The objective of this study was to further investigate the relationship between the arterial media thickness : lumen diameter (M:L) ratio and the arterial contractile response to noradrenaline (NA) under isobaric conditions. Femoral resistance arteries from normally perfused hindlimbs of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats were used as well as those taken from hindlimbs subject to partial ligation of the proximal external iliac artery.

A silk ligature (0.37 mm i.d.) was placed around one external iliac artery of male SHR and WKY rats at 5 weeks of age under anaesthesia (inhalation of 5 % halothane in air). At 20-24 weeks of age rats were humanely killed by stunning followed by cervical dislocation and branches (~180-250 µm i.d.) of the femoral artery from either the unligatured (Unlig) or ligatured (Lig) hindlimb were dissected post-mortem and mounted onto two glass microcannulae in an arteriograph. Arteries were pressurised to their estimated in vivo pressure (SHR Unlig, 116 mmHg; SHR Lig, 64 mmHg; WKY Unlig, 90 mmHg; WKY Lig, 53 mmHg) and bathed in physiological salt solution at 37 °C, pH 7.4. Once pressurised, the vessels developed spontaneous myogenic tone and were then challenged with increasing concentrations of NA (1 nM-10 µM). The bathing solution was then replaced with a calcium-free physiological saline solution, in which the vessels relaxed completely and structural measurements were made using a calibrated micrometre eyepiece and light microscopy. Data are presented as means ± S.E.M. (n). Statistical comparisons were made by ANOVA followed by Student’s unpaired t test modified by the False Discovery Rate test for multiple comparisons.

M:L (%) of SHR Unlig (11.91 ± 0.77 (9)) were greater (P < 0.01) than WKY Unlig (8.68 ± 0.73 (10)) and significantly reduced (P < 0.01) in arteries distal to the ligature within both rat strains (SHR Lig, 6.10 ± 0.40 (10); WKY Lig, 4.16 ± 0.46 (10)). Maximal contractile responses to NA (µm) were not significantly different between the groups (SHR Unlig 116 ± 9 (9), SHR Lig 133 ± 13 (10), WKY Unlig 129 ± 16 (10), WKY Lig 109 ± 17 (10). Thus in no comparison did the arteries with greater M:L contract to a greater extent in response to noradrenaline.

These data provide further evidence that the increased M:L in hypertension does not impart an exaggerated contractile function in response to vasoconstrictor agonists.

All procedures carried out on the animals were performed according to Institutional Guidelines and under the Cruelty to Animals Act (1876) as amended by SI 17/94 to comply with the EC Directive 86/609/EEC.

This work was funded by the Health Research Board of Ireland.