Endogenous nitric oxide buffers blood pressure variability between 0.2 and 0.6 Hz in the conscious rat

Shear stress is a potent stimulus for the formation and release of nitric oxide (NO). It seems, therefore, possible that a short-term increase in arterial blood pressure (ABP), which leads to a concomitant rise in endothelial shear stress, enhances NO release. The latter elicits a relaxation of vascular smooth muscle cells that, in turn, counteracts the initial rise in blood pressure (BP). Thus this chain of events may constitute a negative feedback loop reducing BP variability (BPV). To test this hypothesis, BP-time series were determined via telemetry in freely moving conscious Sprague-Dawley rats. Because it was reported recently that NO effects on ABP are more pronounced in females, the experiments were performed on 2 groups consisting of 10 female and 11 male animals. This was done under control conditions and after fixing NO plasma levels via an intravenous bolus of 15 mg/kg body wt N(G)-nitro-L-arginine methyl ester together with a continuous infusion of nitroprusside (15 +/- 0.8 microg/min). This combined infusion maintained mean ABP and heart rate at physiological levels, thus avoiding as much as possible interferences with other reflexes, e.g., the baroreflex. To quantitate BPV, fast Fourier transforms of the BP-time series were determined. The absolute power in the frequency range below 1 Hz increased during fixed NO to approximately 350% vs. control animals (female control, 2.1 x 10(9) +/- 1.5 x 10(8) mmHg2 vs. fixed NO, 8.0 x 10(9) +/- 1.3 x 10(9) mmHg2, P < 0.005; male control, 3.4 x 10(9) +/- 4.6 x 10(8) mmHg2 vs. fixed NO, 8.3 x 10(9) +/- 2.0 x 10(9) mmHg2, P < 0.05). This was mainly caused by a substantial rise in the power ranging from 0.2 to 0.6 Hz, which increased roughly fourfold in both females and males. It is concluded that the NO system is a potent buffer of spontaneous BP oscillations in the freely moving rat. This system is most efficient in buffering frequencies within the range of 0.2-0.6 Hz and shows no gender-specific differences with respect to its BP buffering capacity.