Cyclic intermittent hypoxia enhances renal sympathetic response to ICV ET-1 in conscious rats

Abstract

To test the hypothesis that central changes in sympathoregulation might contribute to sympathoexcitation after cyclic intermittent hypoxia (CIH) we exposed male Sprague-Dawley rats to CIH or to room air sham (Sham) for 8h/d for 3 weeks. After completion of the exposure we assessed heart rate, mean arterial pressure and renal sympathetic nerve activity in conscious animals before and after intracerebroventricular (i.c.v.) administration of endothelin-1 (ET-1, 3 pmol). CIH-exposed animals had a significantly greater sympathetic response to ET-1 than did Sham-exposed animals (CIH 137.8+/-15.6% of baseline; Sham 112.2+/-10.0% of baseline; CIH vs. Sham, P=0.0373). This enhanced sympathetic response to i.c.v. ET-1 was associated with greater expression of endothelin receptor A (ETA) protein in the subfornical organs of CIH-exposed relative to Sham-exposed rats. We conclude that 3-week CIH exposure enhances central ET-1 receptor expression and the sympathetic response to i.c.v. ET-1 suggesting central endothelin may contribute to the sympathetic and hemodynamic response to cyclic intermittent hypoxia.

Figure 1

Coronal section of rat brain showing the technique used to punch tissue of the subfornical organ (SFO) for Western blot analysis. Left panel (A) shows SFO in close-up, and middle panel (B) shows full coronal section with SFO identified. Right panel (C) shows section after punch is obtained with a blunt Luer adaptor.

Figure 2

Coronal section of rat brain showing confirmation of injection site. Alcian Blue dye was injected into the left lateral ventricle over 1 min. Brain sections were mounted on slides and counterstained with 1% aqueous neutral red staining procedures for histological confirmation of injection sites. In this section Blue dye outlines the lateral ventricle and the subfornical organ (SFO) is identified.

Figure 3

Screen shots from representative Sham-exposed (Sham) and cyclic intermittent hypoxia-exposed (CIH) rats. Upper channel is arterial pressure. Other channels, in descending order, are: mean arterial pressure (MAP); raw renal sympathetic nerve (RNA); integrated sympathetic nerve; and heart rate. Recordings are discontinuous with intracerebroventricular (i.c.v.) injection of endothelin-1 (ET-1) indicated by the vertical line in each panel. Tracings after injection are from approximately 5 min after i.c.v. injection. One sec is indicated by the horizontal line in each panel.

Figure 4

Western blot of showing enhanced expression of ETA receptor in the subfornical organ (SFO) of rats exposed to cyclic intermittent hypoxia-exposed (CIH) relative to room air sham (Sham) for 3 weeks. Homogenized tissue of SFO pooled from 10 rats in each group was used. A. Representative Western blot. B. Ratio of ETA to actin with Sham ratio considered 100%. In tissue from CIH animals, ratio of ETA to actin was significantly different from Sham (*P < 0.05). Data of Densitometric analysis were from three individual Western blots running in triplicates.