1. Endothelium-dependent acetylcholine-mediated relaxations of small coronary arteries (∼200 μm internal diameter) from 20 weeks old spontaneously hypertensive rats (SHR) and normotensive Wistar—Kyoto (WKY) controls were compared under pressurized no-flow conditions after the development of myogenic tone or constriction with the thromboxane A2 mimetic U46619.

2. Relaxations of WKY and SHR arteries following development of myogenic tone did not differ and were not significantly influenced by indomethacin alone (10 μmol/l) or in combination with Nω-nitro-l-arginine (l-NNA, 0.1 mmol/l). Maximum relaxations were significantly attenuated by 30 mmol/l K+ in the SHR, from 85 ± 7% (n = 11) to 20 ± 8% (n = 8), P < 0.001, and in the WKY from 86 ± 5% (n = 9) to 39 ± 14% (n = 8), P < 0.01.

3. Relaxations following constriction with U46619 were also similar in both rat strains. Maximum relaxations were 50 ± 11% (n = 8) in SHR and 60 ± 7% (n = 6) in WKY. Indomethacin did not influence these relaxations. The combination of indomethacin and l-NNA attenuated relaxations in WKY (P < 0.01), but in the SHR the attenuation did not achieve statistical significance (P = 0.07) compared with controls; the maximum responses were reduced to 25 ± 7% (n = 8) and 14 ± 11% (n = 6) in the SHR and WKY respectively, but only in the WKY was this reduction significant (P < 0.05).

4. These data demonstrate that, under control conditions, SHR and WKY coronary arteries relax equally effectively, regardless of mode of contraction, and also that the mechanism of acetylcholine-mediated relaxation differs according to the mode of contraction. Acetylcholine relaxes myogenic tone by a K+-sensitive mechanism in both WKY and SHR, consistent with a role for endothelium-derived hyperpolarizing factor; NO contributes substantially to the relaxation of U46619-induced tone by acetylcholine in the WKY, but to a diminished extent in the SHR.

5. These data indicate that the choice of vasoconstrictor agent is of critical concern when assessing mechanisms of endothelium-dependent relaxation and abnormalities thereof in hypertension.

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