Interaction between alpha 2- and beta 2-adrenergic receptors in rabbit ciliary processes

Pharmacological evidence for heterogeneity of ocular $aL2adrenoceptors

Previous studies have shown that ocular alpha 2 adrenoceptors are located prejunctionally on sympathetic neurons and postjunctionally on cells in the iris/ciliary body. While the activation of alpha 2 adrenoceptors at each site has been postulated to alter aqueous humor dynamics, little is known about the pharmacological characteristics of these receptors or their role in the modulation of anterior segment function. The purpose of the current study was to determine the possible heterogeneity of ocular alpha 2 adrenoceptors using relatively selective alpha 2 adrenoceptor agonists and antagonists to examine ocular pre- and postjunctional alpha 2 adrenoceptors. Prejunctional alpha 2 effects were evaluated by means of the cat nictitating membrane (CNM) preparation. Postjunctional alpha 2 effects were evaluated by means of the cAMP assay in rabbit iris root/ciliary body. In the CNM, the administration of UK-14, 304 (UK) produced a dose-related inhibition of neuronally mediated contractions. Pretreatment with the alpha 2 antagonist rauwolscine caused a 1 to 2 log unit right shift in the dose-response curve of UK in the CNM. However, pretreatment with alpha 2 antagonist SKF 104078 had no demonstrable effect on UK-induced inhibition of neuronally mediated contractions of the CNM. In the rabbit iris root/ciliary body, UK produced a concentration-dependent inhibition of cAMP accumulation on isoproterenol- and VIP-induced cAMP production. Pretreatment of iris root/ciliary bodies with SKF 104078 or rauwolscine reversed the inhibitory effect of UK on isoproterenol- and VIP-induced accumulation of cAMP. These data provide the first evidence that the pre- and postjunctional alpha 2 adrenoceptors represent pharmacologically distinct subpopulations of receptors in the eye.(ABSTRACT TRUNCATED AT 250 WORDS)

Paradoxical effect of phentolamine on aqueous flow in the rabbit

PURPOSE

The aim of this study was to assess the effects of acute systemic, nonselective alpha-adrenergic blockade on aqueous flow.

METHODS

This study used pentobarbital-anesthetized rabbits (n=7), in which the following parameters were measured: mean arterial pressure, carotid blood flow, heart rate, intraocular pressure (IOP), orbital venous pressure (OVP), ciliary blood flow, and aqueous flow (AqFlow). Measurements were made before and after an intravenous administration of phentolamine (0.1 mg/kg).

RESULTS

Phentolamine caused significant decreases in IOP -23%+/-2%; P<0.01), OVP (-28%+/-12%; P<0.05), and AqFlow (-33%+/-6%; P<0.01). The other parameters were not significantly altered. The rapidity of the OVP and IOP responses were noteworthy, being essentially complete 60 s after the phentolamine injection.

CONCLUSIONS

A subpressor dose of phentolamine has complex effects on ocular hydrodynamics. The initial IOP decrease is too fast to be explained by aqueous dynamics or ocular rigidity, and so is most likely a result of the disgorgement of choroidal blood volume caused by decreased venous pressure outside the eye. The more prolonged ocular hypotensive effect is explained by the decrease in AqFlow, and perhaps a decrease in episcleral venous pressure or increase in uveoscleral outflow. However, the inhibition of aqueous production is odd, as lost prejunctional inhibition of norepinephrine release and unopposed beta-receptor activation should have increased aqueous production.

Effect of beta-adrenergic agents on intracellular potential of rabbit ciliary epithelium

PURPOSE

To study the effects of beta-adrenergic agents on intracellular potential (Vm) of the isolated and intact rabbit ciliary epithelium.

METHODS

Vm was measured on the isolated intact ciliary epithelium superfused with adrenergic agents and cyclic AMP modulators.

RESULTS

The nonselective beta-adrenergic agonist isoproterenol depolarized Vm in a dose-dependent fashion. beta-adrenergic antagonists alone had no effect on baseline Vm. The isoproterenol response was blocked by the nonselective antagonist timolol (5 x 10(-5) M). The selective beta 2-antagonist ICI 118-551 caused a greater inhibition (IC50 approximately 7 x 10(-7)) than the selective beta 1-antagonist betaxolol (IC50 approximately 6 x 10(-6)). The isoproterenol response was also significantly (p < 0.03) blocked by the non-selective alpha-antagonist phentolamine. Cyclic AMP and phosphodiesterase inhibitors significantly decreased Vm. Pretreatment with these inhibitors potentiated the agonist-induced depolarization. Barium, a blocker of Ca(2+)-sensitive K+ channels, significantly decreased baseline Vm. Barium pretreatment blocked the beta-agonist and cAMP induced depolarization of Vm, suggesting that the K+ current is necessary for the observed isoproterenol response. Pretreatment with the cotransport inhibitor bumetanide had no effect on the isoproterenol-induced response.

CONCLUSIONS

The beta-adrenergic agonist isoproterenol affects ionic transport processes across the ciliary epithelium (beta 2 > beta 1). This effect is likely mediated through adenylate cyclase coupled to adrenoreceptors and requires the presence of the K+ current. Blockage of the isoproterenol-induced decrease in Vm by a nonselective alpha-adrenergic antagonist indicates an interaction between the two adrenergic systems in the ciliary epithelium.

A low conductance chloride channel in the basolateral membranes of the non-pigmented ciliary epithelium of the rabbit eye

PURPOSE

Chloride efflux plays an important role in aqueous humor production. Chloride currents have been described in bovine non-pigmented ciliary epithelium (NPE), in transformed cultured human NPE, and in bovine volume-activated chloride channels described in the latter. It is the basolateral membranes of the NPE of ocular ciliary processes that comprise the exit pathway for the process of aqueous secretion performed by the double syncytial layer of ciliary epithelium, however. Therefore, we studied both cell-attached, and, excised, inside-out patches from basolateral membranes of the NPE.

METHODS

Cell-attached and cell-free excised patches were formed from the basolateral membranes of NPE and single channel currents recorded with a Dagan 3900A patch clamp amplifier.

RESULTS

A low conductance channel of 14 pS was observed and recorded in 30% of cell attached patches and in 35% of excised inside out patches under symmetrical conditions (160 mM chloride). This channel displayed a nearly linear current-voltage relationship, with an open probability that was not voltage-dependent. The channel was chloride-selective: N-methyl-D-glucamine (NMDG) used as cation did not alter the current profile nor the reversal protein. Further, with inside-out patches, the reversal potential was close to zero (0.3 +/- 0.5 mV (10) in symmetrical (160 mV) chloride, but shifted to -32.3 +/- 0.5 mV (5) when the concentration of chloride in the bathing solution was reduced to 40 mM while the recording pipette was held at 160 mM. This value approaches the theoretical equilibrium potential of chloride for these conditions. The channel anion permeability sequence was: I- > NO3- > or = Br- > Cl- > gluconate- approximately equal to aspartate-. Three different chloride-channel blockers inhibited the channel activity.

CONCLUSION

A low conductance channel, selective for chloride, and, modulated by beta adrenergic and VIP stimulation, based on it sensitivity to exogenously added cAMP, ATP and the catalytic subunit of PKA, is present in the exit basolateral membranes of rabbit NPE and contributes to the process of aqueous humor formation.

In vivo cAMP level in rabbit iris-ciliary body after topical epinephrine treatment

PURPOSE

The present study estimates the in vivo cyclic AMP (cAMP) level in the rabbit iris-ciliary body after topical treatment with epinephrine.

METHODS

Epinephrine 1% was applied unilaterally to pigmented rabbits. At selected time points between 10 min and 6 hr, a 2.5-mm diameter iris-ciliary body, in the treated eye, was irradiated with 2 J diode laser (2 W for 1 sec). Instant heat generated by the irradiation deactivated the cellular enzymes used in the synthesis and degradation of cAMP. Rabbits were sacrificed immediately, and samples of the iris-ciliary body were isolated from the irradiated area, the non-irradiated area, and from the contralateral, untreated eye. Levels of cAMP in these samples and in the aqueous humor were determined by radioimmunoassay. Further experiments were performed in rabbits pretreated with an alpha 2-adrenergic antagonist, yohimbine, and a beta-adrenergic antagonist, timolol, 30 min before the epinephrine treatment. Unilateral laser photocoagulation of the iris-ciliary body was performed at 10 min and 1 hr after the epinephrine treatment.

RESULTS

The cAMP level in the photocoagulated iris-ciliary body did not change in parallel to the change in the aqueous humor. An increase of aqueous humor cAMP was observed for 6 hr. However, a reduction of cAMP level in the photocoagulated iris-ciliary body was observed at 1 hr after the epinephrine treatment. In yohimbine-pretreated rabbits, the reduction of cAMP level in the photocoagulated iris-ciliary body was absent. Pretreatment with timolol had no effect.

CONCLUSIONS

These observations suggest that, at 1 hr after the epinephrine treatment, the cellular signal via the alpha 2-adrenergic receptors in the iris-ciliary body, which reduces the intracellular cAMP level, is stronger than that via the beta-adrenergic receptors.

Effects of timolol, terbutaline and forskolin on IOP, aqueous humour formation and ciliary cyclic AMP levels in the bovine eye

We have studied the effects of terbutaline, timolol, forskolin and 8-bromo cyclic AMP on aqueous humour formation, intraocular pressure and on ciliary epithelial cyclic AMP levels, either in presence or in absence of IBMX, using the bovine isolated arterially perfused eye, excised ciliary processes and cultured ciliary epithelium. Both terbutaline, a beta-adrenoceptor agonist, and timolol, a beta-adrenoceptor antagonist, caused significant reduction in aqueous humour formation and intraocular pressure but produced no effect on ciliary epithelial cyclic AMP content in the absence of IBMX. Even a three times higher dose of terbutaline was entirely ineffective in producing any effect on ciliary cyclic AMP in the perfused eye. On the other hand, terbutaline at the IOP-reducing dose, produced a significant increase in cyclic AMP when injected after 30 min perfusion with IBMX. Incubation of excised ciliary processes or cultured ciliary epithelial cells with terbutaline (10(-6) to 10(-4) M) produced concentration-dependent increases in cyclic AMP, in both tissues, even in the absence of IBMX. Forskolin, which stimulates cyclic AMP synthesis without interacting with cell surface receptors, was found to produce highly significant increases in ciliary cyclic AMP content both in presence and in absence of IBMX but had no effect on aqueous humour formation in the isolated eye. IBMX perfused at concentrations of 1 mM or 10 microM had no effect on basal levels of ciliary cyclic AMP but the 1 mM concentration produced a marked and significant reduction in IOP. Direct application of 8-bromo cyclic AMP, a cell permeable analogue, more resistant to hydrolysis by phosphodiesterases, had also no effect on aqueous humour formation in the perfused eye. It is concluded that in the bovine arterially perfused eye, the correlation between the aqueous humour formation rate and ciliary epithelial cyclic AMP content is unclear.

Inflammation induced changes in adenosine 3',5'-cyclic monophosphate production by ciliary epithelial cell bilayers

Despite extensive evidence implicating the cytokines interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNF alpha) in the intraocular inflammatory response, little is known about their effects on signal transduction in anterior uveal tissue. Since these cytokines have been shown to alter the adenylyl cyclase system in nonocular tissues, we tested the hypothesis that IL-1 beta and TNF alpha affect the anterior uvea by altering production of the intracellular second messenger adenosine 3',5'-cyclic monophosphate (cAMP) in ciliary epithelial bilayers. This was accomplished by measuring the levels of cAMP in bilayers ex vivo, following intraocular inflammation induced by intravitreal injection of IL-1 beta, TNF alpha or bacterial endotoxin, and in vitro, following exposure to IL-1 beta, TNF alpha or bacterial endotoxin. Although cAMP production was enhanced in bilayers from IL-1 beta-, TNF alpha- or endotoxin-inflamed eyes, ex vivo, exposure of normal bilayers to IL-1 beta (15 U ml-1), TNF alpha (20 U ml-1), or a low concentration of endotoxin (0.01 microgram ml-1) for 4 hr, in vitro, had no effect on cAMP production. The inability of IL-1 beta, TNF alpha, or the low concentration of endotoxin to increase cAMP production by bilayers, in vitro, suggests that the enhanced cAMP production observed with inflamed bilayers, ex vivo, was not due to a direct action of these inflammatory agonists on the ciliary epithelial bilayer. Although direct exposure to cytokines or endotoxin did not change cAMP production, treatment with IL-1 beta, TNF alpha, or a higher concentration of endotoxin (1 microgram ml-1) did affect signal transduction mechanisms.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of cyclic AMP production in adult human ciliary processes

Cyclic AMP production in intact ciliary processes from elderly human donors is subject to stimulatory and inhibitory control by various agents. Stimulation of cAMP production is observed with forskolin, vasoactive intestinal peptide, or the beta-adrenergic agonist isoproterenol. Inhibition of forskolin-stimulated cAMP production is observed with endothelin-2 or PAC. The inhibitory effect of PAC is blocked by the specific alpha 2-adrenergic antagonist, yohimbine. Endothelin-2 has no effect on basal cAMP production. These data document the positive and negative regulation of cAMP responses in adult human ciliary processes and support the idea that cAMP is a key intermediate in the regulation of aqueous humor formation.

Role of G-proteins in ciliary process adenylyl cyclase responses of the albino rabbit eye

After intravitreal injections of cholera or pertussis toxin (CTX or PTX, 0.5 -1 microgram/eye) into the albino rabbit eye, the in vitro responses of ciliary process adenylyl cyclase (AC) to isoproterenol, vasoactive intestinal peptide (VIP), and forskolin (FSK) were increased 21-40% for PTX, but for CTX-injected eyes AC responses to fluoroaluminate, VIP and FSK decreased 70-50%. The increased responses after PTX suggests that this toxin blocked an inhibitory Gi control of AC that is present in the control tissue. However, prolonged (> 24 hr) in vivo exposure to CTX appears to down-regulate the AC enzyme. In contrast to the in vivo findings, AC responsiveness was unaffected by PTX pre-treatment of membranes in vitro, while CTX pre-treatment increased basal activity (+600%), and the FSK response (+30%), but decreased responsiveness to fluoroaluminate, VIP and isoproterenol by 88-56%. Treatment of ciliary process membranes with 32P-NAD and CTX or PTX followed by SDS-PAGE autoradiography of labelled proteins gave two bands for the G-protein alpha-subunits of Gs (45, 56 kDa) and one broad band centered at 40 kDa for Gi-type subunits respectively. Western blots using specific antibodies showed the presence of Gi type I or III, but no detectable Gi type II or Go in rabbit ciliary processes. We conclude that the changes in adenylyl cyclase enzyme responses after intraocular CTX or PTX may not correlate with cAMP levels and intraocular pressure effects. However, the in vitro biochemical data on AC responses and on G-proteins provide evidence for dual regulation of ciliary process AC by activating and inhibitory G-proteins.

Allicin-induced hypotension in rabbit eyes

The intent of this work was to examine the actions of allicin on 1) intraocular pressure (IOP) in normal and unilaterally sympathectomized (SX) rabbits; 2) cAMP accumulation in the rabbit iris-ciliary body (ICB) and cultured nonpigmented epithelial (NPE) ciliary body cells; and 3) 3H-norepinephrine (NE) release by calculating fractional tritium overflow in response to electrical field stimulation (EFS, 5 Hz, 12 V/cm) in isolated, perfused rabbit ICBs. Allicin, one of the active compounds produced by garlic, was evaluated on IOP and it was determined that allicin (1, 2.5, or 10 micrograms), topically, but not the precursor, alliin (10 micrograms), lowered the IOP unilaterally in normal rabbits. Allicin (10 micrograms) reduced the IOP by 6 +/- 1 mmHg (n = 4) in normal rabbits at 2 hrs (maximum response) whereas no change occurred in sympathectomized rabbit eyes. Moreover, allicin (0.01, 0.1, or 1 microM) caused 40, 40, or 52% inhibition, respectively, of 3H-NE overflow in response to EFS. Isoproterenol (ISO, 1 microM) stimulated cAMP accumulation by 3.6 and 9 fold in isolated rabbit ICB and cultured NPE cells, respectively. Allicin (1 microM) had no effect on basal cAMP level while it inhibited ISO-stimulated cAMP accumulation by 40% and 23% in ICB and NPE cells, respectively. This study suggests that allicin lowered IOP, in part, by dual actions at the neuroeffector junction.

Effects of adrenergic agents on transepithelial electrical measurements across the isolated iris-ciliary body

Transmembrane electrical measurements were performed on the isolated rabbit iris-ciliary body to study direct effects of adrenergic drugs on the ciliary epithelium. Alpha-adrenergic agonists (epinephrine, norepinephrine, or phenylephrine) lowered the short-circuit current (SCC) in a dose-dependent fashion relative to which chamber side the drug was added: simultaneous addition to both chambers greater than blood side only greater than aqueous side only. Pretreatment (5 x 10(-5) M) with the non-selective beta-adrenergic antagonist timolol had no effect while the non-selective alpha-adrenergic antagonist, phentolamine, completely prevented the alpha agonist-induced decrease in SCC. The alpha-adrenergic response was mediated by the alpha 1 subtype since prazosin, but not yohimbine, blocked the induced reduction in SCC. The beta-adrenergic agonist isoproterenol caused a dose-dependent decrease in the SCC. The decrease was similar when the drug was added to only the blood side or to both sides of the chamber. Addition to only the aqueous chamber had no effect. Pretreatment with beta-adrenergic antagonists blocked the isoproterenol response: non-selective = selective beta 2 greater than selective beta 1. The isoproterenol-induced decrease in SCC was also blocked by non-selective alpha-adrenergic antagonists. The response was mediated by the alpha 1 subtype since prazosin, but not yohimbine, blocked the isoproterenol response. This suggests that isoproterenol interacted with the alpha 1-adrenergic sensitive pathway in the rabbit ciliary process.

Stimulatory and inhibitory cyclic AMP responses in rabbit ciliary processes after cervical ganglionectomy

Cyclic AMP production in response to agonists which act at a variety of receptors to either stimulate or inhibit cyclic AMP production has been studied in intact, dissected ciliary processes from rabbit eyes after unilateral surgical removal of the cervical ganglion. Cyclic AMP responses to stimulatory ligands vasoactive intestinal peptide (VIP), isoproterenol, and forskolin and inhibitory agonists neuropeptide Y (NPY), the synthetic somatostatin analogue SMS 201-995, and alpha-adrenergic agents were investigated in tissues from normal eyes and compared to the same responses in tissues from sympathetically denervated eyes. Neither stimulated cyclic AMP production nor inhibition of stimulated cyclic AMP production was significantly different in tissues from denervated vs. normal eyes. Inhibition of VIP-stimulated cyclic AMP production by epinephrine and paraaminoclonidine in tissues from both normal and denervated eyes was blocked by the alpha 2-adrenergic antagonist yohimbine but not by the alpha 1-adrenergic antagonist prazosin. These data indicate that the VIP, NPY, somatostatin, and alpha 2- and beta 2-adrenergic receptors which regulate cyclic AMP production in rabbit ciliary processes are postjunctional and suggest that ligands known to modulate cyclic AMP levels in this tissue may exert effects on aqueous humor formation independently of adrenergic innervation.

Chronic electrical stimulation of sympathetic nerves: effects on blood-aqueous barrier

The effect of electrical stimulation of the sympathetic nerves on the blood-aqueous barrier was investigated in rabbits. The permeability of the barrier was assessed during either acute, chronic or following chronic nerve stimulation. During acute and chronic stimulation of the sympathetic nerves, fluorescein entered the anterior chamber at a rate significantly slower than in control eyes. After chronic stimulation, both the rate of entry of fluorescein and the aqueous humor protein concentration were much greater than in control eyes indicating breakdown of the blood-aqueous barrier. Treatment with the non-steroidal anti-inflammatory drugs, indomethacin and suprofen, completely blocked the breakdown of the blood-aqueous barrier. These results indicate that sympathetic nerve stimulation can cause the local synthesis of prostaglandins and that these can affect the blood-aqueous barrier.

Effects of timolol on terbutaline- and VIP-stimulated aqueous humor flow in the cynomolgus monkey

The effects of timolol on terbutaline- and VIP-stimulated aqueous humor flow were investigated in cynomolgus monkeys, with a labeled albumin dilution method. The maximal increase in aqueous humor flow caused by intracameral (100 micrograms/ml) or intravenous (0.4 micrograms/kg/min) administration of terbutaline was about 100%. The effect of intravenously infused terbutaline was completely abolished by intracameral administration of timolol, 0.1 mg/ml. The same dose of timolol also abolished the effect of intravenously infused VIP, 50 ng/kg/min. Intravenous administration of timolol, 0.2 mg/kg, had no effect on VIP-stimulated aqueous humor flow, when VIP (90 micrograms) was given intracamerally, but abolished completely the effect of intracameral terbutaline, 100 micrograms/ml. The results suggest that the effect of intravenously infused VIP on aqueous humor flow is secondary to activation of the sympathetic nervous system, while the effect of intracameral administration of VIP is a direct effect on the ciliary epithelium. The maximal aqueous humor flow achieved with terbutaline is comparable to that in conscious cynomolgus monkeys.

Neuropeptide Y and somatostatin inhibit stimulated cyclic AMP production in rabbit ciliary processes

The interaction of adrenergic and peptide receptors linked to adenylate cyclase and the inhibition by bioactive peptides of stimulated cyclic AMP production has been investigated in intact, excised rabbit ciliary processes. Cyclic AMP production stimulated by isoproterenol, vasoactive intestinal peptide, or forskolin was inhibited by the biologically active peptides neuropeptide Y, somatostatin, and the synthetic somatostatin analogue SMS 201-995. IC50s determined from dose-response curves of inhibition are consistent with the known abilities of these ligands to modulate cyclic AMP and physiological responses in other tissues. Inhibition by neuropeptide Y or SMS 201-995 was unaffected by the specific alpha 2-adrenergic antagonist yohimbine, which shows that peptide inhibition is not occurring via peptide binding to the inhibitory alpha 2-adrenergic receptor. These results suggest that endogenous peptides may participate in modulation of cyclic AMP production and subsequent physiological events influenced by cyclic AMP levels in rabbit ciliary processes by inhibiting stimulated cyclic AMP synthesis.

Inhibitory effects of neuropeptide Y on adenylate cyclase of rabbit ciliary processes

The inhibitory effect of neuropeptide Y (NPY) was studied on the adenylate cyclase (AC) activity in homogenates of rabbit ciliary processes and compared with that of the alpha 2-adrenergic agonist clonidine (CLN). NPY inhibited basal AC activity as well as AC activity stimulated by isoproterenol (ISO), vasoactive intestinal polypeptide (VIP) or forskolin (FSK). The extent of this inhibition corresponded well to the inhibition elicited by CLN. The inhibitory effects of NPY and CLN appeared to be nonadditive. AC activity stimulated by ISO was considerably more sensitive to the effects of either NPY or CLN than basal, VIP- or FSK-stimulated AC activity. It was inferred that NPY inhibitory effects were mediated by the activation of NPY receptors coupled negatively to the catalytic unit of AC via the inhibitory Gi protein. Moreover, involvement of NPY in physiological modulation of AC activity in ciliary processes and in the regulation of aqueous humor formation and intraocular pressure is suggested.

Inhibitory effects of clonidine and dopamine on adenylate cyclase of rabbit ciliary processes

The inhibitory effects of the alpha 2-adrenergic agonist clonidine and that of dopamine were studied on the adenylate cyclase activity in homogenates of ciliary processes. Clonidine inhibited in a dose-dependent manner basal adenylate cyclase activity as well as that stimulated by isoproterenol or forskolin. However, the adenylate cyclase activity stimulated by isoproterenol was sensitive to at least one order lower inhibitory concentrations of clonidine than basal or forskolin-stimulated adenylate cyclase. Dopamine inhibited adenylate cyclase stimulated by isoproterenol considerably less potently than clonidine. The slope of the dopamine dose-response curve was, however, similar to that of the dose-response curve of clonidine. The inhibitory effects of clonidine and dopamine were antagonized by an alpha 2-adrenergic antagonist, yohimbine, in a manner suggesting a competitive nature of this interaction. On the contrary, the inhibitory effects of neither clonidine nor dopamine were prevented by an alpha 1-adrenergic antagonist, prazosin. In addition, the effect of dopamine was not antagonized by the D2-antagonist, tiapride. Taken together, these results strongly indicate that both clonidine and dopamine exert their inhibitory effects by the stimulation of alpha 2-adrenergic receptors. Accordingly, they provide experimental evidence that both basal and drug-stimulated adenylate cyclase activity of ciliary processes can be inhibited via stimulation of alpha 2-adrenergic receptors. The substantially higher sensitivity of isoproterenol-stimulated than basal or forskolin stimulated adenylate cyclase to alpha 2-adrenergic inhibition seems to be a unique feature of this enzyme of ciliary processes. It is suggested that this may reflect an involvement of alpha 2-adrenergic receptors in the physiological feedback mechanism preventing the over-stimulation of adenylate cyclase of ciliary processes during excessive adrenergic drive.

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.

Alpha 2-adrenergic and VIP receptors in rabbit ciliary processes interact

The interaction between alpha 2-adrenergic and VIP receptors has been studied by examining inhibition of VIP-stimulated cyclic AMP production by adrenergic agonists in intact, excised rabbit ciliary processes. Epinephrine, norepinephrine, isoproterenol, dopamine, and the specific alpha 2-adrenergic agonists clonidine and p-aminoclonidine exhibit dose-dependent inhibition of VIP-stimulated cyclic AMP production. I50s, clonidine (0.05 microM) = p-aminoclonidine (0.05 microM) congruent to epinephrine (0.1 microM) less than norepinephrine (2.0 microM) less than isoproterenol (15 microM) = dopamine (15 microM), are consistent with the characteristic binding affinities of these adrenergic agonists for alpha 2-adrenergic receptors. Inhibition of VIP-stimulated cyclic AMP production by clonidine, epinephrine, isoproterenol, and dopamine is blocked by yohimbine but not by prazosin. These data establish the alpha 2-adrenergic specificity of the inhibitory effects observed. We have previously shown that beta 2-adrenergic receptor-mediated stimulation of cyclic AMP production in rabbit ciliary processes is also inhibited by postjunctional alpha 2-adrenergic receptors. These studies support the idea that the catecholamines may regulate aqueous humor formation by inhibiting stimulation of cyclic AMP production via postjunctional alpha 2-adrenergic receptors in ciliary processes.

Inhibition of adenylate cyclase in bovine ciliary process and rabbit iris ciliary body to alpha 2-adrenergic agonists

Alpha 2-adrenergic inhibition of adenylate cyclase in bovine ciliary processes and rabbit iris ciliary body (ICB) was studied. With bovine ciliary process membrane, it was found that cAMP production in the presence of 1 microM isoproterenol was increased with increasing NaCl concentrations from 0 to 200 mM. Clonidine, an alpha 2-adrenergic agonist, produced a NaCl concentration-dependent inhibition of cAMP production in the presence of isoproterenol with a maximum inhibition at 200 mM NaCl (P less than 0.05). NaCl concentrations had no effect on basal adenylate cyclase activities and activity in the presence of clonidine alone. The alpha 2-adrenergic agonists, lofexidine, clonidine and p-amino-clonidine were tested for their ability to inhibit isoproterenol-stimulated adenylate cyclase in bovine ciliary process membrane in the presence of 200 mM NaCl. Their dose-response curves showed that they had similar IC50's but the maximum inhibition differed among these agonists. Clonidine was found to be a partial agonists producing 55% of the inhibition obtained with lofexidine. The specificity of inhibition of isoproterenol-stimulated adenylate cyclase by alpha 2-agonists and blockade by pertussis toxin was examined by adenine labelling in rabbit ICB. The results demonstrate that alpha 2-adrenergic receptors exert specific inhibitory effects on adenylate cyclase activity in rabbit ICB, which are mediated by an inhibition guanine nucleotide regulatory protein, Gi.

Characterization of alpha 2-adrenergic receptors, negatively coupled to adenylate cyclase, in rabbit ciliary processes

Inhibition of ciliary process adenylate cyclase was studied on rabbit membrane preparations. When considered individually, epinephrine, GTP and NaCl did not inhibit adenylate cyclase activity. On the other hand, when present together, epinephrine, GTP (10(-5) M) and NaCl (200 mM) acted synergistically to cause a 27% inhibition of basal activity. A similar inhibition was observed with 1-norepinephrine. Clonidine and BHT 920, two alpha 2-agents were found to be partial agonists causing 63% and 82% as much inhibition as epinephrine. Phenylephrine, an alpha 1-agonist did not inhibit adenylate cyclase activity at concentrations up to 10(-4) M. Yohimbine and phentolamine prevented the inhibition of adenylate cyclase by epinephrine, while prazosin was ineffective. Alpha 2-receptor selectivity in rabbit ciliary processes and their negative coupling to an adenylate cyclase via a NaCl-dependent GTP binding protein, Ni, is thus well established.

Alpha 1-adrenoceptors in the albino rabbit ciliary process

3H-Prazosin and 3H-rauwolscine binding sites were identified in a membrane suspension prepared from albino rabbit iris + ciliary body. Scatchard analysis of saturation binding experiments demonstrated that both 3H-prazosin and 3H-rauwolscine bind to a single population of binding sites with KD values of 0.87 nM and 5.33 nM, respectively. Bmax values of 65.7 and 198 fmol/mg protein were obtained for 3H-prazosin and 3H-rauwolscine, respectively. Displacement studies by several adrenergic agonists and antagonists indicated that 3H-prazosin and 3H-rauwolscine labelled alpha 1- and alpha 2-adrenoceptors, respectively, in the iris + ciliary body. Epinephrine, norepinephrine and phenylephrine were able to stimulate the synthesis of 3H-inositol phosphates in ciliary processes labelled with 3H-inositol, with EC50 values of 2.4, 12 and 10 microM, respectively. The corresponding maximum stimulations of basal activity were 433, 430 and 283%, respectively. Phenylephrine behaved like a partial agonist in this assay. The norepinephrine response could be potently antagonized by prazosin (Ki = 27 nM), with rauwolscine being 285-fold less potent. An epithelial cell suspension was prepared from the ciliary process. Stimulation of phosphatidylinositol turnover by norepinephrine (0.1 mM) was observed, and this could be blocked by prazosin (10 microM), thus, indicating the presence of alpha 1-adrenoceptors, coupled to phosphatidylinositol turnover, in epithelial cells of the rabbit ciliary process.

Calcium does not act as a second messenger for adrenergic and cholinergic agonists in corneal epithelial cells

The role of changes in intracellular [Ca2+]i as a second messenger in response to either adrenergic or cholinergic agonists was determined in isolated bovine corneal epithelial cells. [Ca2+]i was measured in suspensions of cells loaded with either of the fluorescent indicators quin2 or indo-1, as well as in single cells loaded with fura-2. Fluorescence from the cell suspensions was measured in a spectrofluorometer while single cell fluorescence was measured using a modified fluorescence microscope with a photon counting photometer. Cells were loaded with these dyes by incubation in Ringer's (pH 8.1) containing 2-50 microM of the acetoxymethyl ester of the indicator. Fluorescence was measured before and after exposure to either, one of the adrenergic agonists isoproterenol, phenylephrine or epinephrine, or the cholinergic agonist carbachol. The resting [Ca2+]i level from the quin2 experiments was 115 nM +/- 41 nM (SEM) (n = 23) whereas with fura-2 it was 71 +/- 10 nM (n = 30). In no case did we see any change in [Ca2+]i within 15 min after addition of any agonist but we were able to observe increased calcium when 0.5 microM ionomycin was added to either the same or untreated cells. The disparity in the resting levels determined by the two methods may result from various calibration problems. Our results indicate that changes in [Ca2+]i have no second messenger role in response to these agonists.