Evaluation of IBMX-enhanced ocular hypotension after adrenergic agonists in the rabbit eye.

BACKGROUND: 3-Isobutyl-1-methylxanthine (IBMX), a phosphodiesterase inhibitor, enhanced the reduction of intraocular pressure more after administration of norepinephrine and epinephrine than after isoproterenol. The question arises of whether or not the IBMX-induced enhancement of ocular hypotension is exclusively due to beta 2-adrenoceptor/cAMP stimulation. METHODS: In groups of eight rabbits the ocular hypotensive responses after selective adrenergic agonists were studied in the presence and absence of phosphodiesterase inhibition with IB-MX. RESULTS: Pretreatment with IBMX 1%, applied topically, did not enhance the ocular hypotensive responses after phenylephrine (alpha 1), B-HT920 (alpha 2) and dobutamine (beta 1). The ocular hypotensive responses induced by salbutamol (0.001-0.5%) and higher concentrations of terbutaline were significantly enhanced by IBMX. Combined treatments of terbutaline 0.01% and B-HT920 0.2%, dobutamine 3% and phenylephrine 2%, and dobutamine 3% and B-HT920 0.2% were not associated with enhanced ocular hypotensive responses in the presence of IBMX. The only combination that was associated with a significant enhancement of ocular hypotension when combined with 1% IBMX was phenylephrine 2% and terbutaline 0.01%. A subthreshold dose of phenylephrine 0.1% further increased the enhanced ocular hypotensive responses induced by salbutamol 0.025, 0.2 and 0.5% in combination with IBMX. CONCLUSIONS: Phosphodiesterase inhibition with IB-MX enhances the ocular hypotensive effect induced by catecholamines not only by beta 2-adrenoceptor/cAMP stimulation, but also by simultaneous alpha 1-adrenoceptor stimulation.

Adenylyl cyclase in human and bovine trabecular meshwork.

PURPOSE: To determine the basic characteristics and responses of adenylyl cyclase in trabecular tissues. Because the second messenger cyclic adenosine monophosphate can lower intraocular pressure by increasing outflow facility, it is of interest to know which signalling pathways stimulating adenylyl cyclase are involved. METHODS: Adenylyl cyclase activity of bovine and human trabecular meshwork membrane fractions and of whole tissue homogenates (bovine) to forskolin, manganese, fluoroaluminate, isoproterenol, prostaglandins (PGE1, PGE2, PGF2 alpha), and vasoactive intestinal peptide, were evaluated. RESULTS: In bovine trabecular meshwork particulate fractions, adenylyl cyclase was stimulated 3.3- and 2.6-fold over basal by 60 and 2 microM forskolin, respectively, 2.2-fold by fluoroaluminate, and 1.5-fold by PGE1 and PGE2, whereas no or a very week response was obtained with PGF2 alpha, isoproterenol, and vasoactive intestinal peptide. PGE1-induced stimulation was dose-dependent and G-protein-dependent, which provides evidence for EP receptor-mediated activation. Whole tissue homogenates of bovine trabecular meshwork did not differ from the particulate fractions. In human trabecular meshwork membrane fractions adenylyl cyclase stimulation was more pronounced, 12.4- and 5.5-fold by 60 and 2 microM forskolin, respectively, 8.2-fold by fluoroaluminate, and 3-fold by PGE1 and PGE2. PGF2 alpha had no effect. Significant stimulation was obtained with isoproterenol (2.8-fold) and with vasoactive intestinal peptide (1.8-fold). CONCLUSIONS: Human and bovine trabecular meshwork can be stimulated at all known activation levels of adenylyl cyclase. The human adenylyl cyclase system, especially receptor-coupled activity, is more sensitive than that of bovines. Beta-adrenoreceptor stimulation, PGE2, and vasoactive intestinal peptide may have a local physiologic function by activating adenylyl cyclase in human trabecular meshwork.

Effects of cyclic nucleotide analogs on intraocular pressure and trauma-induced inflammation in the rabbit eye.

In this study the effects of cell-permeable 8-bromo-cAMP and 8-bromo-cGMP on intraocular pressure (IOP) and puncture-induced inflammatory response were investigated. Both 8-bromo-cAMP and 8-Bromo-cGMP reduced IOP when given subconjunctivally, but not topically. Subconjunctival administration of 8-bromo-cAMP induced a moderate disruption of the blood-aqueous barrier (BAB); in addition, subconjunctival 8-bromo-cAMP, but not topical 8-bromo-cAMP or subconjunctival 8-bromo-cGMP, reduced the disruption of the BAB and elevation of the aqueous PGE2 level after puncture trauma. It is concluded that the effects of 8-bromo-cAMP depend on the mode of administration, since this determines the concentration of 8-bromo-cAMP reached in the aqueous humor. It is suggested that 8-bromo-cAMP can partially suppress a slight inflammatory response by interference with the release of arachidonic acid from the tissues surrounding the aqueous humor.

Isobutylmethylxanthine enhances adrenergic-induced ocular hypotension in rabbits and beagles.

Isobutylmethylxanthine (IBMX), a phosphodiesterase/adenosine receptor inhibitor, was combined with norepinephrine (nE), epinephrine (Epi) and isoproterenol, respectively, to evaluate their effect on intraocular pressure (IOP). Application of topical IBMX alone had no measurable effect on IOP. When IBMX was combined with nE or Epi the ocular hypotension in rabbits and beagles increased. The Emax for nE alone was 2.9 +/- 0.4 mmHg, for Epi alone 7.3 +/- 0.5 mmHg and for isoproterenol alone 5.1 +/- 0.3 mmHg. The EC50 was 0.2 +/- 0.05% (nE), 0.05 +/- 0.01% (Epi) and 0.003 +/- 0.001% for isoproterenol. When given in combination with 1% IBMX the Emax for nE was 7.4 +/- 1.7 mmHg, for Epi 9.0 +/- 0.8 mmHg and for isoproterenol 6.1 +/- 0.3 mmHg. The corresponding values for EC50 were 0.07 +/- 0.03% (nE), 0.02 +/- 0.006% (Epi) and 0.002 +/- 0.001% for isoproterenol. Combining 1% IBMX with 0.1% Epi increased the aqueous humour cyclic AMP-levels at 1, 3 and 5 hr in rabbits. The results of this study demonstrate that a phosphodiesterase/adenosine receptor inhibitor such as IBMX enhances the reduction in IOP induced by adrenergic agonists.

Increase in ocular blood flow induced by isobutylmethylxanthine and epinephrine.

Alterations in regional blood flow of the eye were studied in rabbits using the radioactively labelled microsphere technique. The animals were topically treated with 1% IBMX, 0.1% epinephrine in combination with 1% isobutylmethyl-xanthine (IBMX), or with 0.1% epinephrine alone. After IBMX there was a tendency towards an increase in blood flow in the iris, ciliary processes and sclera, whereas the choroidal blood flow tended to decrease. After epinephrine the iridial blood flow increased about 50% at 6 hr. After IBMX combined with epinephrine a biphasic response was obtained: an initial decrease in blood flow of the iris, the ciliary processes and to some extent the choroid was followed by a marked and long lasting increase in blood flow from 2.5 to 7.5 hr in the iris (up to 220%), ciliary processes (123%) and sclera (115%). The choroidal blood flow was not increased. The marked increase in blood flow of the iris and the ciliary processes indicates that the reduction of intraocular pressure seen after topical treatment with a combination of epinephrine and IBMX, is not based on a vascular mechanism. The increase of ocular blood flow after a combination of IBMX and epinephrine is probably based on increased cAMP secondary to inhibition of phosphodiesterases. This may indicate the presence of beta-adrenergic receptors in the ocular vasculature.

Soluble forms of isobutylmethylxanthine enhance the ocular hypotension induced by catecholamines.

Isobutylmethylxanthine (IBMX) suspended in 0.5% hydroxypropyl methylcellulose has been shown to enhance the reduction in intraocular pressure (IOP) induced by epinephrine and norepinephrine in rabbits and beagles. In the present study, the effects of two soluble forms of IBMX, i.e. the ethylenediamine salt (IBMX-ED) and IBMX bound to cyclodextrin in saline (IBMX-CD), were studied in rabbits. When used alone, 1% IBMX-ED did not affect the IOP, but the hypotensive response to 0.1% epinephrine was enhanced dose-dependently by combination of the catecholamine with IBMX-ED. Although 1% IBMX-CD applied alone also failed to influence the IOP, the hypotensive response to epinephrine, dipivalyl epinephrine and norepinephrine was enhanced by combination of these substances with IBMX-CD. Soluble IBMX-ED as well as soluble IBMX-CD and the IBMX suspension enhanced, to a similar extent and in a dose-dependent manner, the IOP reduction induced by catecholamines. IBMX-CD, which has a neutral pH, may be a suitable form of IBMX for future long-term experiments.