Contribution of Vascular Nitric Oxide to Basal Blood Pressure in Conscious Spontaneously Hypertensive Rats and Normotensive Wistar Kyoto Rats

1. The aim of this study was to clarify the extent to which vascular nitric oxide contributes to basal blood pressure in conscious spontaneously hypertensive rats and normotensive Wistar Kyoto rats.

2. The contribution of vascular nitric oxide to maintenance of blood pressure was estimated by measuring the pressor response to an intravenous injection of nitric oxide synthase inhibitor, N^ω-l-arginine methyl ester, given after serial injections of captopril, vasopressin V₁-receptor antagonist (V₁-antagonist) and ganglion blocker (pentolinium) in conscious spontaneously hypertensive and Wistar Kyoto rats aged 20–28 weeks. To estimate the ‘amplifier property’ of hypertrophied vasculature in spontaneously hypertensive rats, which is known to modulate pressor responses, the lower blood pressure plateau after serial injections of captopril, V₁-antagonist and pentolinium and the maximum blood pressure elicited by subsequent injection of increasing doses of phenylephrine were also measured.

3. The serial injections of captopril, V₁-antagonist and pentolinium decreased mean arterial pressure from 164 ± 9 mmHg to 67 ± 2 mmHg and from 117 ± 2 mmHg to 49 ± 1 mmHg in spontaneously hypertensive and Wistar Kyoto rats respectively. The subsequent injection of N^ω-l-arginine methyl ester restored mean arterial pressure almost to its control levels in both spontaneously hypertensive and Wistar Kyoto rats. The absolute changes in mean arterial pressure elicited by N^ω-l-arginine methyl ester were significantly greater in spontaneously hypertensive than in Wistar Kyoto rats (P < 0.01), but there was no significant difference in the responses to N^ω-l-arginine methyl ester when they were expressed as percentages of either the lower blood pressure plateau or maximum blood pressure.

4. These results indicate that basal blood pressure in both spontaneous hypertensive and Wistar Kyoto rats is maintained by a balance between vascular nitric oxide and major pressor systems. They also suggest that the vasodilatory effect of vascular nitric oxide does not differ between spontaneously hypertensive and Wistar Kyoto rats, and that the increased pressor effect of N^ω-l-arginine methyl ester in spontaneously hypertensive rats is due to a vascular amplifier mechanism.