INCREASED WALL-LUMEN RATIO OF MESENTERIC VESSELS FROM THE SPONTANEOUSLY HYPERTENSIVE RAT IS NOT ASSOCIATED WITH INCREASED CONTRACTILITY UNDER ISOBARIC CONDITIONS

We investigated the morphological (wall-lumen ratio) and contractile c haracteristics of distal mesenteric arteries from spontaneously hypert ensive rats (SHR) and Wistar-Kyoto (WKY) controls at a distending pres sure of 63% of the mean aortic pressure of each rat using a pressure a rteriograph. The wall-lumen ratios obtained were compared with those o btained at a pressure of 100 mm Hg. Experiments were carried out at 5 and 20 weeks. Mean aortic pressure of SHR was significantly increased at 5 weeks compared with that of WKY and was further increased by 20 w eeks. At 63% of mean aortic pressure, no difference in the wall-lumen ratio of the arteries was observed between strains at 5 weeks; at 20 w eeks, the wall-lumen ratio of SHR arteries was significantly increased compared that in WKY arteries. The wall-lumen ratio of SHR vessels di d not differ at 63% mean aortic pressure compared with 100 mm Hg at ei ther 5 or 20 weeks, whereas this parameter was significantly reduced i n WKY vessels at 100 mm Hg compared with 63% mean aortic pressure at 5 and 20 weeks. In the presence of spontaneous myogenic tone, there was a borderline reduction in the lumen diameter of SHR vessels compared with WKY vessels and with increasing norepinephrine concentrations at 5 weeks. At 20 weeks, lumen diameter between strains did not differ in the presence of myogenic tone nor with increasing norepinephrine conc entrations. Similar results were obtained when vessels from both rat s trains were pressurized to 80 mm Hg. Thus, the increased wall-lumen ra tio in the distal mesenteric arteries from adult SHR compared with the se from WKY is not associated with an increased contractility under is obaric conditions when studied at physiological distending pressure.