Metabolic flux of cyclic GMP and phototransduction in rabbit retina

Efficacy and selectivity of phosphodiesterase-targeted drugs in inhibiting photoreceptor phosphodiesterase (PDE6) in retinal photoreceptors

PURPOSE

Phosphodiesterase (PDE) inhibitors are important therapeutic agents, but their effects on photoreceptor PDE (PDE6) and photoreceptor cells are poorly understood. The potency and selectivity of various classes of PDE inhibitors on purified rod and cone PDE6 and on intact rod outer segments (ROS) were characterized.

METHODS

The inhibition constant (K(i)) of isozyme-selective PDE inhibitors was determined for purified rod and cone PDE6. Perturbations of cGMP levels in isolated ROS suspensions by PDE inhibitors were quantitated by a cGMP enzyme-linked immunoassay.

RESULTS

Most PDE5-selective inhibitors were excellent PDE6 inhibitors. Vardenafil, a potent PDE5 inhibitor (K(i) = 0.2 nM), was the most potent PDE6 inhibitor tested (K(i) = 0.7 nM). Zaprinast was the only drug that inhibited PDE6 more potently than did PDE5. PDE1-selective inhibitors were equally effective in inhibiting PDE6. In intact ROS, PDE inhibitors elevated cGMP levels, but none fully inhibited PDE6. Their potency for elevating cGMP levels in ROS was much lower than their ability to inhibit the purified enzyme. Competition between PDE5/6-selective drugs and the inhibitory gamma-subunit for the active site of PDE6 is proposed to reduce the effectiveness of drugs at the enzyme-active site.

CONCLUSIONS

Several classes of PDE inhibitors inhibit PDE6 equally as well as the PDE family to which they are targeted. In intact ROS, high PDE6 concentrations, binding of the gamma-subunit to the active site, and calcium feedback mechanisms attenuate the effectiveness of PDE inhibitors to inhibit PDE6 and disrupt the cGMP signaling pathway during visual transduction.

Creatine transporter localization in developing and adult retina: importance of creatine to retinal function

Creatine and phosphocreatine are required to maintain ATP needed for normal retinal function and development. The aim of the present study was to determine the distribution of the creatine transporter (CRT) to gain insight to how creatine is transported into the retina. An affinity-purified antibody raised against the CRT was applied to adult vertebrate retinas and to mouse retina during development. Confocal microscopy was used to identify the localization pattern as well as co-localization patterns with a range of retinal neurochemical markers. Strong labeling of the CRT was seen in the photoreceptor inner segments in all species studied and labeling of a variety of inner neuronal cells (amacrine, bipolar, and ganglion cells), the retinal nerve fibers and sites of creatine transport into the retina (retinal pigment epithelium, inner retinal blood vessels, and perivascular astrocytes). The CRT was not expressed in Müller cells of any of the species studied. The lack of labeling of Müller cells suggests that neurons are independent of this glial cell in accumulating creatine. During mouse retinal development, expression of the CRT progressively increased throughout the retina until approximately postnatal day 10, with a subsequent decrease. Comparison of the distribution patterns of the CRT in vascular and avascular vertebrate retinas and studies of the mouse retina during development indicate that creatine and phosphocreatine are important for ATP homeostasis.

Viagra (sildenafil citrate) and ophthalmology

Viagra (sildenafil citrate) improves penile erections in men with erectile dysfunction (ED) by selectively inhibiting cGMP-specific phosphodiesterase type 5 (PDE5), which is present in all vascular tissue. It also exerts a minor inhibitory action against PDE6, which is present exclusively in rod and cone photoreceptors. At higher doses, sildenafil causes mild and transient visual symptoms in a minority of patients (mainly blue tinge to vision, increased brightness of lights). Therefore, the effects of sildenafil on the visual system have been investigated in a wide variety of clinical and preclinical studies. In preclinical studies, sildenafil shows transient reversible effects on electrical response to light. In long-term toxicology studies in which animals were exposed to high multiples of the maximum human therapeutic dose, detailed examinations have revealed no adverse effects on the structure or function of the eye. The effects of sildenafil have been systematically investigated in visual function studies in volunteers and in patients with eye disease; sildenafil does not affect visual acuity, visual fields, and contrast sensitivity. The only definite effect is transient, mild impairment of color discrimination occurring around the time of peak plasma levels. In long-term studies, no long-term effects of sildenafil on the visual system have been observed. Postmarketing, sildenafil has been prescribed to over 15 million men with ED. Isolated examples of a variety of visual adverse events have been reported. No consistent pattern has emerged to suggest any long-term effect of sildenafil on the retina or other structures of the eye. Based on this experience, intermittent, short-term, partial inhibition of PDE5 or PDE6 by sildenafil is unlikely to induce any long-term visual change.

Free cyclic AMP increases in PC12 cells on depolarization

The temporal dynamics of the intracellular second messenger cyclic AMP (cAMP) were monitored in living PC12 cells by digital fluorescence ratio imaging using FlCRh, a single-excitation dual-emission cAMP indicator. When the cells were depolarized by exposure to high K+, the free cAMP concentration was elevated, and then slowly decreased back to resting levels when the depolarizing stimulus was removed. Furthermore, the cAMP elevation due to depolarization decreased with successive depolarizations.

Inhibition of nitric oxide synthase alters light responses and dark voltage of amphibian photoreceptors

We studied the effects of competitive inhibitors of nitric oxide synthase (L-NMMA and L-NNA) on dark voltage and flash responses of retinal rods of the frog. Substances were applied intracellularly via whole-cell patch-clamp electrodes while the membrane voltage was recorded simultaneously. During recording the exchange of substances by diffusion between cytosol and pipette medium affects the cell's function. Under control conditions this exchange is reflected by a slow hyperpolarization of the dark voltage with time and a prolongated flash response recovery, which is mainly due to a loss of nucleotides. Application of L-NMMA and L-NNA accelerated the spontaneous hyperpolarization of the membrane voltage during the course of an experiment, while the recovery of the flash responses was slowed down. The effects observed upon intracellular application of NO-synthase inhibitors were opposite to those observed previously upon application of sodium nitroprusside. Sodium nitroprusside was much less effective when the intracellular calcium level was decreased by application of EGTA at the same time. It is reasonable to assume that the observed effects are linked to nitric oxide synthase and to a NO-dependent soluble guanylate cyclase. The results suggest that the activity of NO-synthase in photoreceptor cells has an influence on concentration and metabolic flux of cGMP in photoreceptors, which may be of relevance for flash response recovery and adaptation processes. It is likely that the regulation of the soluble guanylate cyclase requires a physiological level of calcium.

Steady state feedback in mammalian phototransduction illustrated by a nomogram

Published data characterizing the four reactions responsible for the Ca(2+)-mediated negative feedback in mammalian rod phototransduction were used to generate graphs which are combined in a circular fashion so that the y-axis of one serves as the x-axis of the next. The nomogram thus created makes it possible to determine by inspection the steady state situation in darkness, and the quasi-steady state situations that pertain shortly after exposure to light of different intensities. The results predicted by the nomogram suggest that Ca(2+)-mediated negative feedback is responsible for the Weber-Fechner relationship between stimulus and response.

Regulation of intracellular cyclic GMP concentration by light and calcium in electropermeabilized rod photoreceptors

This study examines the regulation of cGMP by illumination and by calcium during signal transduction in vertebrate retinal photoreceptor cells. We employed an electropermeabilized rod outer segment (EP-ROS) preparation which permits perfusion of low molecular weight compounds into the cytosol while retaining many of the features of physiologically competent, intact rod outer segments (ROS). When nucleotide-depleted EP-ROS were incubated with MgGTP, time- and dose-dependent increases in intracellular cGMP levels were observed. The steady state cGMP concentration in EP-ROS (0.007 mol cGMP per mol rhodopsin) approached the cGMP concentration in intact ROS. Flash illumination of EP-ROS in a 250-nM free calcium medium resulted in a transient decrease in cGMP levels; this occurred in the absence of changes in calcium concentration. The kinetics of the cGMP response to flash illumination of EP-ROS were similar to that of intact ROS. To further examine the effects of calcium on cGMP metabolism, dark-adapted EP-ROS were incubated with MgGTP containing various concentrations of calcium. We observed a twofold increase in cGMP steady state levels as the free calcium was lowered from 1 microM to 20 nM; this increase was comparable to the behavior of intact ROS. Measurements of guanylate cyclase activity in EP-ROS showed a 3.5-fold increase in activity over this range of calcium concentrations, indicating a retention of calcium regulation of guanylate cyclase in EP-ROS preparations. Flash illumination of EP-ROS in either a 50- or 250-nM free calcium medium revealed a slowing of the recovery time course at the lower calcium concentration. This observation conflicts with any hypothesis whereby a reduction in free calcium concentration hastens the recovery of cytoplasmic cGMP levels, either by stimulating guanylate cyclase activity or by inhibiting phosphodiesterase activity. We conclude that changes in the intracellular calcium concentration during visual transduction may have more complex effects on the recovery of the photoresponse than can be accounted for solely by guanylate cyclase activation.

Modulation of a sustained calcium current by intracellular pH in horizontal cells of fish retina

A sustained high voltage-activated (HVA), nifedipine- and cadmium-sensitive calcium current and a sustained calcium action potential (AP) were recorded from horizontal cells isolated from catfish retina. pH indicator dyes showed that superfusion with NH4Cl alkalinized these cells and that washout of NH4Cl or superfusion with Na-acetate acidified them. HVA current was slightly enhanced during superfusion of NH4Cl but was suppressed upon NH4Cl washout or application of Na-acetate. When 25 mM HEPES was added to the patch pipette to increase intracellular pH buffering, the effects of NH4Cl and Na-acetate on HVA current were reduced. These results indicated that intracellular acidification reduces HVA calcium current and alkalinization increases it. Sustained APs, recorded with high resistance, small diameter microelectrodes, were blocked by cobalt and cadmium and their magnitude varied with extracellular calcium concentration. These results provide confirmatory evidence that the HVA current is a major component of the AP and indicate that the AP can be used as a measure of how the HVA current can be modified in intact, undialyzed cells. The duration of APs was increased by superfusion with NH4Cl and reduced by washout of NH4Cl or superfusion with Na-acetate. The Na-acetate and NH4Cl washout-dependent shortening of the APs was observed in the presence of intracellular BAPTA, a calcium chelator, IBMX, a phosphodiesterase inhibitor, and in Na-free or TEA-enriched saline. These findings provide supportive evidence that intracellular acidification may directly suppress the HVA calcium current in intact cells. Intracellular pH changes would thereby be expected to modulate not only the resting membrane potential of these cells in darkness, but calcium-dependent release of neurotransmitter from these cells as well. Furthermore, this acidification-dependent suppression of calcium current could serve a protective role by reducing calcium entry during retinal ischemia, which is usually thought to be accompanied by intracellular acidosis.

Species differences in the response of mammalian photoreceptor cyclic GMP and PIII to a reduction in calcium

Changes in the content of cyclic GMP (cGMP), induced by exposure of isolated, dark-adapted mouse, cat and dog retinas to media depleted of calcium, have been compared with the amplitude of the trans-retinal PIII. Major differences exist in the time-course and magnitude of effects between the species and, in the cat and dog, changes in PIII (potentially a reflection of free cGMP in photoreceptor outer segments) do not correlate with those occurring in total cGMP. The observations imply species variation, not only in the enzymes maintaining cGMP homeostasis in photoreceptors, but also in phototransduction and allied processes.

The magnitude, time course and spatial distribution of current induced in salamander rods by cyclic guanine nucleotides

1. Cyclic GMP was introduced into isolated salamander rods through a tight-seal electrode attached to the inner segment while the outer segment was held in a suction electrode; nucleotide-induced membrane current was recorded by both electrodes. After 3-15s of nucleotide exposure the cells were stimulated with intense, brief flashes, which suppressed 90-95% of the induced membrane current. 2. The magnitude of the induced light-sensitive current depended little on the pipette cyclic GMP concentration in the range 10-20 nM: the mean whole-cell current magnitude was 1256 +/- 160 pA (mean +/- 2 S.E.M., n = 41). 3. Experiments and analyses addressed hypotheses about the nature of the magnitude limitation on the induced current. It was shown that the spatial distribution of nucleotide, the residual series resistance of the whole-cell electrode, and the diminution of the ion gradients driving the induced current did not limit the current magnitude by more than 20%. 4. In contrast, the hypothesis that outer segment internal longitudinal resistance severely limits the magnitude of the cyclic GMP-induced current was supported by experiments in which various lengths of the outer segment were drawn into the suction electrode. These showed that the ratio of nucleotide-induced light-sensitive current collected by the suction electrode to that collected by a whole-cell electrode decreased steeply as a function of outer segment length excluded from the suction electrode, having an apparent space constant of 5-7 microns. 5. A cable model of the rod was developed and used to analyse the magnitude of the nucleotide-induced currents. The data are consistent with an outer segment longitudinal resistance of 1-4 M omega microns-1, and a maximum (space-clamped) light-sensitive current density of 313 pA microns-1, equivalent to a total induced current of 7200 pA (23 microns outer segment). 6. A diffusion model was developed and combined with the non-linear cable model to provide an account for the time course of the induced membrane currents. The results are consistent with an effective longitudinal diffusion coefficient of cyclic GMP in the outer segment of 3-10 microns2 s-1, and Hill coefficient of 2-3 for the cyclic GMP gating of the light-sensitive conductance. 7. 8-Bromo-cyclic GMP also caused the light-sensitive membrane current to increase to about the same magnitude as did cyclic GMP.(ABSTRACT TRUNCATED AT 400 WORDS)

Atrial natriuretic factor increases cyclic GMP and cyclic AMP levels in a directly photosensitive pineal organ

Atrial natriuretic factor (ANF) stimulates accumulation of cyclic GMP in a photosensitive organ, as evidenced for the first time in cultured trout pineals. Stimulation was rapid (within a few min), dose-dependent, and stronger in organs cultured in darkness than in those cultured under light. After 30 min in the dark, (i) cyclic AMP levels were slightly increased at 10(-7) mole/l of ANF, (ii) cyclic GMP and cyclic AMP increased dramatically after inhibition of the phosphodiesterases by isobutylmethylxanthine (IBMX), (iii) ANF and IBMX effects were more than additive on cyclic GMP, (iv) pertussis toxin decreased the cyclic GMP response to ANF. These responses were affected by light. The possibility that cyclic GMP might be a second messenger of both light and chemical (ANF) inputs, in pineal photoreceptor cells, is hypothetized.