Cytochemical localization of adenyl cyclase in the rabbit ciliary body

Drug-dependent Ca2+ mobilization in organ-cultured rabbit ciliary processes

This study was conducted to determine whether drug-dependent changes in cytosolic Ca2+ concentration take place in the ciliary nonpigment epithelial cells of rabbits under more physiological conditions. Iris-ciliary body from pigmented rabbits in organ-culture was loaded with a Ca(2+)-sensitive fluorescent dye, fura-2, and a video-imaging system with an image analyzer was employed. Using this method fluorescence from nonpigmented epithelial cells can be analyzed without interference from fluorescence from pigmented ciliary epithelial cells. Among the drugs studied, norepinephrine and carbachol induced Ca2+ transients in the nonpigmented epithelial cells of organ-cultured ciliary processes. Epinephrine, isoproterenol, dopamine, neuropeptide Y, and substance P at the concentration of 10(-6) to 10(-3) M failed to elicit a response. The cytosolic free Ca2+ concentration of the cells in the resting state, as determined by an in vitro calibration curve, was 166 nM. The peak free cytosolic Ca2+ concentration induced by norepinephrine was about 263 nM, and that induced by carbachol was more than 1,000 nM. The carbachol-induced response was larger in magnitude and longer in duration than that induced by norepinephrine. Not uncommonly, the carbachol-induced response lasted more than 15 min. The response was diminished in both peak height and duration by chelation of extracellular Ca2+. Atropine abolished the response showing the response being mediated by a muscarinic receptor.

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.

Effects of adrenergic drugs on intracellular electrical potential difference of rabbit ciliary epithelial cells

The effects of adrenergic drugs on intracellular electrical potential difference (PDI) of rabbit ciliary epithelial cells were investigated. Epinephrine as well as norepinephrine hyperpolarized the PDI at lower concentrations (10(-6) M) and depolarized the PDI at higher concentrations (greater than 10(-5) M). Isoproterenol produced a depolarization of PDI and phenylephrine caused a hyperpolarization only. After pretreatment with propranolol, the change of PDI by isoproterenol was minimal. Selective agonists and antagonists were used to further characterize adrenergic effects on the PDI. Both beta 1 and beta 2 agonists caused a depolarization of PDI while both beta 1 and beta 2 antagonists produced a hyperpolarization. Alpha 1 antagonist depolarized the PDI and alpha 2 antagonist hyperpolarized the PDI. Such electrophysiological effects of the adrenergic drugs confirm the presence of alpha and beta adrenoceptors in the rabbit ciliary epithelial cells.

Mechanically stripped pigmented and non-pigmented epithelium of the shark ciliary body: morphology and transepithelial electrical properties

Sections of intact ciliary epithelium and mechanically stripped non-pigmented (NPE) and pigmented (PE) cell layers of adult sharks (Squalus acanthias) were mounted in Ussing-type chambers (area 0.1 cm2). Addition of 10(-5) M forskolin to the aqueous side of intact epithelium significantly increased short-circuit current (Isc) within 15 min and a maximum of approx. 30 microA cm-2 was reached after 45-60 min. Transepithelial potential difference (V) increased from -0.8 mV (aqueous side negative as compared with blood/stromal side) to -1.5 mV, whereas resistance (R) was unchanged (50 omega cm2). Forskolin was without effect when applied to the blood side. In stripped PE preparations (R 15 omega cm2), 10(-5) M forskolin applied to the apical side induced a qualitatively similar change of Isc and V compared with the intact tissue. The forskolin-induced effects were fully reversed by 10(-4) M bumetanide and were not dependent on pretreatment of the tissue with 10(-3) M BaCl2. In stripped NPE preparations resistance was usually less than 10 omega cm2 and was not stable. This is consistent with the morphologic observation that although tight junctions were still demonstrable in stripped NPE cells, the apical membranes were damaged. In preparations taken for light and electron microscopy the stripped PE layer revealed intact epithelial cells. In particular, the basal thirds of the stripped PE cells were in very close contact with each other. These attachment zones may have the appearance of tight junctions. Thus the PE cells of the shark ciliary epithelium can be successfully isolated for transepithelial transport studies. The adenylate cyclase system is present in PE cells, and transepithelial transport of chloride may be regulated by intracellular cAMP.

Histochemistry of adenylate cyclase activity in the anterior segment of the eye: a methodological evaluation with biochemical background

A histochemical technique for adenylate cyclase activity suitable for both light and electron microscopy is described. This technique is based on the Sr2+ capture reaction using adenylylimidodiphosphate as the substrate. The precipitated strontium phosphate is converted to lead phosphate, which is electron-dense for electron microscopy and can also be made visible by light microscopy. Different fixatives and fixation times were tested; paraformaldehyde at a concentration of 1% for 5 min demonstrated the enzyme activity best. The enzyme was protected and-or stimulated during all stages with isoproterenol, 5-guanylylimidodiphosphate, and dimethyl sulfoxide (DMSO). Numerous capturing agents (Pb2+, Co2+, Ce2+, Ba2+, Cd2+, and Sr2+) were tested. Strontium chloride (10 mM) produced the best result, inhibiting the enzyme less than 50%. The others gave inconsistent results. By this technique, adenylate cyclase activity was localized on the cell membranes of the deep layers of the rabbit corneal epithelium, on the endothelium, and on the cell membranes of the non-pigmented epithelium in the ciliary processes. These sites correspond well with earlier autoradiographic localization of beta-adrenergic receptors in the rabbit eye.

Timolol inhibits adenylate cyclase activity in the iris-ciliary body and trabecular meshwork of the eye and blocks activation of the enzyme by salbutamol

The enzymatic activity of adenylate cyclase in homogenates and membrane-rich fractions prepared from rabbit iris-ciliary bodies and bovine trabecular meshwork was found to be inhibited by timolol. Treatment of iris-ciliary body homogenates with Triton X-305 resulted in abolition of the inhibitory effect of the drug on the activity of the enzyme. The stimulatory effect of salbutamol on the enzyme was also susceptible to blockade by timolol. It is suggested that the hypotensive action of timolol on intraocular pressure results from structural and functional changes induced on the plasma membranes of the iris-ciliary body and trabecular meshwork by the thiadiazole group of the molecule, and, also, from the occupation of the adrenergic receptors of the iris-ciliary body by the tert-butylamino-2-hydroxypropoxy part of the compound.

Effects of forskolin, prostaglandin F2 alpha, and Ba2+ on the short-circuit current of the isolated rabbit iris-ciliary body

To gain information on the role of cyclic AMP (cAMP), ion transport and cell membrane permeability on aqueous humor formation, agents with well-known effects on transport properties in other epithelia were tested on the isolated rabbit iris-ciliary body. Forskolin stimulated the short-circuit current (SCC) by 37.5% when added to the aqueous-side solution. Forskolin was ineffective when added to the blood-side solution or when HCO3- was absent from the bathing solutions. The effect of forskolin confirms the presence of adenylate cyclase in the ciliary epithelium and the involvement of cAMP in ion transport. In HCO3- -rich media, 5 X 10(-5) M prostaglandin F2 alpha (PGF2 alpha), produced a prompt 25% increase in the SCC when added to the aqueous-side, and a small stimulatory SCC response when added to the blood-side. No change in SCC occurred when PGF2 alpha was added to either side of a HCO3- -free bathing solution. It is implied that cAMP acts on a HCO3- dependent transport system. These results are consistent with the previously observed stimulation of the SCC by 8Br-cAMP. BaCl2, 2.5 mM, on the aqueous-side increased the SCC by 240.5%, but reduced the SCC by 26% when added to the blood-side solution. The Ba2+ effects indicate the presence of high conductance K+ channels in the basolateral membranes of both the pigmented and non-pigmented cell layers.

Stimulation of adenylate cyclase of ciliary processes in response to decreased inflow of aqueous humour

Intravitreal injection of ouabain was used to induce unilateral hypotony and to study the relationship of adenylate cyclase (AC) and sodium-potassium activated adenosine triphosphatase (NaK-ATPase) both of which are involved in the production of aqueous humour. After preliminary experiments, days 3 and 5 were chosen as representative times when IOP was maximally reduced and stable following ouabain injection. NaK-ATPase and adenylate cyclase activities were measured biochemically in the same homogenates of isolated ciliary processes (CP). Biochemical measurements showed that 46% of NaK-ATPase activity was inhibited after 3 days, and about 78% of NaK-ATPase was inhibited 5 days after ouabain injection. At the beginning of NaK-ATPase inhibition there was a significant stimulation of adenylate cyclase activity of the CP. The highest activities were seen 2 and 3 days after ouabain injection. The suggestion is made that these 2 enzymes are interdependent.

Effects of adrenergic agonists and cyclic AMP on the short-circuit current across the isolated rabbit iris-ciliary body

Epinephrine, isoproterenol and norepinephrine (10(-6) M - 10(-4) M) reduced the short-circuit current (SCC) across the isolated rabbit iris-ciliary body when added to the blood side (pigmented layer) bathing solution but were without effect when added to the aqueous side (non-pigmented layer) bathing solution. The effect was observed in both HCO-3-rich and HCO-3-free solutions, although it was smaller in the latter. Propranolol and phentolamine (10(-5) M) partially blocked the action of the adrenergic agonists, 8Br-cAMP (1 mM), on the other hand, stimulated the SCC when added to either bathing solution. The apparent contradiction between these results is explained on the basis of separate cyclic AMP compartmentalization in the two epithelial cell layers and the lack of adrenoceptors in the non-pigmented layer.

Interaction of timolol and caffeine on intraocular pressure

The clinical interaction of 0.5% timolol maleate and 400 mg of oral caffeine on the intraocular pressure of 20 normotensive volunteers was studied. Caffeine administration did not significantly alter the reduction in intraocular pressure in timolol treated eyes. An antagonism, however, was observed in the fellow timolol untreated eyes with caffeine. The crossover reduction in intraocular pressure noted in the control group was significantly depressed in the caffeine group. A proposed mechanism for these observations is discussed.

Pharmacological characteristics of beta-adrenergic-sensitive adenylate cyclase in non pigmented and in pigmented cells of bovine ciliary process

Adenylate cyclase activity was studied on the two epithelial cell types of bovine ciliary process after digestion by trypsin and purification by density gradient centrifugation. cAMP formation after isoproterenol stimulation showed that non pigmented cells were particularly enriched in adenylate cyclase activity. Furthermore pharmacological characteristics of this enzyme indicated that adenylate cyclase-associated beta adrenergic receptors were of beta 2 type in each kind of epithelial cells. These results support the hypothesis that beta adrenergic compounds modulate aqueous humor production through a direct effect on ciliary epithelium and enrichment of beta adrenergic sensitive adenylate cyclase activity in non pigmented epithelial cells suggests that these particular cells may play an important role in aqueous humor secretion.

Human ciliary epithelia in monolayer culture

The ciliary epithelia of human (one to 12 months old) ciliary processes were isolated by trypsin and EDTA, cultured in Dulbecco's Modified Eagle Medium (DMEM) with 5% fetal calf serum and examined by phase and electron microscopy. The primary cultures were maintained for three to four months. Only a few non-pigmented epithelia adhered and none of them proliferated. After the first passage the culture seemed to consist of only the pigmented epithelia. Most cells were densely pigmented at first, then became less pigmented during successive proliferations. Half of the cells remained densely pigmented after the first subculture, another half remained less pigmented. The cells started to lose their pigment granules at four to six weeks in culture. After three months of culture, the cell sheets became entirely unpigmented. In thin section, most of the pigment granules in the cells at two weeks in culture were pre-melanosomes, and half of them were at the earliest stage of pre-melanosomes. Monolayer cells possessed basement membranes. At 14 weeks in culture, most cells established an apparent polarity, contained well-developed Golgi apparatus and rough endoplasmic reticulum and intermediate filaments, but no pigment granules. A bundle of intermediate filaments was found in the perinuclear cytoplasm. Multilayer cells presented a typical apex-to-apex and base-to-base configuration , and the extracellular material was detected only in the base-to-base intercellular spaces. Our culture system provided differentiated cells derived from the pigment epithelia of human ciliary processes.

Laboratory and clinical studies on the mechanism of action of timolol

Timolol is now widely used in the treatment of glaucoma but its mechanism of action is unclear. The drug decreases aqueous humor formation by the ciliary processes and is a potent beta-adrenergic antagonist. However, the physiological basis for such a drug to decrease intraocular pressure has not been clearly demonstrated.

Ocular effects of beta-adrenergic agents. XII Jules Stein Lecture

The ocular effects of beta-adrenergic agonists and antagonists are reviewed. The permeability of the rabbit corneal epithelium was estimated for various instilled drugs, and difference in pharmacokinetics of timolol between the albino and pigmented rabbit demonstrated. The effects of the drugs on the cornea and the lens epithelium, and also the distribution of the adrenergic receptors in the intraocular muscles were discussed. The beta-adrenergic receptors are present in the ciliary processes of the rabbit as shown through studies by biochemical pharmacology and cytochemical electron microscopy. Adrenergic effects on the blood flow and the vascular permeability of the anterior uvea were discussed. The effects of adrenergic drugs on aqueous humor dynamics differ considerably among rabbits, monkey and the human subjects, and they were discussed separately for these species. The mechanism underlying the hypotensive effects of beta-adrenergic drugs is complex, but the available data indicate that in the human eye the hypotensive effects of epinephrine are attributed mainly to a decrease in the outflow resistance and the effects of timolol are principally to a reduction of aqueous humor formation.

Ultracytochemistry of cholera-toxin binding sites in ciliary processes

Cholera toxin reduces the rate of aqueous humor in concentrations (10-11M) that do not disturb the morphology of the aqueous-humor forming epithelial cells of the ciliary processes of the rabbit eye. The search for an endogenous mediator of aqueous-humor formation comparable to cholera toxin in its mode of operation prompted us to map the distribution of cell surface receptors for cholera toxin in the ciliary processes of the eyes of rabbits. Cytochemical studies were carried out with the use of conjugates of cholera toxin to fluorescein isothiocyanate (CT-FITC) and to horseradish peroxidase (CT-HRP), and of the B subunit of cholera toxin to horseradish peroxidase (B-HRP). Multiple fluorescent CT-FITC binding sites were observed on the outer nonpigmented epithelial layer near the crests of the processes. Processes incubated with CT-HRP in vitro showed surface staining of 30-40% of the nonpigmented epithelial cells. A prominent reaction product was observed along the basal and lateral plasma membranes of these cells. In vivo studies carried out after arterial infusion of B-HRP showed a reproducible dense reaction product between the apical surfaces of the pigmented epithelium (PE) and of the nonpigmented epithelium (NPE) facing each other. Aggregations of reaction product were observed with the electron microscope in the extracellular space between the apices of PE and NPE. The apical plasma membrane of the endothelium of the blood vessels near the crests of the ciliary processes was stained after either in vivo or in vitro exposure to peroxidase conjugates. These findings indicate that the cell-surface receptors which mediate the action of cholera toxin on aqueous humor formation are very likely localized in the apical plasma membranes of the epithelium of the ciliary processes.

A radioautographic study on the incorporation of 14C-bupranolol (beta-blocking agent) into the rabbit eye

Localization of instilled 14C-bupranolol was studied in the rabbit eye by means of light microscopic radioautography. After the administration of 50 microliter of 14C-bupranolol into the eyes of two rabbits, both eyes were enucleated at intervals of 15 and 30 min respectively, and radioautographic procedures were carried out on cryostat sections by means of both dry-mounting and wet-mounting methods. Although with the former it was possible to demonstrate many radioautographic silver grains in the specimens, with the latter it was not possible to detect their localization. On the dry-mounting procedure, radioautographic grains were observed mainly in the epithelium of the ciliary body 15 and 30 min after the administration of 14C-bupranolol. The numbers of grains were counted on the 15 and 30 min specimens. They were decreased in the conjunctiva, and cornea 30 min after the administration. On the contrary, they were remarkably increased in the epithelium of the ciliary body. From these results it is concluded that the 14C-bupranolol first penetrates into the epithelium of the conjunctiva and cornea, then moves rapidly to the ciliary body, and that only very small amount of 14C-bupranolol is changed into the insoluble form. Also, these results suggest that bupranolol plays a role in the control of aqueous production in the ciliary processes.

An attempt at localizing adenylate cyclase in rat calvaria. Influence of sodium fluoride and parathyroid hormone

To show adenylate cyclase (AC) activity in rat calvaria, it is necessary first to decalcify the specimen. In hard tissues, several enzymes (adenosine triphosphatase (ATPase), alkaline phosphatase (APase), adenylate cyclase (AC) and perhaps pyrophosphatase (PPiase) are able to degrade adenosine triphosphate (ATP). The presence of sodium fluoride (NaF) in the incubation medium reduces the quantity of precipitate formed, compared to that observed using a NaF-free incubation medium. Levamisole, used under the same conditions, gives similar results. Possibly NaF inhibits pyrophosphohydrolase and/or phosphatases which mask the AC activity. Adenylylimidophosphate (AMP-PNP), which is a specific AC substrate, confirms the results obtained with ATP. AC activity is demonstrated cytochemically in the osteoblast and preosteoblast membranes, at the junction between two osteoblasts and along the cytoplasmic processes of the osteoblast which penetrate into the osteoid matrix. The osteocytes never show a precipitate, except those which present some osteoblastic features and then only on the membrane facing the osteogenic layer. An intracellular reaction is also evident and is discussed. Parathyroid hormone (PTH) does not reveal new sites of AC activity but increases the quantity of precipitate observed.

Effects of adrenergic drugs on aqueous cAMP and cGMP and intraocular pressure

The following variously selective adrenergic agonists were tested for their effects on the concentration of adenosine 3',5' cyclic monophosphate and guanosine 3',5- cyclic monophosphate in the aqueous humor of treated and fellow eyes of rabbits one hour after topical unilateral application (2 mg, base): Epinephrine (alpha, beta 1.beta 2), phenylephrine alpha), isoproterenol (beta 1, beta 2), tazolol (beta 1) and terbutaline (beta 2). All drugs produced a significant increase in cAMP in the treated eyes and all but terbutaline produced a significant increase in the fellow eye. Terbutaline alone caused an increased in cGMP although a similar dose of pilocarpine, a cholinergic agonist, was ineffective in changing cGMP levels. We conclude that (1) diverse adrenoceptor stimulation can increase cAMP, but (2) not necessarily in direct relation to an agent's hypotension induced by pilocarpine is not accompanied by an increase in aqueous cGMP.