Evaluation of the Modulation of Organic Cation Transporter (OCT) in the Tear Disposition of its Substrates in Rabbits

Abstract

Purpose: : Endogenous cationic compounds are known to exist in the tear fluid. Therefore, the positioning and modulation of Organic Cation Transporters (OCT) in cornea, conjunctiva and lacrimal gland gain interest in understanding the precorneal disposition of xenobiotics for their pharmacological and toxicological significance. This study was conducted to evaluate the role of OCT in the fate of topical and intravenous substrates in their precorneal disposition using their blockers as pharmacological tools. Gene expression studies were also conducted to identify the presence of the subtype of OCT in tear glands.

Methods: : Albino rabbits of either sex weighing 1.5-2.0 kg were used for the study. Tear kinetics of OCT substrates (tetraethylammonium (TEA), metformin) after topical and intravenous administration were assessed in the presence and absence of blockers (quinidine & atropine) applied topically. Tear samples were collected using Schirmer’s strips at various time intervals and subjected for the simultaneous quantification of substrates and blockers using LC-MS/MS. Invivo Gamma Scintigraphy was also used to image the fate of 99Tc labelled TEA with or without OCT transporter blockade in rabbits. Rabbit lacrimal tissue samples were subjected for the evaluation for the expression of OCT1, OCT2 & OCTN2 genes using RT-PCR.

Results: : Topical pretreatment (30 min prior to substrate) of OCT blockers significantly decreased the pre-corneal clearance of OCT substrates thereby increased their tear levels after topical administration. Surprisingly, intravenous administration of TEA reached significant concentration in the tear after single intravenous administration at the Tmax of 60 min and was significantly inhibited by the blockers. Gene expression studies revealed the presence of OCTN2 in lacrimal glands whereas OCT 1 & 2 were undetected.

Conclusions: : OCTs are functionally active in the precorneal disposition of cationic substrates. OCTs present in corneal epithelium and conjunctiva are positioned from apical to basolateral and basolateral to apical in tear glands. For the first time, this study revealed that intravenously administered xenobiotics (OCT substrates) are capable of reaching precorneal area through tear secretion and can have physiological and pharmacological relevance.

Keywords: ion transporters • cell membrane/membrane specializations • lacrimal gland