Role of P-glycoprotein in ocular clearance of rhodamine 123 in rabbits

Upregulation of P-glycoprotein expression by ophthalmic drugs in different corneal in-vitro models

OBJECTIVES

The purpose of this study was to analyse P-glycoprotein (P-gp) expression in different human in-vitro cornea models (HCE-T epithelial model and Hemicornea construct) after stimulation with P-gp substrates (rhodamine 123, levofloxacin and acebutolol).

METHODS

The influence of P-gp substrates on mRNA expression was analysed using reverse transcriptase polymerase chain reaction (PCR) and real-time PCR. The effect of stimulation on the transporter functionality was estimated with a digoxin efflux assay. The Caco-2 cell line was used as positive control.

KEY FINDINGS

The reverse transcriptase PCR results showed an increase in band intensity compared with the control medium for all substrates. The real-time PCR for the Caco-2 and HCE-T epithelial model yielded a similar outcome, in which all tested substrates upregulated P-gp. In contrast, the Hemicornea construct showed no significant increase in the mRNA expression after stimulation. Both in-vitro models possessed similar drug transport profiles after stimulation. A significantly increased efflux of digoxin was measured after 24 and 72 h of stimulation with levofloxacin and acebutolol.

CONCLUSIONS

The expression and functionality of the P-gp in corneal tissue can be influenced through time exposure with specific substrates. However, the exact mechanism still requires further elucidation.

Expression and function of P-glycoprotein in rats with carbon tetrachloride-induced acute hepatic failure

Acute hepatic failure was induced experimentally in rats by intraperitoneal injection of 2.5 mL kg(-1) carbon tetrachloride (CCl4), and the effects on the expression and function of P-glycoprotein in the liver, kidney and brain were evaluated. The CCl4 injection significantly increased the indicators of hepatic function (glutamate oxaloacetate transaminase, glutamate pyruvate transaminase), but not of renal function (blood urea nitrogen, glomerular filtration rate). In rats with acute hepatic failure, the hepatic P-glycoprotein concentration increased 1.5-fold and the ATP concentration decreased to approximately 40% that in control rats. In contrast, P-glycoprotein concentrations in the kidney and brain and ATP concentrations in the kidney remained unchanged. The in-vivo P-glycoprotein function in these tissues was suppressed as evaluated by biliary and renal secretory clearances and brain distribution of rhodamine 123, a P-glycoprotein substrate. These findings suggest that factors other than P-glycoprotein concentration are involved in the systemic suppression of P-glycoprotein function in diseased rats. In Caco-2 cells, plasma collected from CCl4-treated rats exhibited a greater inhibitory effect on P-glycoprotein-mediated transport of rhodamine 123 than that from control rats, suggesting the accumulation of an endogenous P-glycoprotein substrate/inhibitor in the plasma of diseased rats. In fact, the plasma concentration of corticosterone, an endogenous P-glycoprotein substrate, increased 2-fold in CCl4-treated rats compared with control rats. It was demonstrated that P-glycoprotein function is systemically suppressed in rats with CCl4-induced acute hepatic failure, not only in the target organ (liver), but also in other organs (kidney and brain), although the P-glycoprotein concentration remained unchanged in the kidney and brain, and increased in the liver. In the systemic suppression of the P-glycoprotein function in the diseased state, the alteration of plasma concentrations or components of endogenous P-glycoprotein-related compounds, such as corticosterone, would likely be involved.

Expression and function of P-glycoprotein in rats with glycerol-induced acute renal failure

The effect of glycerol-induced acute renal failure on P-glycoprotein expression and function was evaluated in rats. The in vivo function of P-glycoprotein was evaluated by measuring renal secretory and biliary clearance and brain distribution of rhodamine 123 (Rho-123), a P-glycoprotein substrate, under a steady-state plasma concentration. In acute renal failure rats, the P-glycoprotein level increased 2.5-fold in the kidney, but not in the liver and brain. In contrast, P-glycoprotein function in these tissues was suppressed. Interestingly, not only the renal but also the biliary clearance of Rho-123 was correlated with the glomerular filtration rate. In Caco-2 cells, plasma from renal failure rats exhibited a greater inhibitory effect on P-glycoprotein-mediated transport of Rho-123 than did plasma from control rats. In conclusion, P-glycoprotein function was systemically suppressed in acute renal failure, even though the level of P-glycoprotein remained unchanged or rather increased. This may be due to the accumulation of some endogenous P-glycoprotein substrates/modulators in the plasma in disease states.