA photon-1 micron 2 respectively. 3. The time-to-peak of the dim flash response and the recovery of membrane current after a flash of any intensity are fastest in red-sensitive and slowest in blue-sensitive cones. 4. In blue- and UV-sensitive cones the flash response peaks progressively earlier as the flash strength is increased, as in rods. In red-sensitive cones, however, bright flash responses take longer to peak than dim flash responses. 5. In all three cone types, voltage clamping at -40 mV reduces the time-to-peak of the response to a bright flash, showing that the rising phase of the bright flash response is normally limited by the time constant of the cell. Under voltage clamp, all cones show a decrease in time-to-peak with increasing flash intensity. 6. Voltage clamping red-sensitive cones reveals two components of the rising phase of the response to a bright flash. Most of the current is rapidly suppressed by a bright flash, and represents the closure of light-sensitive channels. The residual current decays with a mean time constant of 20 ms, and is probably attributable to the decline of electrogenic Na(+)-Ca2+, K+ exchange. The amplitude of this exchange current suggests that the proportion of the dark current carried by calcium ions is greater in red-sensitive cones than in rods of the same species. 7. In UV-sensitive cones, a prominent oscillation of light-sensitive current is observed during the recovery from flashes of intermediate intensity. A similar, but slower and less prominent oscillation is usually seen in blue-sensitive cones. 8. When a red-sensitive cone is voltage clamped an oscillation similar to those in the other two cone types is revealed. An underswing of up to 2 pA is also observed after recovery from intermediate or bright flashes in the majority of red-sensitive cones, and voltage clamping increases the amplitude of this underswing.(ABSTRACT TRUNCATED AT 400 WORDS)

The sodium-calcium exchanger of bovine rod photoreceptors: K(+)-dependence of the purified and reconstituted protein

The K(+)-dependence of the rod photoreceptor sodium-calcium exchanger was investigated using the Ca2(+)-sensitive dye arsenazo III after reconstitution of the purified protein into proteoliposomes. The uptake of Ca2+ by Na(+)-loaded liposomes was found to be greatly enhanced by the presence of external K+ (EC50 approximately 1 mM) in a Michaelis-Menten manner, suggesting that one K+ ion is involved in the transport of one Ca2+ ion. We also found a minimal degree of Ca2+ uptake in the total absence of K+. Other alkali cations, notably Rb+ and, to a lesser extent, Cs+, were also able to stimulate Na(+)-Ca2+ exchange. We also investigated the K(+)-dependence of the photoreceptor Na(+)-Ca2+ exchanger by determining the effects of electrochemical K+ gradients on the Na(+)-activated Ca2+ efflux from proteoliposomes. We found that, under conditions of membrane voltage clamp with FCCP, inwardly directed electrochemical K+ gradients (i.e., K0+ greater than Ki+) inhibited, whereas an outwardly directed electrochemical K+ gradient (i.e., Ki+ greater than K0+) enhanced, Na(+)-dependent Ca2+ efflux, consistent with the notion that K+ is cotransported in the same direction as Ca2+. The investigation of the reconstituted exchanger at physiological (i.e. Ki+ = 110 mM, K0+ = 2.5 mM) potassium concentrations revealed that the Na(+)-dependence of Ca2(+)-efflux was highly cooperative (n = 3.01 from Hill plots), indicating that at least three, but possibly four, Na+ ions are exchanged for one Ca2+ ion. Under these conditions the reconstituted exchanger showed a Km for Na+ of 26.1 mM, and a turnover number of 115 Ca2+.s-1 per exchanger molecule. Our results with the purified and reconstituted sodium-calcium exchanger from rod photoreceptors are therefore consistent with previous reports (Cervetto, L., Lagnado, L., Perry, R.J., Robinson, D.W. and McNaughton, P.A. (1989) Nature 337, 740-743; Schnetkamp, P.P.M., Basu, D.K. and Szerencsei, R.T. (1989) Am. J. Physiol. 257, C153-C157) that the sodium-calcium exchanger of rod photoreceptors cotransports K+ under physiological conditions with a stoichiometry of 4 Na+:1 Ca2+, 1K+.

A new cornea-positive component to the ERG of the aspartate-treated frog retina?

Using isolated bullfrog retinas treated with aspartate, we have found a new cornea-positive photo-response (positive response). The positive response could be detected only when the retina was stimulated by a dim flash. The peak amplitude and the rate of initial rise of the positive response were intensity dependent. The spectral sensitivity of the positive response peaked at 500 nm. The general characteristics of the response were different from those of the PIII response. The positive response was closely related to the extracellular Na+ and Ca2+ concentration and completely abolished by 50 microM La3+. On the basis of the present findings, it was suggested that some types of calcium channels or transporters are involved in the generation of the positive response.

Sodium-calcium exchange in cultured bovine pulmonary artery endothelial cells

1. Intracellular free calcium ([Ca2+]i) was measured in cultured bovine pulmonary artery endothelial cell monolayers loaded with the fluorescent calcium indicator Fura-2. 2. Resting [Ca2+]i was 112 +/- 10 nM. Application of ouabain (20 microM) was without effect on [Ca2+]i for periods of up to 1 h. Monensin (10 microM) resting [Ca2+]i to 145 +/- 32 nM over approximately 2 min. In the presence of ouabain (20 microM), 10 microM-monensin increased [Ca2+]i to 146 +/- 15 nM. 3. Removal of extracellular sodium was without effect in resting cells or cells exposed to ouabain alone. However, in the presence of monensin, replacement of extracellular Na+ with Li+ resulted in a prompt increase in [Ca2+]i to a peak of 280 +/- 37 nM, which then returned towards resting levels. When Na+ was removed in the presence of both ouabain and monensin, [Ca2+]i reached a peak of 585 +/- 53 nM. 4. When extracellular Na+ was replaced with K+, to achieve simultaneous Na+ removal and depolarization, [Ca2+]i reached a peak of 568 +/- 63 nM, compared with a peak of 462 +/- 38 nM when Li+ was used as a Na+ substitute in paired experiments. The transient increase in [Ca2+]i evoked by sodium removal peaked earlier when K+ was used as the sodium substitute, showing that depolarization increased the rate of calcium influx into the cell when sodium was removed from the bathing medium. 5. Removal of extracellular K+ had no effect on the low-Na(+)-evoked increase in [Ca2+]i. 6. Returning extracellular Na+ during the increase in [Ca2+]i resulting from Na+ removal increased the rate of return of [Ca2+]i towards basal levels. In the absence of Na+, [Ca2+]i took 41 +/- 5 s to decline from 400 to 200 nM, and this was reduced to 26 +/- 6 s (n = 4, S.E.M.) when Na+ was returned to the bathing solution. 7. These results indicate endothelial cells possess a voltage-dependent Na(+) -Ca2+ exchange mechanism in the surface membrane. However, this mechanism does not appear to be of primary importance in the maintenance of resting [Ca2+]i since cells were able to restore a low [Ca2+]i in the absence of extracellular Na+. The evidence for the existence of a Na(+) -Ca2+ exchanger in the surface membrane of endothelial cells and the possibility that this mechanism may contribute to calcium entry and/or extrusion during agonist-evoked responses is discussed.

Ca2+ extrusion across plasma membrane and Ca2+ uptake by intracellular stores

The aim of this review is to summarize the various systems that remove Ca2+ from the cytoplasm. We will initially focus on the Ca2+ pump and the Na(+)-Ca2+ exchanger of the plasma membrane. We will review the functional regulation of these systems and the recent progress obtained with molecular-biology techniques, which pointed to the existence of different isoforms of the Ca2+ pump. The Ca2+ pumps of the sarco(endo)plasmic reticulum will be discussed next, by summarizing the discoveries obtained with molecular-biology techniques, and by reviewing the physiological regulation of these proteins. We will finally briefly review the mitochondrial Ca(2+)-uptake mechanism.

Characterization of the process of sodium-calcium exchange in pancreatic islet cells

Na(+)-Ca2+ exchange may play a role in Ca2+ extrusion from the pancreatic B-cell. The characteristics of the process working in its reverse mode were examined in normal rat pancreatic islet cells. Isosmotical replacement of extracellular Na+ by sucrose induced a concentration-dependent increase in 45Ca uptake, displaying a pharmacological sensitivity compatible with an uptake mediated by Na(+)-Ca2+ exchange. Glucose, up to 2.8 mM, stimulated reverse Na(+)-Ca2+ exchange. Likewise, membrane depolarization activated the process but only under raised intracellular Na+ activity. In conclusion, the B-cell Na(+)-Ca2+ exchange displays properties similar to those observed in other cells: reversibility and sensitivity to membrane potential. When working in its reverse mode the exchanger displays a quite large capacity. The role played by the exchanger in the process of insulin release warrants further investigation.

Calcium ion-regulated phospholipase C activity in bovine rod outer segments

Bovine retinal rod outer segment membranes are enriched in a phosphoinositide-specific phosphodiesterase (phospholipase C) activity strictly modulated by free calcium ion concentration. The enzyme(s) was highly active on phosphatidylinositol 4,5-bisphosphate: maximal hydrolysis rate was attained at 10(-5)M Ca2+ and accounted for 91 +/- 4 nmoles hydrolyzed/min/mg of protein. The results support the notion that in vivo the enzyme(s) is regulated so as to conform to the phototransduction events.

Rapid induction of acetylcholine receptor aggregates by a neural factor and extracellular Ca2+

A soluble fetal brain extract (EBX) induces acetylcholine receptor (AChR) aggregation in cultured rat myotubes within 4 hr at 36 degrees C in a defined medium containing 1.8 mM (normal) extracellular Ca2+ (Olek et al., 1983). The activity of EBX was Ca2+ dependent; reducing extracellular Ca2+ significantly inhibited EBX-induced AChR aggregation and a 15-50% increase in extracellular Ca2+ synergistically enhanced the activity of EBX. Synergism was specific for Ca2+ as increases in other divalent cations (Ba2+, Co2+, Mg2+, Mn2+, Sr2+) had no effect. A large increase (300-500%) in extracellular Ca2+ alone also induced AChR aggregation within 4 hr at 36 degrees C. An equivalent increase in other cations (Ba2+, Co2+, Mg2+, Mn2+, Sr2+) did not promote AChR aggregation. An initial 15-min pulse of increased extracellular Ca2+ alone or with EBX was adequate to induce AChR aggregation. Aggregates induced by EBX, Ca2+ alone, or EBX/Ca2+ were found predominantly on the top surface of the myotube. These treatments did not detectably alter preexisting aggregates present at substrate contact sites on the bottom surface of myotubes. AChR aggregation induced by any treatment was not inhibited by cycloheximide, Ca2+ channel blockers, or protease inhibitors but was blocked by Co2+ and sodium azide.

The effects of quinidine on sodium-dependent calcium efflux in isolated rod photoreceptors of the salamander retina

The effect of quinidine on the membrane current generated by the Na:Ca, K exchange has been investigated in the outer segment of isolated rod photoreceptors from the retina of the larval tiger salamander. The inward exchange current associated with the efflux of Ca2+ was selectively recorded by introducing a Ca2+ load through the light-sensitive channels, and then shutting these channels with a bright light. Quinidine (20-1000 microM) reduced the magnitude of the exchange current and slowed its decay during the removal of a Ca2+ load. Quinidine did not alter the form of the relation between the exchange current and the total concentration of exchangeable calcium remaining within the outer segment. [Ca]T, showing that it does not change the affinity of the exchange mechanism for internal Ca2+. The relation between exchange current inhibition and the quinidine concentration could be described by a simple Michaelis relation with a Ki of 287 microM and a maximum inhibition of 50%. The incomplete block of the Na:Ca, K exchange current by quinidine shows that it does not act by simple competition with external Na+, and suggests that the inhibition of the exchange by quinidine may be non-specific.

Calcium and the mechanism of light adaptation in vertebrate photoreceptors

Vertebrate photoreceptors transduce the absorption of light into a hyperpolarizing change in membrane potential. The mechanism of transduction is becoming fairly well understood and has been shown to occur via a G protein-coupled decrease in cyclic GMP. Attention is now turning to the way the enzymatic machinery in the outer segment of the photoreceptor cell is modulated during light adaptation. Recent studies show that light adaptation cannot occur if changes in the concentration of cytoplasmic free calcium in the outer segment are prevented, suggesting that calcium functions as a second messenger in sensitivity regulation.

Visual transduction in cones of the monkey Macaca fascicularis

1. Visual transduction in macaque cones was studied by measuring the membrane current of single outer segments projecting from small pieces of retina. 2. The response to a brief flash of light was diphasic and resembled the output of a bandpass filter with a peak frequency near 5 Hz. After the initial reduction in dark current there was a rebound increase which resulted from an increase in the number of open light-sensitive channels. The response to a step of light consisted of a prominent initial peak followed by a steady phase of smaller amplitude. 3. Responses to dim light were linear and time-invariant, suggesting that responses to single photons were linearly additive. From the flash sensitivity and the effective collecting area the peak amplitude of the single photon response was estimated as about 30 fA. 4. With flashes of increasing strength the photocurrent amplitude usually saturated along a curve that was gentler than an exponential but steeper than a Michaelis relation. The response reached the half-saturating amplitude at roughly 650 photoisomerizations. 5. The response-intensity relation was flatter in the steady state than shortly after a light step was turned on, indicating that bright light desensitized the transduction with a delay. This desensitization was not due to a reduction in pigment content. In the steady state, a background of intensity I lowered the sensitivity to a weak incremental test flash by a factor 1/(1 + I/IO), where IO was about 2.6 x 10(4) photoisomerizations s-1, or about 3.3 log trolands for the red- and green-sensitive cones. 6. Bleaching exposures produced permanent reductions in flash sensitivity but had little effect on the kinetics or saturating amplitude of subsequent flash responses. The sensitivity reductions were consistent with the expected reductions in visual pigment content and gave photosensitivities of about 8 x 10(-9) microns2 (free solution value) for the red- and green-sensitive pigments. During a steady bleaching exposure the final exponential decline of the photocurrent had a rate constant given by the product of the light intensity and the photosensitivity. 7. In some cells it was possible to measure a light-induced increase in current noise. The power spectrum of the noise resembled the spectrum of the dim flash response and the magnitude of the noise was consistent with a single photon response roughly 20 fA in size. 8. The membrane current recorded in darkness was noisy, with a variance near 0.12 pA2 in the band 0-20 Hz.(ABSTRACT TRUNCATED AT 400 WORDS)

Kinetics, stoichiometry and role of the Na-Ca exchange mechanism in isolated cardiac myocytes

Compelling evidence has existed for more than a decade for a sodium/calcium (Na-Ca) exchange mechanism in the surface membrane of mammalian heart muscle cells which exchanges about three sodium ions for each calcium ion. Although it is known that cardiac muscle contraction is regulated by a transient increase in intracellular calcium ([Ca2+]i) triggered by the action potential, the contribution of the Na-Ca exchanger to the [Ca2+]i transient and to calcium extrusion during rest is unclear. To clarify these questions, changes in [Ca2+]i were measured with indo-1 in single cardiac myocytes which were voltage clamped and dialysed with a physiological level of sodium. We find that Ca entry through the Na-Ca exchanger is too slow to affect markedly the rate of rise of the normal [Ca2+]i transient. On repolarization, Ca extrusion by the exchanger causes [Ca2+]i to decline with a time constant of 0.5 s at -80 mV. The rate of decline can be slowed e-fold with a 77-mV depolarization. Calcium extrusion by the exchanger can account for about 15% of the rate of decline of the [Ca2+]i transient (the remainder being calcium resequestration by the sarcoplasmic reticulum (SR]. The ability of the cell to extrude calcium was greatly reduced on inhibiting the exchanger by removing external sodium, which itself led to an increase in resting [Ca2+]i. This finding is in contrast to the suggestion that calcium extrusion at rest is mediated mainly by a sarcolemmal Ca-ATPase.(ABSTRACT TRUNCATED AT 250 WORDS)

In squid nerve fibers monovalent activating cations are not cotransported during Na+/Ca2+ exchange

Squid axons display a high activity of Na+/Ca2+ exchange which is largely increased by the presence of external K+, Li+, Rb+ and NH+4. In this work we have investigated whether this effect is associated with the cotransport of the monovalent cation along with Ca2+ ions. 86Rb+ influx and efflux have been measured in dialyzed squid axons during the activation (presence of Ca2+i) of Ca2+o/Na+i and Ca2+i/Ca2+o exchanges, while 86Rb+ uptake was determined in squid optic nerve membrane vesicles under equilibrium Ca2+/Ca2+ exchange conditions. Our results show that although K+o significantly increases Na+i-dependent Ca2+ influx (reverse Na+/Ca2+ exchange) and Rb+i stimulates Ca2+o-dependent Ca2+ efflux (Ca2+/Ca2+ exchange), no sizable transport of rubidium ions is coupled to calcium movement through the exchanger. Moreover, in the isolated membrane preparation no 86Rb+ uptake was associated with Ca2+/Ca2+ exchange. We conclude that in squid axons although monovalent cations activate the Na+/Ca2+ exchange they are not cotransported.

Effects of metabolic blockade on the regulation of intracellular calcium in dissociated mouse sensory neurones

1. Impaired intracellular Ca2+ concentration ([Ca2+]i) regulation may underlie alterations in neuronal function during hypoxia or hypoglycaemia and may initiate cell damage. We have used the Ca2(+)-sensitive fluorophore, Fura-2, to study the regulation of [Ca2+]i in neurones isolated from mouse dorsal root ganglia. Mean resting [Ca2+]i was 163 +/- 11 nM (mean +/- S.E.M., n = 38). 2. Depolarization by exposure to 20 or 30 mM-K+ caused a rapid Co2(+)- and Cd2(+)-sensitive rise in [Ca2+]i, which subsequently declined with a time course usually fitted by the sum of two exponential functions. 3. Interference with mitochondrial function (by CN- or FCPP) or with glycolysis (by glucose removal) all raised [Ca2+]i by up to 220%. Addition of FCCP in the presence of CN- further increased [Ca2+]i. The response to CN- was still seen in the absence of extracellular Ca2+, although it attenuated rapidly, indicating release from an intracellular store. 4. Either CN- or glucose removal increased the rise in [Ca2+]i induced by K+ 2- to 3-fold and slowed recovery, suggesting interference with sequestration or extrusion of [Ca2+]i. 5. Resting [Ca2+]i rose when external Na+ was replaced by Li+ or N-methyl-D-glucamine, demonstrating the presence of a Na(+)-Ca2+ exchange process. However, Na+ replacement had only a slight effect on the handling of a Ca2+ load. 6. We conclude that (i) Ca2+ is released into the cytoplasm from intracellular organelles when energy supplies are reduced: (ii) that the extrusion or sequestration of Ca2+ entering the cell during electrical activity is rapidly impaired by interference with mitochondrial metabolism: and (iii) Na(+)-Ca2+ exchange makes only a small contribution to intracellular Ca2+ homeostasis. 7. [Ca2+]i would thus be expected to rise in vivo during hypoxia or hypoglycaemia and may initiate alterations in neuronal function. However, if a rise in Ca2+ is an important cause of cell damage in cerebral hypoxaemia, the combination of excitation and hypoxia will lead to the largest increases in [Ca2+]i, while hypoxia alone appears to cause only a small increase in [Ca2+]i in quiescent cells.

Combined force and voltage measurement in rapidly superfused guinea pig heart cells

We describe the construction and use of a setup that allows the rapid exchange of the solution surrounding an isolated guinea pig heart cell while simultaneously measuring the isometric force and membrane potential (Em). Cells were stably attached, by means of poly-L-lysine, to a force transducer which was adapted from one previously used for a study of frog atrial cells [N. Shepherd and F. Kavaler.Am. J. Physiol. 251 (Cell Physiol. 20): C653-C661, 1986]. The modified transducer is simple to construct and use and can be readily added to existing patch-clamp setups. The strength of attachment of a cell to the transducer exceeded the strength of the gigaseal in all of the experiments. The membrane potential was measured by means of patch electrodes and a high-impedance voltage follower. Rapidly changing extracellular K concentration [( K]o) from 5.4 to 10.8 mM caused a positive change of Em by 16.5 +/- 1.4 mV with a half-time (t1/2) of 27 +/- 4 ms. Replacing calcium in the perfusate by magnesium instantly abolished the contraction and shortened the action potential. Twitch tension returned stepwise to the control value on return of calcium to the perfusate. Our initial observations show that the patch electrode can be used successfully in conjunction with the isometric force transducer and rapid extracellular solution changes for studies of excitation and contraction coupling in isolated mammalian heart cells.

Regulation and deregulation of cardiac Na(+)-Ca2+ exchange in giant excised sarcolemmal membrane patches

A plasmalemmal Na(+)-Ca2+ exchange mechanism is an important electrogenic determinant of contractility in cardiac cells. As in other cell types, calcium influx by Na(+)-Ca2+ exchange is secondarily activated by cytoplasmic calcium and probably ATP, but these modulatory mechanisms are either absent or altered in isolated cardiac sarcolemmal vesicles. Involvement of a calcium-dependent protein kinase in exchange regulation has been suggested but not verified. Here I describe measurements of outward Na(+)-Ca2+ exchange current, corresponding to calcium influx, in giant excised sarcolemmal patches from guinea pig myocytes. The exchange current is stimulated by both calcium and Mg-ATP from the cytoplasmic face, evidently through separate mechanisms. Activation by cytoplasmic calcium takes place within seconds, is reversible, and does not require ATP. Stimulation by Mg-ATP reverses only slowly over greater than 10 min, or not at all. Unexpectedly, a substantial decrease in exchange current occurs during activation by cytoplasmic sodium, which seems to reflect an inactivation process rather than ion concentration changes or a 'first pass' exchange cycle. This apparent inactivation, and the modulations by cytoplasmic calcium and Mg-ATP, are all abolished by brief treatment of the cytoplasmic surface with chymotrypsin, leaving the exchanger in a maintained state of high activity. Therefore, limited proteolysis deregulates Na(+)-Ca2+ exchange and could contribute to the loss of secondary regulation of the exchange in isolated sarcolemmal vesicles.

Identification of the sodium-calcium exchanger as the major ricin-binding glycoprotein of bovine rod outer segments and its localization to the plasma membrane

After neuraminidase treatment the Na+/Ca2+ exchanger of bovine rod outer segments was found to specifically bind Ricinus communis agglutinin. SDS gel electrophoresis and Western blotting of ricin-binding proteins purified from rod outer segment membranes by lectin affinity chromatography revealed the existence of two major polypeptides of Mr 215K and 103K, the former of which was found to specifically react with PMe 1B3, a monoclonal antibody specific for the 230-kDa non-neuraminidase-treated Na+/Ca2+ exchanger. Reconstitution of the ricin affinity-purified exchanger into calcium-containing liposomes revealed that neuraminidase treatment had no significant effect on the kinetics of Na+/Ca2+ exchange activation by sodium. We further investigated the density of the Na+/Ca2+ exchanger in disk and plasma membrane preparations using Western blotting, radioimmunoassays, immunoelectron microscopy, and reconstitution procedures. The results indicate that the Na+/Ca2+ exchanger is localized in the rod photoreceptor plasma membrane and is absent or present in extremely low concentrations in disk membranes, as we have previously shown to be the case for the cGMP-gated cation channel. Previous reports describing the existence of Na+/Ca2+ exchange activity in rod outer segment disk membrane preparations may be due to the fusion of plasma membrane components and/or the presence of contaminating plasma membrane vesicles.

Is potassium co-transported by the cardiac Na-Ca exchange?

It has been suggested that the stoichiometry of the electrogenic Na-Ca exchange is 3Na:1Ca. Recently, however, it was reported in rod outer segments that the stoichiometry of Na-Ca exchange is not 3Na:1Ca but 4Na:1Ca + 1K. In cardiac cells, the reversal potential has always been measured in the absence of K or at a very low K concentration. We have, therefore, re-examined the reversal potential of the Na-Ca exchange current by whole-cell voltage clamp in single guinea-pig ventricular cells in the presence of K on both sides of the membrane. The Na-Ca exchange current reversed at potentials close to the calculated values for 3Na:1Ca stoichiometry even in the presence of K. The magnitude of the Na-Ca exchange current did not change in 1 and 10 mM [K]o. We therefore conclude that K is not co-transported by cardiac Na-Ca exchange and its stoichiometry is 3Na:1Ca.

Cyclic GMP and calcium: the internal messengers of excitation and adaptation in vertebrate photoreceptors

The roles of cyclic GMP (cGMP) and calcium (Ca2+) in vertebrate rod phototransduction are reviewed, with the emphasis on developments since the discovery of the cGMP-activated conductance of the rod outer segment. The first hypothesis subjected to critical examination is that cGMP acts as the sole internal messenger of excitation. This hypothesis is evaluated with a formal, quantitative model of the biochemical actions of cGMP. Application of the model shows a remarkable agreement between independent electrophysiological and biochemical measurements of the resting dark amounts of (1) total cGMP (2) free cGMP (3) fraction of open cGMP-activated channels and (4) the rate of cGMP hydrolysis. The second hypothesis examined is that Ca2+ acts as an internal messenger in rod light adaptation. Recent electrophysiological evidence has shown minimization of the normal light-induced reduction of free Ca2+ prevents rods from exhibiting the change in sensitivity and speed characteristic of light adaptation. Physiological effects, formerly attributed to a role of calcium as an excitational messenger are shown to be consistent with a biochemical model in which Ca2+ serves as the cytoplasmic signal in a powerful feedback loop that acts to restore the concentration of cGMP both during and after exposure to light. Residual problems facing the "cGMP cascade theory of phototransduction" are reviewed. Issues are itemized that will have to be resolved quantitatively before it will be possible to develop a fully comprehensive theory of photoreceptor excitation, restoration and adaptation combining the roles of Ca2+ and cGMP.

Kinetics and stoichiometry of coupled Na efflux and Ca influx (Na/Ca exchange) in barnacle muscle cells

Coupled Na+ exit/Ca2+ entry (Na/Ca exchange operating in the Ca2+ influx mode) was studied in giant barnacle muscle cells by measuring 22Na+ efflux and 45Ca2+ influx in internally perfused, ATP-fueled cells in which the Na+ pump was poisoned by 0.1 mM ouabain. Internal free Ca2+, [Ca2+]i, was controlled with a Ca-EGTA buffering system containing 8 mM EGTA and varying amounts of Ca2+. Ca2+ sequestration in internal stores was inhibited with caffeine and a mitochondrial uncoupler (FCCP). To maximize conditions for Ca2+ influx mode Na/Ca exchange, and to eliminate tracer Na/Na exchange, all of the external Na+ in the standard Na+ sea water (NaSW) was replaced by Tris or Li+ (Tris-SW or LiSW, respectively). In both Na-free solutions an external Ca2+ (Cao)-dependent Na+ efflux was observed when [Ca2+]i was increased above 10(-8) M; this efflux was half-maximally activated by [Ca2+]i = 0.3 microM (LiSW) to 0.7 microM (Tris-SW). The Cao-dependent Na+ efflux was half-maximally activated by [Ca2+]o = 2.0 mM in LiSW and 7.2 mM in Tris-SW; at saturating [Ca2+]o, [Ca2+]i, and [Na+]i the maximal (calculated) Cao-dependent Na+ efflux was approximately 75 pmol#cm2.s. This efflux was inhibited by external Na+ and La3+ with IC50's of approximately 125 and 0.4 mM, respectively. A Nai-dependent Ca2+ influx was also observed in Tris-SW. This Ca2+ influx also required [Ca2+]i greater than 10(-8) M. Internal Ca2+ activated a Nai-independent Ca2+ influx from LiSW (tracer Ca/Ca exchange), but in Tris-SW virtually all of the Cai-activated Ca2+ influx was Nai-dependent (Na/Ca exchange). Half-maximal activation was observed with [Na+]i = 30 mM. The fact that internal Ca2+ activates both a Cao-dependent Na+ efflux and a Nai-dependent Ca2+ influx in Tris-SW implies that these two fluxes are coupled; the activating (intracellular) Ca2+ does not appear to be transported by the exchanger. The maximal (calculated) Nai-dependent Ca2+ influx was -25 pmol/cm2.s. At various [Na+]i between 6 and 106 mM, the ratio of the Cao-dependent Na+ efflux to the Nai-dependent Ca2+ influx was 2.8-3.2:1 (mean = 3.1:1); this directly demonstrates that the stoichiometry (coupling ratio) of the Na/Ca exchange is 3:1. These observations on the coupling ratio and kinetics of the Na/Ca exchanger imply that in resting cells the exchanger turns over at a low rate because of the low [Ca2+]i; much of the Ca2+ extrusion at rest (approximately 1 pmol/cm2.s) is thus mediated by an ATP-driven Ca2+ pump.(ABSTRACT TRUNCATED AT 400 WORDS)