Influence of intermittent hypoxia on myocardial and hepatic P-glycoprotein expression in a rodent model

Abstract

Study objective: Patients with obstructive sleep apnea who receive drug therapy for cardiovascular disease may experience resistant hypertension, arrhythmias, or more severe heart failure, and many of the drugs used to treat these conditions are substrates for P-glycoprotein (P-gp) transporters. Therefore, we sought to determine if intermittent hypoxia, which mimics obstructive sleep apnea, would upregulate myocardial and hepatic P-gp expression and Abcb1a and Abcb1b messenger RNA (mRNA) expression (genes that encode for P-gp) in an animal model.

Design: Prospective, randomized, blinded, parallel-design animal study.

Setting: University research laboratory.

Animals: Thirty adult, male Sprague-Dawley rats.

Intervention: Rats were assigned to either 2 weeks of intermittent hypoxia exposure similar to sleep apnea (12 rats) or no hypoxia exposure (controls, 18 rats).

Measurements and main results: After intermittent hypoxia or normoxia exposure, the rats were anesthetized. Heart and liver were harvested, and small samples were taken from the left ventricle (heart) and the liver for analysis. Expression of P-gp was measured by Western blotting, whereas Abcb1a and Abcb1b mRNA expression was assessed by real-time polymerase chain reaction. Band density of myocardial (but not hepatic) P-gp expression (standardized by beta-actin) was significantly higher in hypoxic rats than in control rats (p=0.03). Quantitative polymerase chain reaction revealed that myocardial and hepatic Abcb1a and myocardial Abcb1b mRNA expression were significantly increased in hypoxic rats compared with controls (p<0.05).

Conclusion: Myocardial P-gp expression and myocardial and hepatic Abcb1a mRNA expression were significantly increased after 2 weeks of intermittent hypoxia. Hypoxia-induced increases in P-gp expression may partially explain drug-resistant cardiovascular disease in patients with obstructive sleep apnea.

Figure 1

Mean (dashed horizontal line) and median (solid horizontal line) band density (pixels) of P-glycoprotein in homogenized heart (left) and liver (right) samples from rats exposed to two weeks of intermittent hypoxia and normoxia (control) are depicted in the boxplots. Myocardial Pgp protein expression was significantly increased (p=0.03) in rats exposed to hypoxia compared to rats exposed to room air. Hepatic Pgp expression was similar between hypoxia rats and controls. Outliers are depicted as dots above and below each boxplot.

Figure 2. Western Blots of Myocardial and Hepatic P-glycoprotein Expression

Top - Western blots of P-glycoprotein in homogenized heart (left) and liver (right) in rats exposed to intermittent hypoxia and control conditions. Myocardial P-glycoprotein was significantly more abundant (darker bands) in hypoxic rats compared to control rats. Hepatic P-glycoprotein was similar between hypoxic and control rats. Bottom – Western blots of beta-actin in heart (left) and liver (right) for hypoxic and control rats. Beta-actin expression was similar between hypoxia and control rats in both heart and liver samples.

Figure 3

Quantitative real time reverse-transcriptase PCR analysis of Abcb1a (gray bars) and Abcb1b (white bars) mRNA expression in heart and liver tissue of rats exposed to intermittent hypoxia compared to control rats. Data are shown as mean±SEM. Abcb1a was significantly upregulated in both heart and liver tissue, while Abcb1b was significantly upregulated in heart tissue of rats exposed to intermittent hypoxia. * p<0.05 compared to control