Aqueous humor / IOP regulation

Category: transport

Overview

Intraocular pressure (IOP) reflects aqueous humor production (ciliary body) - outflow (trabecular meshwork + uveoscleral). Glaucoma Rx classes attack both arms. (1) Decrease production: β-blockers (timolol, betaxolol) — block ciliary β-receptors that drive aqueous secretion; α2-agonists (brimonidine, apraclonidine) — Gi-coupled inhibition of cAMP-driven production; carbonic anhydrase inhibitors (dorzolamide topical, acetazolamide systemic). (2) Increase outflow: prostaglandin F2α analogs (latanoprost, bimatoprost, travoprost, tafluprost) — uveoscleral outflow facilitation by remodeling extracellular matrix; rho-kinase inhibitors (netarsudil) — trabecular meshwork; cholinergic miotics (pilocarpine — older) — open trabecular meshwork via ciliary muscle contraction.

Organ Systems

• nervous

Pathway Steps

1. ciliary-body → aqueous-humor-production — via β-adrenergic + CA-mediated bicarbonate secretion drives net fluid. The ciliary body secretes aqueous humor (largely by carbonic anhydrase-dependent active transport), nourishing the avascular cornea and lens and setting intraocular pressure. Reducing production lowers IOP — the mechanism of carbonic-anhydrase inhibitors and β-blockers in glaucoma.
2. aqueous-humor → trabecular-meshwork-outflow — via majority outflow via canal of Schlemm; minor via uveoscleral pathway (PGF2α target). Aqueous humor drains mainly through the trabecular meshwork into Schlemm’s canal; resistance here sets IOP and is dysfunctional in primary open-angle glaucoma. Increasing outflow (prostaglandin analogs, Rho-kinase inhibitors, pilocarpine) is the dominant modern strategy to lower pressure.

Known Modulators

• latanoprost (activator) — FP prostaglandin receptor (uveoscleral outflow). first PGF2α analog (Xalatan); once-nightly drops; iris pigmentation + eyelash growth + periorbital changes
• bimatoprost (activator) — FP prostaglandin receptor. PGF2α analog; same MOA + side effects as latanoprost; cosmetic eyelash form (Latisse)
• timolol eye (inhibitor) — β1 + β2 (ciliary body — reduces aqueous production). topical β-blocker; systemic absorption can cause bradycardia/bronchoconstriction; punctal occlusion reduces
• brimonidine (activator) — α2-adrenergic (reduces aqueous production + increases uveoscleral outflow). α2-agonist eye drops; pediatric CNS depression risk (BBB-permeant in young kids)
• dorzolamide (inhibitor) — carbonic anhydrase II (topical CAI). topical CAI; reduces bicarbonate-driven aqueous production; metallic taste even from topical
• acetazolamide (inhibitor) — carbonic anhydrase (systemic). systemic CAI; acute glaucoma + altitude sickness + epilepsy adjunct; cross-link: renal_tubular_transport for diuretic role

References

• PMID 22262082