1. Pressure/flow relationships were determined in the in situ blood-perfused superior mesenteric and hindquarters vascular beds of spontaneously hypertensive rats and Wistar-Kyoto normotensive rats before and after destruction of the endothelium with detergent. The effects of indomethacin on the regression of pressure on flow were also investigated in the spontaneously hypertensive rats, as were the endothelium-dependent relaxations in response to carbachol in the mesenteric bed.

2. In the spontaneously hypertensive rats the regression line of pressure on flow in the two vascular beds was both steeper and more elevated than in the Wistar-Kyoto rats, showing that there was greater resistance to flow in the hypertensive animals. Destruction of the endothelium significantly increased the slope of the regression in both Wistar-Kyoto and spontaneously hypertensive rats: the increases in the Wistar-Kyoto rats were 2.4 ± 0.3 fold (mesenteric) and 2.0 ± 0.5 fold (hindquarters) which were comparable with the respective increases of 1.6 ± 0.3 fold and 1.8 ± 0.3 fold in the spontaneously hypertensive rats.

3. Indomethacin (5 mg/kg, intravenously) had no effect on the pressure/flow relations in either of the vascular beds of the spontaneously hypertensive rats.

4. The dose-response curves for the endothelium-dependent vasodilatation in response to carbachol were not significantly different in spontaneously hypertensive and Wistar-Kyoto rats.

5. The results suggest that tonic release of endothelium-derived relaxing factor has similar effects in modulating resistance vessel tone in vivo in both hypertensive and normotensive rats. Further, endothelium-dependent vasodilatation does not appear to be impaired in the mesenteric vasculature in spontaneously hypertensive rats, and there appears to be no significant modulation of mesenteric vascular resistance by tonic release of cyclo-oxygenase products in spontaneously hypertensive rats.

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