Blood-eye barriers in the rat: correlation of ultrastructure with function

The function of different vascular beds in the rat eye and brain was evaluated by measuring the transfer of a vascular tracer, 14C-alpha-amino-isobutyric acid, from blood to tissue. The density of vascular pores was measured in electron micrographs of perfusion-fixed, age-matched tissue to determine whether the differences in tracer transfer were paralleled by differences in ultrastructure. Tracer transfer in retina was approximately four times that in brain of the same animal. The transfer constant was not changed by the inclusion of cold alpha-amino-isobutyric acid, showing that transport across retinal vessels is not saturable, and indicating that, as in brain, transport is due to passive diffusion. Ultrastructurally, retinal vessels have a higher density of interendothelial junctions and of endothelial vesicles, both of which suggest higher vascular permeability. However, pericytes, which contribute to a second line of defence in the blood-brain barrier, are approximately four times as numerous in retina as in brain, and we suggest that in the retina, they act to compensate for a more permeable endothelial barrier. Ciliary body vessels had a high transfer of tracer, probably as a consequence of the fenestrations in their walls. Iridial vessels had a relatively low transfer of tracer, similar to that in retina even though a proportion of the interendothelial junctions in iridial vessels had expanded junctional clefts suggestive of open paracellular channels. However, both iris and ciliary body may lose tracer to the anterior chamber fluid, leading us to underestimate the vascular permeability in these sites.

The effect of cycloplegia on measurement of the ocular components

PURPOSE

The purpose of this study was to examine the effect of cycloplegic agent on the measurement of refractive error and the ocular components.

METHODS

We compared two commonly used topical cycloplegic agents, 1% tropicamide and 1% cyclopentolate, for their effect on the measurement of refractive error (by Canon R-1 autorefraction), accommodative response (by Canon R-1 autorefraction and by the conventional, subjective "pushup" method), crystalline lens power (by video phakometry and by calculation), and axial ocular dimensions (by A-scan ultrasonography) in 20 emmetropic to moderately hyperopic children.

RESULTS

Comparison of refractive error at each drug's reported time of maximum cycloplegia (30 minutes for tropicamide and 60 minutes for cyclopentolate) showed that distance autorefraction in the vertical meridian differed by +0.20 +/- 0.30 diopters (D) (P = 0.008). The average difference was +0.07 +/- 0.10 mm for anterior chamber depth (P = 0.004), -0.03 +/- 0.05 mm for crystalline lens thickness (P = 0.025), -0.65 +/- 0.69 D for phakometrically measured crystalline lens power (P < 0.001), +0.03 +/- 1.55 D for calculated crystalline lens power (P = 0.94), and -0.09 +/- 0.19 mm for vitreous chamber depth (P = 0.062, all paired t tests; positive signs denote greater values with cyclopentolate). Residual accommodation was 0.47 and 0.67 D greater with tropicamide when measured by autorefraction and the pushup method (P = 0.013 and 0.08 respectively, paired t test). All significant differences were consistently in the direction of poorer cycloplegia with tropicamide.

CONCLUSIONS

Although tropicamide, as expected, showed poorer cycloplegia compared to cyclopentolate, the degree of difference appeared to be small, with minimal effect on the measurement of distance refractive error and the ocular optical components.

Dihydropyridine-sensitive Ca2+ spikes and Ca2+ currents in rabbit ciliary body epithelial cells

Intracellular microelectrode and whole-cell patch-clamp recordings were used to investigate a Ba(2+)-induced regenerative depolarization and its underlying Ba2+ current in the ciliary body epithelial cells of the rabbit eye. Exposure of these epithelial cells to 4-10 mmol l-1 Ba2+ depolarized the membrane potential and caused the generation of one or more spikes, before the membrane potential reached a steady-state level. The spikes, but not the slow phase of depolarization, could be blocked with Co2+ (2 mmol l-1), Gd3+ (25 mumol l-1), La3+ (20 mumol l-1), Cd2+ (10 mumol l-1), verapamil (30 mumol l-1) and nifedipine (1 mumol l-1). Tetrodotoxin at 100 nmol l-1 had no effect. In the absence of Na+, but in the presence of external Ba2+, step depolarization of the membrane potential activated an inward current that could be blocked with Co2+ (2 mmol l-1), Cd2+ (10 mumol l-1) and nifedipine (1 mumol l-1), but not with Ni2+ (50 mumol l-1) or omega-conotoxin (1-10 mumol). This inward current could be enhanced with the dihydropyridine agonist (+/-)BAY K 8644 (1 mumol l-1). The inactivation characteristics of the inward current (v1/2 = -38.7 mV, k = 12.6 mV) is most like that seen in neurons. These findings indicate that the epithelial cells of the ciliary body possess dihydropyridine-sensitive, voltage-activated Ca2+ channels.

A physical model demonstrating Schachar's hypothesis of accommodation

Schachar's hypothesis of accommodation states that there is increased zonular tension during accommodation and the observed in vivo changes in lenticular curvature that occur during accommodation are the result of zonular forces. To demonstrate that there can be steepening of the central curvature of the lens with increased zonular tension, profile photographs of an equatorially unstretched and stretched gelatin-filled balloon and profile photographs from the literature of an unstretched and equatorially stretched human lens were digitized. Their radii of curvature were determined. We found that equatorial stretching of both the gelatin-filled balloon and the human lens produced central curvature steepening that was consistent with Schachar's hypothesis of accommodation.

Characterization of muscarinic receptor involvement in human ciliary muscle cell function

Muscarinic agonist-induced contraction of the ciliary muscle is generally believed to increase aqueous outflow facility and effect accommodation. We used cultured human ciliary muscle cells as a model to study the muscarinic receptor subtype(s) involved in the contractile response of the muscle. Thus, a single cell contraction assay for these muscle cells was developed. And since agonist-induced contraction of smooth muscles is expected to involve the activation of phospholipase C (PLC), we also monitored the PLC activity in these cells. Carbachol caused contraction of the muscle cells in a dose-dependent and time-dependent manner with an estimated EC50 of 1-3 microM. The contractile effect of 100 microM carbachol was antagonized by pretreatment of atropine (1 microM) and 4DAMP (10 nM, antagonist selective for the M1 and M3 receptors) but not by pirenzepine (10 microM, antagonist selective for the M1 receptor), suggesting the involvement of the M3 but not the M1 muscarinic receptor. M3 receptor is also essential for the carbachol-induced PLC activation in the ciliary muscle cells, as indicated by the activity profiles of receptor subtype selective antagonists. For example, the stimulative effect of carbachol (EC50 = 20 microM) was antagonized by pirenzepine (pKi = 6.8), HHSiD (pKi = 7.6), 4DAMP (pKi = 9.5) and methoctramine (pKi < 6). Thus, these results indicate that an M3-like receptor subtype is essential in mediating the muscarinic agonists-induced functional changes, such as PLC activation or muscle contraction, in the ciliary muscle.

In vitro alteration of human lens curvatures by radial stretching

The shapes of two human lenses of different ages (27 and 46 years old) were altered by applying a stretching force to the ciliary muscle. The force was in the outward radial direction and was produced by an electronically driven stretching instrument which is described. The lenses were photographed for different degrees of stretch, and their anterior and posterior curvatures analysed. All curves could be fitted to second order polynomial functions. The posterior curvatures did not appear to alter with stretch and were almost identical in both lenses. Although the anterior surfaces of both lenses changed with application of the stretching force, that of the younger lens showed a considerably greater alteration. Most changes in sagittal thickness occurred with radial stretching of up to 1.0 mm. The results of this work are compared to those of other studies on both functional and in vitro lenses.

Tissue culture of rabbit ciliary body epithelial cells on permeable supports

The aqueous humor is produced by the epithelium of the ciliary body, a complex structure encircling the anterior segment of the eye. Aqueous humor production occurs by active transport, but the mechanism of this process is not understood. To produce a preparation in which active transport can be investigated, we have attempted to prepare cultures suitable for measurements of ion and water flux. We have grown rabbit ciliary body epithelial cells on permeable supports, coated with several extracellular matrix proteins. We then examined the ability of these proteins to promote the growth of a differentiated layer of epithelial cells. Non-pigmented and pigmented cells formed sheets of contiguous cells when grown on a variety of support media. The most successful substrate was a permeable support produced by Falcon/Becton Dickinson coated with a mixture of collagen IV, laminin and heparan sulfate. Under these conditions, cultures could be maintained for several months, but pigmented cell cultures did not develop a measurable transepithelial resistance (TER), and the TER of non-pigmented cell cultures was typically only 20-30 omega cm2. Much higher TERs exceeding 200 omega cm2 could be measured from non-pigmented cell cultures 3-5 days after plating, but these high values were unstable. Examination of the cultures with electron microscopy revealed that the cells were partially differentiated with the formation of a basal lamina and intercellular junctions. Labelling with a specific monoclonal antibody marker for tight junction protein (ZO-1) suggested that non-pigmented cell cultures showed extensive tight junction formation. The low TER of the non-pigmented cell cultures appears therefore not to be due to the lack of tight junctions but rather to the presence of spaces between cells.

Prejunctional receptors and second messengers for angiotensin II in the rabbit iris-ciliary body

Angiotensin II has been shown to act prejunctionally to facilitate sympathetic neutrotransmission in various tissues including the iris-ciliary body. In the present study, we characterized the prejunctional angiotensin II receptor subtype and its signal transduction pathway in the rabbit iris-ciliary body. Angiotensin II caused concentration-dependent facilitation of electrically evoked [3H]-norepinephrine overflow from the isolated, superfused rabbit iris-ciliary body without affecting basal tritium efflux. Responses to angiotensin II were antagonized by saralasin and DuP753 but not by PD123177 indicating that prejunctional angiotensin II receptors of the AT1-subtype mediate the facilitation of evoked [3H]-norepinephrine release. The non-selective cyclic nucleotide phosphodiesterase inhibitor, isobutylmethyl xanthine enhanced the angiotensin II response whereas the cAMP-specific phosphodiesterase inhibitor, RO-20-1724 had no effect. In the presence of 8-bromo-cGMP, responses elicited by angiotensin II were significantly (P < 0.01) greater than that caused in the absence of 8-bromo-cGMP. In contrast, 8-bromo-cAMP had no effect on the angiotensin II-induced response. Guanylate cyclase inhibitors, methylene blue and LY83583 abolished angiotensin II-induced enhancement of [3H]-norepinephrine overflow without affecting basal tritium efflux. Taken together, these results suggest that cGMP could be involved in the angiotensin II response. Neither phospholipase C inhibitors (neomycin, 2-nitro-4-carboxyphenyl-N,N-diphenyl carbamate and phenylmethylsulfonyl fluoride) nor an inhibitor of protein kinase C (staurosporine) had any significant effect on the angiotensin II response, indicating that metabolites of inositol phospholipid metabolism or activation of protein kinase C are not involved in the response to this peptide.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of choroidal and ciliary nerve transection on choroidal blood flow, retinal health, and ocular enlargement

Our previous studies suggested that reduced choroidal blood flow (CBF) occurs with manipulations that yield myopic eye growth and that these reductions are primarily a consequence of the ocular enlargement. We could not entirely rule out the possibility, however, that reductions in CBF are at least to some extent antecedent and causal to the ocular enlargement. We therefore in the present study examined the effects on eye size of artificially reducing CBF by unilaterally transecting the choroidal nerves of the ciliary ganglion in four-day-old chicks. For comparison, we also transected the ciliary nerves in a second group of chicks or transected both ciliary and choroidal nerves in a third group of chicks. The effects of the nerve transections were evaluated in comparison to the effects of the orbital surgery itself (without nerve transection) in a fourth group termed the sham-operated control group. Two weeks after transection, CBF was measured using laser Doppler velocimetry, the ocular axial, nasotemporal and dorsoventral lengths were measured, and the eyes weighed. The results showed that CBF in birds with either choroidal nerve cuts or choroidal plus ciliary nerve cuts was greatly reduced in the treated eye (20-40% of nontreated eye). The treated eyes of these birds also showed gross depigmentation and histologically evident loss of the outer retina, most typically in the temporal retina. Birds with ciliary nerve cuts showed increased CBF in both eyes (131% right eye and 154% left eye compared to shams). Since ciliary nerve cuts yield fixed dilated pupils, increased CBF with ciliary nerve cuts appears consistent with the previously reported involvement of the choroidal nerves within a neural circuit subserving light-mediated upregulation of CBF. Clear effects on eye size were observed in the treated eyes in each group. The sham surgery alone yielded slight enlargement of the right eye compared to left eye, particularly in the axial dimension. In the choroidal nerve and the both nerve cut groups, nasotemporal and dorsoventral elongation were slightly diminished in the treated eyes compared to the sham-treated eyes. In contrast, enlargement of the right eye was slightly enhanced in the ciliary nerve cut group compared to the sham-treated eyes. The overall results suggest that large decreases in CBF do not enhance myopic eye growth, although large increases in CBF may.

Membrane potential oscillation from a novel combination of ion channels

Single pigmented epithelial cells from the ciliary body of the eye were studied using the whole cell voltage and current clamp, permeabilized patch recording, and patch-clamp recording. These cells can produce two types of oscillation. Both are slow, with a period in the range of 1-2 min; one has a low amplitude and oscillates between -60 and -80 mV, and the second is larger, with biphasic hyperpolarizing and depolarizing phases. The latter was seen when the membrane potential was driven negative by a constant current and results from the interplay between the inward rectifier K+ channel and a hyperpolarizing-activated cation channel. The hyperpolarization is caused by the constant current acting on a decreasing conductance as the inward rectifier inactivates, and the depolarization drive results from the activation of cation channels. It is suggested that the constant current would be provided by the Na+ pump in vivo, and such an interplay of channels and pumps could drive the uptake of cations in absorbing epithelia or provide an increased driving force for chloride exit in secretory epithelia.

Expression of the rasT24 oncogene in the ciliary body pigment epithelium and retinal pigment epithelium results in hyperplasia, adenoma, and adenocarcinoma

We examined eye lesions in five lines of transgenic mice carrying the human rasT24 oncogene driven by the type I gamma glutamyl transferase (gamma GT) promoter. In three lines, hyperplasia developed as early as 11.5 days postconception in the outer neuroectodermal layer, which gives rise to ciliary body and retinal pigment epithelium. At birth, the eyes from many animals contained adenomas, and by day 27, mice developed invasive adenocarcinomas originating in the region of the ciliary body. Microphthalmia, cataracts, and chronic nongranulomatous inflammation involving the anterior and/or posterior segments of the eye were also found. gamma GT is detectable histochemically as early as 11.5 gestational days in the outer neuroectodermal layer and after birth is more abundant in the ciliary body than in the retinal pigment epithelium. Using a reverse transcriptase-polymerase chain reaction, we found that type I (but not types II or III) gamma GT RNA is made by the mouse eye; the gamma GT(I)rasT24 transgene transcription product was detected in the eyes of all five transgenic lines. The sequential progression of hyperplasia to invasive neoplasms in the ciliary body in response to gamma GT(I)rasT24 expression differs from the process in the kidney of these animals in which tubular hyperplasia and microadenomas with little evidence of progression are the major lesions.

Biophysics of the cochlea: linear approximation

Several deficiencies affecting previous "box" models of the cochlea are overcome in this paper. Both mechanical and hydrodynamical aspects are treated at a level adequate to the complexity of realistic cochlear structures. The dynamics of the cochlea as a passive physical system, in the linear approximation, is described by an integral equation. It is further shown that this equation describes the properties of the working cochlea, provided a force term that accounts for hair cell motility is included. Numerical solutions for different degrees of outer hair cells activity, obtained by matrix methods in the frequency domain, are presented. Amplitudes and phases of the computer-simulated traveling waves are in fair agreement with basilar membrane responses to tones measured in various experimental conditions.

[The effect of intraocular lens (IOL) length and design on anterior chamber depth under contraction and relaxation of the ciliary muscle]

We compared the effect of intraocular lens (IOL) length and its design on the anterior chamber depth (ACD) change during contraction and relaxation of the ciliary muscle. We performed phacoemulsification or extra capsular cataract extraction using continuous circular capsulorhexis on 100 eyes, and implanted in 23 eyes a short one-piece IOL, in 22 eyes a short three-piece IOL, in 25 eyes a long one-piece IOL, and in 30 eyes a long three-piece IOL. All IOLs were fixed in the capsular bag. ACD changes after administration of 2% pilocarpine and 1% cyclopentolate were measured with a pachymeter between 3 and 6 months postoperatively. The compatibility of the IOLs with ciliary muscle contraction and relaxation was, in descending order, the short one-piece IOL, the short three-piece IOL, the long one-piece IOL, and the long three-piece IOL. The results indicated that short IOLs are more physiologically fixed in the capsular bag, with less stress on the zonules of Zinn.

Age does not affect contractile responses of the isolated rhesus monkey ciliary muscle to muscarinic agonists

In primates, ciliary muscle contraction causes accommodation and facilitates aqueous outflow. In living rhesus monkeys, accommodative, outflow facility, and ciliary muscle movement responses to cholinergic agonists all decline with age. We developed an apparatus to determine in vitro whether the latter is related to intra- or extra-ciliary muscle factors, and whether ciliary muscle contraction in the coronal (putatively more accommodation-relevant) and longitudinal (putatively more facility-relevant) vectors can be dissociated pharmacologically. In fresh ciliary muscle strips, carbachol and aceclidine each induced dose-dependent contraction in the longitudinal and coronal vectors. With neither drug was there any apparent dissociation of the responses in the two vectors. Atropine pretreatment completely prevented a supramaximal dose of carbachol from inducing ciliary muscle contraction in either vector. Ciliary muscle strips responded to several cholinergic agonists as well on day 2 (24-32 hours post-enucleation) as on day 1 (1-9 hours post-enucleation) when kept in a cell culture medium at 4 degrees C. By light microscopy, the general architecture of the ciliary muscle, the muscle bundles, and the single muscle cells appeared normal; however, cellular and nuclear swelling were apparent following the 32-hour culturing period. Contractile responses to near-maximal doses of carbachol and aceclidine did not vary markedly with age in either vector, suggesting that the age-related decrease in ciliary muscle mobility in vivo is due to extra-muscular restrictive factors rather than diminished muscular contractility.

Single-cell contraction assay for human ciliary muscle cells. Effect of carbachol

PURPOSE

The authors developed an assay to observe the contraction of a single human ciliary muscle cell.

METHODS

Cultured human ciliary muscle cells were partially detached from the culture dish by incubation with a nonenzymatic dissociation buffer and treated with carbachol or pilocarpine. Contraction was quantified by measuring the cross-sectional surface areas of the cells.

RESULTS

Carbachol decreased the cell surface area in a time-dependent manner. Contraction was observed within 1 min after the addition of carbachol and completed in less than 15 min. The effect of carbachol was dose dependent. For example, at 10 min after treatment with 10 mumol/l carbachol, the relative surface areas of cells decreased to 47% +/- 4% (mean +/- standard error of the mean, n = 7, with surface area at 0 min defined as 100%). The relative surface areas were 74% +/- 4% (n = 7) after 1 mumol/l and 100% +/- 9% (n = 7) after 0.1 mumol/l carbachol treatment. This contractile effect was antagonized by pretreatment with atropine, a specific muscarinic antagonist.

CONCLUSIONS

A simple method was established to study the functional changes of human ciliary muscle cells.

Heterogeneity of endothelium-dependent regulation in ophthalmic and ciliary arteries

PURPOSE

Endothelial cells modulate vascular tone by releasing the vasodilator nitric oxide (NO) or the vasoconstrictor endothelin-1. From one vascular bed to another and between vessels of different diameter, heterogeneities of endothelium-dependent regulatory mechanisms exist. Hence, the current study compared the effects of NO and endothelin-1 in the porcine ophthalmic artery and one of its branches, the ciliary artery.

METHODS

Porcine eyes were obtained at the slaughterhouse. The ophthalmic and ciliary arteries were dissected free under a microscope and suspended in myograph systems (95% O2 and 5% CO2, 37 degrees C) for isometric tension recording.

RESULTS

In both vessels, bradykinin stimulated the release of NO, but the sensitivity to bradykinin increased with decreasing vascular diameter. By contrast, the basal release of NO became less efficient in inhibiting contractions to serotonin and endothelin-1 in ciliary versus ophthalmic artery. Endothelin-1 induced potent contractions that were more pronounced in ciliary than in ophthalmic artery. Serotonin-induced contractions also were more efficient in ciliary artery but less than to those to endothelin-1. Contractions to serotonin were inhibited in both blood vessels by the 5-HT2 serotonergic antagonist ketanserin.

CONCLUSIONS

Thus endothelium-derived vasoactive substances are potent regulators of porcine extraocular ophthalmic circulation. Their effects increase with decreasing vascular diameter, suggesting an important role of NO and endothelin-1 in the regulation of ophthalmic circulation. A dysfunction of these regulatory mechanisms could have implications about the pathogenesis of ophthalmic complications seen in diabetes, hypertension, and in certain forms of glaucoma associated with ocular vasospasms.

The Glenn A. Fry Award Lecture 1992: aqueous humor proteins: a key for unlocking glaucoma?

In this report I summarize recently reported studies which establish the existence of a previously undocumented diffusional pathway for plasma-derived proteins from the ciliary body stroma to the anterior chamber of the normal mammalian eye via the iris root. The data support the hypothesis that nearly all of the plasma-derived protein present in the aqueous humor of the anterior chamber arrives via this pathway, thus by-passing the posterior chamber. Additional studies suggest that some of the protein transported via the pathway is shunted directly into the trabecular meshwork and aqueous outflow pathways. Perfusion studies show how the perfused eyes were later immunohistochemically stained to reveal the distribution and qualitative amounts of serum albumin. These studies strongly suggest that the well-known "wash-out" effect, encountered in experimental studies of aqueous outflow, arises predominantly from wash-out of the protein depot that serves as the anterior diffusional pathway for proteins. Taken as a whole, these studies support the hypothesis that protein is a physiologically relevant and important element in the generation of normal aqueous outflow resistance, possibly more important than glycosaminoglycans (GAG's).

Studies on intraocular inflammation produced by intravitreal human interleukins in rabbits

Recombinant human interleukin-1 alpha, interleukin-1 beta (10, 80 or 200 units), interleukin-8 (10 or 40 units) or endotoxin was injected intravitreally into rabbit eyes. Twenty-four hours thereafter aqueous humor protein, leukocyte number, prostaglandin E2, leukotriene B4 and rabbit interleukin-1 beta were measured. In addition, synthesis of prostaglandin E2 and leukotriene B4 in iris-ciliary body and myeloperoxidase (MPO) activity were determined. Recombinant human interleukins 1 alpha and 1 beta, but not interleukin-8 induced signs of uveitis, i.e. protein and leukocytic infiltration into aqueous humor. At 200 unit activities, human interleukin-1 beta was significantly greater than interleukin-1 alpha in causing leukocyte infiltration response. Interleukin-1 alpha did not stimulate the release of prostaglandin E2 or leukotriene B4. In fact, interleukin-1 beta significantly inhibited the synthesis of prostaglandin E2 in iris-ciliary body. Both of these human interleukins caused a release of rabbit interleukin-1 beta in aqueous achieving a level significantly higher than observed after endotoxin injection. This study demonstrates that intravitreal injections of human IL-1 alpha and IL-1 beta induce uveitis by releasing rabbit interleukin-1 beta within the eye.

[Adrenergic influence on the outflow facility and the ciliary muscle of enucleated bovine eyes]

In an effort to understand the aqueous outflow mechanism, the author compared changes in the outflow facility with the response of ciliary muscle, using fresh bovine eyes. Neither epinephrine nor isoproterenol alone increased the outflow facility, regardless of their concentrations. However, theophylline, caffeine, isobutylmethylxanthine (IBMX), and other phosphodiesterase (PDE) inhibitors, increased facility in a dose-dependent manner. Epinephrine alone neither relaxed the tone nor inhibited the nerve-mediated contraction of bovine ciliary muscle. Theophylline and IBMX relaxed the tone and inhibited the nerve-mediated contraction, whereas N-ethylmaleimide, iodoacetamide and iodoacetic acid also inhibited the nerve-mediated contraction, and elevated the tone of the ciliary muscle. All PDE inhibitors tested increased the outflow facility and inhibited the nerve-mediated contractions of the ciliary muscle, however, the effect of such drugs on the ciliary muscle tone varied. These results suggest that PDE inhibition may increase the conventional outflow of aqueous humor. PDE inhibitors has much greater influence on and around the outflow channels, from the viewpoint of change of outflow facility and ciliary muscle contraction.

Adrenergic stimulation of bovine non-pigmented ciliary epithelial cells raises cAMP but has no effect on K+ or Cl- currents

Freshly isolated bovine nonpigmented ciliary epithelial cells were studied using the whole-cell voltage-clamp and permeabilized patch techniques. In a study of 148 cells three classes of cell were found; those containing inward currents alone, 31%; those containing outward currents alone, 37% and those containing both inward and outward currents, 32%. The outward currents exhibited slow, delayed activation, were blocked by tetraethylammonium (TEA+) but were not effected by changes in [Cai] suggesting they are K(+)-currents similar to IK(V), the delayed rectifier. The inward currents were reduced by TEA+ and blocked by Cs+ suggesting they are inward rectifier K(+)-currents. Intracellular cAMP levels were increased by isoprenaline with a Ka of 20 nM. However, the resting membrane potential recorded from whole ciliary processes in intracellular microelectrode studies was not effected by adrenergic stimulation, neither were the leak current, the outward current nor the sustained inward current significantly effected by isoprenaline, noradrenaline or dibutyryl-cAMP in whole-cell and permeabilized patch clamp studies.

Glycocalyx and ciliary interconnections in bullfrog vestibular end organs: normal structure and changes after incubation in frog Ringer's solution

The ultrastructure of ciliary interconnecting systems as well as changes after incubation in frog Ringer's solution of the bullfrog vestibular end organs were investigated by using a scanning electron microscopy. The plasma membrane of the sensory hairs showed rather rough appearance. Side links emerged from the plasma membrane and tightly connected neighboring cilia. The tip links stretched from the tips of the stereocilia to their taller neighbors. These tip links were arranged in a same direction towards the kinocilium. These findings indicate that the side links may keep the cilia arranged in a bundle and the tip links may be involved in sensory cell transduction system. Changes in the surface texture and ciliary interconnecting systems were detected 2 hrs after incubation in frog Ringer's solution. Such changes included an increase in granularity of the surface membrane, fracturing and disappearance of all types of ciliary interconnections, especially side links. The tip links were more durable than side links. This indicates that the physiological studies of bullfrog hair bundle are justified across a 1-hr interval from the time the temporal bones are removed from the animal.

A new theory of human accommodation: cilio-zonular compression of the lens equator

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The inward rectifier K+ current underlies oscillatory membrane potential behaviour in bovine pigmented ciliary epithelial cells

1. Fresh bovine, pigmented ciliary epithelial cells possess an inward rectifier current activated by hyperpolarization. This current was investigated using whole-cell patch-clamp techniques. At the holding potential of -70 mV, and with EK (potassium equilibrium potential) set at -84 mV, a small outward current flowed through the inward rectifier that was sensitive to external K+, becoming more outward in 0.5 mM K+ and progressively more inward in 20 and 50 mM K+. 2. The inward rectifier showed V-EK dependence; increasing [K+]o increased the inward conductance from 1.28 nS in 5 mM K+ to 7.42 nS in 50 mM K+. The conductance at a given V-EK was proportional to the square root of [K+]o. 3. It was blocked by external Cs+ but replacing K+ in the pipette with Cs+ blocked only outward ion movement through the inward rectifier. Inward rectification was also blocked by Ba2+ (85% with Ki (concentration giving half-maximal inhibition) = 3.1 x 10(-5) M) and TEA+ (74% with Ki = 2.9 x 10(-4) M). 4. The activation time constant was voltage dependent, decreasing from 5 ms to 0.7 ms over the voltage range -90 to -170 mV. With increasing hyperpolarization the current exhibited time-dependent decay. The time constant for this process was voltage sensitive but the steady-state inactivation was independent of external [K+]. 5. The current disappeared in culture within 8 days. 6. Solution flow over the cell inactivated the inward rectifier, a property that may be related to [K+]o. 7. In current clamp the cells exhibited an unstable region at a potential of around -70 mV. Once in this region oscillations and regenerative hyperpolarizing potentials were observed. This behaviour was eliminated by treatments that blocked (Cs+, Ba2+) or removed (0.5 mM K+) active inward rectification. 8. It is suggested that these oscillations may represent a process of cation loading, the first step in the secretion of aqueous humour.

Characterization of acetylcholine- and endothelin-induced calcium entry in cultured human ciliary muscle cells

We characterized the effects of acetylcholine and endothelin on cultured human ciliary muscle cells, using the calcium-sensitive dye fura-2 to measure intracellular calcium and intracellular microelectrodes to measure the membrane potential. Both agonists, endothelin and acetylcholine, had a typical biphasic effect on the intracellular calcium concentration. Calcium peaked initially, because of its release from intracellular stores, and then reached a plateau, owing to entry of extracellular calcium. Endothelin-induced calcium entry was almost completely blocked by addition of extracellular La3+ (50 mumol/l) and Ni2+ (1 mmol/l). Acetylcholine-induced calcium entry was likewise almost completely abolished by La3+ and Ni2+. Both endothelin and acetylcholine led to an initial transient hyperpolarization with a subsequent depolarization. The hyperpolarization of the membrane potential had a time course similar to the initial calcium peak, while the depolarization occurred parallel to the calcium plateau. The depolarization induced by both agonists was reduced in the presence of La3+ and Ni2+. Verapamil (10 mumol/l) had no effect on either the calcium entry or the depolarization. Acetylcholine did not induce a [Ca2+]i peak when it was applied during the endothelin-induced [Ca2+]i plateau and vice versa. The [Ca2+]i plateau was not higher with concomitant than with single application of acetylcholine or endothelin. Thus, calcium entry and membrane depolarization induced by acetylcholine and endothelin seem to be mediated by a common La(3+)- and Ni(2+)-sensitive but verapamil-insensitive mechanism.