Endogenous renin and related short-term blood pressure variability in the conscious rat

This study was designed to investigate, by use of spectral analysis, the blood pressure variability changes induced in the conscious rat by activation of plasmatic renin activity. Rats were surgically prepared with a supra-renal catheter inserted via the left carotid artery to perform the infusions, and with a femoral artery catheter to measure blood pressure and heart rate. Secretion of renin was induced using beta-adrenoceptor stimulation produced by isoprenaline. A first group (n=8) was infused with isoprenaline: 0.003, 10, 100 and 300 ng/kg/min, at a rate of 20 microl/min. A second group (n=8) was given a bolus injection of the angiotensin AT(1) receptor antagonist, valsartan (2 mg/kg, i.a.), prior to isoprenaline infusions. The lack of effect of infusion per se was checked in additional animals (n=8) infused with saline only (20 microl/min). Five other groups of animals were prepared with arterial catheters as mentioned previously. Each group received one concentration of infused isoprenaline and samples of blood were collected for further determinations of plasma renin activity and catecholamine concentrations. Blood pressure recordings were analysed using the fast Fourier transform on 2048 points time series (204.8 s). Isoprenaline increased plasma renin activity and did not modify plasma catecholamine concentrations. The low-frequency (0.02-0.2 Hz) component of the systolic blood pressure variability was amplified by isoprenaline (10 ng/kg/min isoprenaline: 4.16+/-0.62 mm Hg(2) vs. 2.90+/-0.44 mm Hg(2) for control value, P<0.05), a concentration that did not alter either blood pressure or heart rate levels. Isoprenaline lowered blood pressure and increased heart rate, starting at concentrations of 100 ng/kg/min. Valsartan, whose principal effect was generation of tachycardia (+25 bpm) modified neither blood pressure levels nor blood pressure variability. Valsartan prevented the amplification of the low-frequency oscillations of systolic blood pressure induced by isoprenaline (10 ng/kg/min isoprenaline: 2.53+/-0.38 mm Hg(2) vs. 2.20+/-0.25 mm Hg(2) for control value (valsartan, ns). We conclude that a moderate increase of plasma renin activity enhanced systolic blood pressure variability in the low-frequency range, without affecting blood pressure and heart rate levels.