Time-Dependent Changes in Catecholamine Turnover in Spontaneously Hypertensive Rats Exposed to Hypoxia

Abstract

In three separate experiments, 4 to 5-week-old spontaneously hypertensive rats (SHR) and normotensive controls (Wistar-Kyoto [WKy]) were exposed to hypobaric hypoxia (simulated altitude = 3658 m) for 3 hr, 3 days, or 3 weeks. Comparable groups were maintained in ambient laboratory conditions (normoxia). Hypoxia prevented the increase in blood pressure noted in 8-week-old normoxic SHR. Right ventricular hypertrophy first occurred after 3 days of hypoxia, and was found in both SHR and WKy. Catecholamine turnover was measured using the tyrosine hydroxylase inhibitor, α-methyl-p-tyrosine. In myocardium, both strains evidenced hypoxia-induced changes in norepinephrine (NE) turnover, which was increased at 3 hr, normalized at 3 days, and increased again at 3 weeks. Reduced basal NE concentration at 3 days indicated a temporary deficit in synthetic capacity, which would allow maintenance of a heightened neuronal output. Catecholamine turnover in right and left ventricles differed little in response to hypoxia, in spite of differential hemodynamic demands on SHR versus WKy or on right versus left ventricle. In contrast to findings in myocardium, significant interactive effects between strain and altitude exposure were noted for adrenal catecholamine turnover. Specifically, hypoxia exerted a suppressive influence in SHR that was not evident in WKy, and this may represent an important component of hypoxia-induced protection against the development of spontaneous hypertension.