MYOGENIC TONE IN CORONARY-ARTERIES FROM SPONTANEOUSLY HYPERTENSIVE RATS

The aims of this study were to investigate myogenic tone in cannulated coronary arteries from SHR (spontaneously hypertensive rats) and WKY (Wistar-Kyoto rats) at 5 and 20 weeks of age under pressurised no-flow conditions. Pressure-diameter relationships (20-200 mm Hg) were deter mined in the presence (active) and absence (passive) of calcium and my ogenic tone at each pressure was calculated. Active, but not passive d iameter-pressure relationships were significantly different between st rains at both ages, Active diameters were similar between strains at p ressures up to 140 mm Hg at 5 and 20 weeks. At higher pressures SHR ar teries generated more tone, Maximum myogenic tone was similar between strains in spite of increased wall thickness in the SHR at both ages. Endothelium denudation increased myogenic tone to a greater extent in the SHR at both ages, Active diameters of endothelium-denuded SHR arte ries were smaller than those of WKY in the range 20-120 mm Hg at 5 wee k and 20-80 and 160-180 mm Hg at 20 weeks. These results demonstrate t hat during both the developmental and established phases of hypertensi on, myogenic tone is not enhanced in intact SHR coronary arteries, exc ept at the highest pressures, Following endothelium removal SHR arteri es generated greater myogenic tone also at lower pressures, These data demonstrate a greater endothelium-mediated antagonism of myogenic ton e in SHR coronary arteries.