Vascular pressure-flow analysis in normal and hypoxemic spontaneously hypertensive rats

Increase in body weight of spontaneously hypertensive rats (SHR) under prolonged behavioral stimulation

We noticed that during 44 weeks of exposure to a number of behavioral tests (such as assessments of emotionality, locomotion, short term memory and visual discrimination learning) the body weight of spontaneously hypertensive rats (SHR) increased above that of normotensive controls (NT). To determine whether this difference was related to "behavioral stimulation" resulting from different testing situations and accompanying handling, body weights were recorded in other SHR and NT groups not subjected to any experiment (unstimulated). Stimulated SHR were significantly heavier than rats from other subgroups (unstimulated SHR as well as stimulated and unstimulated NT rats) which did not differ from one another. The first significant difference appeared after 7 weeks of behavioral training (18th week of life). Several weeks later, and continuing for the duration of study, stimulated SHR were approximately 12% heavier than the other 3 groups. No effect of behavioral testing on blood pressure was seen in either strain.

Attenuation of alpha-adrenergic responsiveness in hypoxic SHR

Chronic exposure to hypoxia reduces the severity of hypertension in SHR. This study explored the possibility that hypoxic moderation of spontaneous hypertension is caused by a decrease in vascular responsiveness. In vitro studies were conducted with thoracic aortic rings obtained from SHR and Wistar-Kyoto (WKY) rats maintained under hypoxic (H; simulated altitude = 3658 m) and normoxic (N; laboratory altitude = 1520 m) conditions. Vessels were removed prior to the rapid development of hypertension (5 weeks of age; 3 days of altitude exposure), during the rapid hypertension-development stage (10 weeks of age; 5 weeks of altitude exposure), and during the established hypertension stage (18 to 20 weeks of age; 11 to 13 weeks of altitude exposure). Dose-response curves were obtained using a non-specific vasoconstrictor (KCl) and an alpha-adrenergic agonist, phenylephrine. At all ages, WKY vessels developed greater maximal contraction to vasoconstrictor stimuli, whereas vessels from the two older groups of SHR were more sensitive (more responsive at lower dosages) to KCl. Hypoxia caused significant (p less than 0.05) attenuation of the contractile responses to phenylephrine in young "pre-hypertensive" SHR, while similar, though less marked, attenuation of phenylephrine responsiveness was evident in young WKY-H. Chronically-reduced responsiveness to phenylephrine was found in vessels from SHR-H but not WKY-H. The lack of hypoxia-induced changes in vessel response to the non-specific vasoconstrictor, KCl, suggests a specific hypoxic attenuation of adrenergic vascular responsiveness. Thus, hypoxia may protect against the development of spontaneous hypertension through attenuation of alpha-adrenergic vasoconstrictor mechanisms.

Neural and local control of arterioles in SHR

This study sought to determine if neural influences and/or alterations in arteriolar responses to oxygen could contribute to an elevated microvascular resistance in spontaneously hypertensive rats (SHR). Diameters of third-order arterioles (3A) and fourth-order arterioles (4A) were measured in the cremaster muscle of 12- to 15-week-old SHR and normotensive Wistar-Kyoto (WKY) controls anesthetized with pentobarbital. The preparation was suffused with physiological salt solution (PSS) equilibrated with various concentrations of oxygen (0% O2, 5% O2, or 10% O2) with and without local neural blockade with 10(-7) g/ml tetrodotoxin (TTX). Total active tone was assessed with 10(-4) M adenosine. SHR 3A (but not 4A) exhibited a smaller resting diameter than WKY, and larger dilations in response to TTX and adenosine. When suffusion solution PO2 was elevated in the presence or absence of TTX, SHR arterioles constricted more than did those of WKY, and SHR 4A exhibited a higher incidence of complete closure. Therefore, both neural influences and local vascular control mechanisms may contribute to an elevated microvascular resistance in SHR.