Diverse effects of renal denervation on ventricular hypertrophy and blood pressure in DOCA-salt hypertensive rats

Cardiac hypertrophy that accompanies hypertension seems to be a phenomenon of multifactorial origin whose development does not seem to depend on an increased pressure load alone, but also on local growth factors and cardioadrenergic activity. The aim of the present study was to determine if sympathetic renal denervation and its effects on arterial pressure level can prevent cardiac hypertrophy and if it can also delay the onset and attenuate the severity of deoxycorticosterone acetate (DOCA)-salt hypertension. DOCA-salt treatment was initiated in rats seven days after uninephrectomy and contralateral renal denervation or sham renal denervation. DOCA (15 mg/kg, sc) or vehicle (soybean oil, 0.25 ml per animal) was administered twice a week for two weeks. Rats treated with DOCA or vehicle (control) were provided drinking water containing 1% NaCl and 0.03% KCl. At the end of the treatment period, mean arterial pressure (MAP) and heart rate measurements were made in conscious animals. Under ether anesthesia, the heart was removed and the right and left ventricles (including the septum) were separated and weighed. DOCA-salt treatment produced a significant increase in left ventricular weight/body weight (LVW/BW) ratio (2.44 ± 0.09 mg/g) and right ventricular weight/body weight (RVW/BW) ratio (0.53 ± 0.01 mg/g) compared to control (1.92 ± 0.04 and 0.48 ± 0.01 mg/g, respectively) rats. MAP was significantly higher (39%) in DOCA-salt rats. Renal denervation prevented (P>0.05) the development of hypertension in DOCA-salt rats but did not prevent the increase in LVW/BW (2.27 ± 0.03 mg/g) and RVW/BW (0.52 ± 0.01 mg/g). We have shown that the increase in arterial pressure level is not responsible for cardiac hypertrophy, which may be more related to other events associated with DOCA-salt hypertension, such as an increase in cardiac sympathetic activity.

Effects of small doses of ouabain on the arterial blood pressure of anesthetized hypertensive and normotensive rats

Ouabain increases vascular resistance and may induce hypertension by inhibiting the Na+ pump. The effects of 0.18 and 18 µg/kg, and 1.8 mg/kg ouabain pretreatment on the phenylephrine (PHE; 0.1, 0.25 and 0.5 µg, in bolus)-evoked pressor responses were investigated using anesthetized normotensive (control and uninephrectomized) and hypertensive (1K1C and DOCA-salt treated) rats. Treatment with 18 µg/kg ouabain increased systolic and diastolic blood pressure in all groups studied. However, the magnitude of this increase was larger for the hypertensive 1K1C and DOCA-salt rats than for normotensive animals, while the pressor effect of 0.18 µg/kg ouabain was greater only in DOCA-salt rats. A very large dose (1.8 mg/kg) produced toxic effects on the normotensive control but not on uninephrectomized or 1K1C rats. Rat tail vascular beds were perfused to analyze the effects of 10 nM ouabain on the pressor response to PHE. In all animals, 10 nM ouabain increased the PHE pressor response, but this increase was larger in hypertensive DOCA-salt rats than in normotensive and 1K1C rats. Results suggested that a) increases in diastolic blood pressure induced by 18 µg/kg ouabain were larger in hypertensive than normotensive rats; b) in DOCA-salt rats, smaller ouabain doses had a stronger effect than in other groups; c) hypertensive and uninephrectomized rats were less sensitive to toxic doses of ouabain, and d) after treatment with 10 nM ouabain isolated tail vascular beds from DOCA-salt rats were more sensitive to the pressor effect of PHE than those from normotensive and 1K1C hypertensive rats. These data suggest that very small doses of ouabain, which might produce nanomolar plasma concentrations, enhance pressor reactivity in DOCA-salt hypertensive rats, supporting the idea that endogenous ouabain may contribute to the increase and maintenance of vascular tone in hypertension.

A low concentration of ouabain (0.18 µg/kg) enhances hypertension in spontaneously hypertensive rats by inhibiting the Na+ pump and activating the renin-angiotensin system

We investigated the effects of low ouabain concentrations on systolic (SAP) and diastolic (DAP) arterial pressures and on pressor reactivity in 3-month-old male spontaneously hypertensive rats (SHR). Arterial blood pressure (BP) and pressor reactivity to phenylephrine (PHE) were investigated before and after 0.18 μg/kg ouabain administration (N = 6). The influence of hexamethonium (N = 6), canrenone (N = 6), enalapril (N = 6), and losartan (N = 6) on ouabain actions was evaluated. Ouabain increased BP (SAP: 137 ± 5.1 to 150 ± 4.7; DAP: 93.7 ± 7.7 to 116 ± 3.5 mmHg; P < 0.05) but did not change PHE pressor reactivity. Hexamethonium reduced basal BP in control but not in ouabain-treated rats. However, hexamethonium + ouabain increased DAP sensitivity to PHE. Canrenone did not affect basal BP but blocked ouabain effects on SAP. However, after canrenone + ouabain administration, DAP pressor reactivity to PHE still increased. Enalapril and losartan reduced BP and abolished SAP and DAP responses to ouabain. Enalapril + ouabain reduced DAP reactivity to PHE, while losartan + ouabain reduced SAP and DAP reactivity to PHE. In conclusion, a small dose of ouabain administered to SHR increased BP without altering PHE pressor reactivity. Although the renin-angiotensin system (RAS), Na+ pump and autonomic reflexes are involved in the effects of ouabain on PHE reactivity, central mechanisms might blunt the actions of ouabain on PHE pressor reactivity. The effect of ouabain on SAP seems to depend on the inhibition of both Na+ pump and RAS, whereas the effect on DAP seems to depend only on RAS.

Effects of ouabain on vascular reactivity

Ouabain is an endogenous substance occurring in the plasma in the nanomolar range, that has been proposed to increase vascular resistance and induce hypertension. This substance acts on the a-subunit of Na+,K+-ATPase inhibiting the Na+-pump activity. In the vascular smooth muscle this effect leads to intracellular Na+ accumulation that reduces the activity of the Na+/Ca2+ exchanger and to an increased vascular tone. It was also suggested that circulating ouabain, even in the nanomolar range, sensitizes the vascular smooth muscle to vasopressor substances. We tested the latter hypothesis by studying the effects of ouabain in the micromolar and nanomolar range on phenylephrine (PE)-evoked pressor responses. The experiments were performed in normotensive and hypertensive rats in vivo, under anesthesia, and in perfused rat tail vascular beds. The results showed that ouabain pretreatment increased the vasopressor responses to PE in vitro and in vivo. This sensitization after ouabain treatment was also observed in hypertensive animals which presented an enhanced vasopressor response to PE in comparison to normotensive animals. It is suggested that ouabain at nanomolar concentrations can sensitize vascular smooth muscle to vasopressor stimuli possibly contributing to increased tone in hypertension.

Endogenous angiotensin II modulates nNOS expression in renovascular hypertension

Nitric oxide (NO) influences renal blood flow mainly as a result of neuronal nitric oxide synthase (nNOS). Nevertheless, it is unclear how nNOS expression is modulated by endogenous angiotensin II, an inhibitor of NO function. We tested the hypothesis that the angiotensin II AT1 receptor and oxidative stress mediated by NADPH oxidase contribute to the modulation of renal nNOS expression in two-kidney, one-clip (2K1C) hypertensive rats. Experiments were performed on male Wistar rats (150 to 170 g body weight) divided into 2K1C (N = 19) and sham-operated (N = 19) groups. nNOS expression in kidneys of 2K1C hypertensive rats (N = 9) was compared by Western blotting to that of 2K1C rats treated with low doses of the AT1 antagonist losartan (10 mg·kg-1·day-1; N = 5) or the superoxide scavenger tempol (0.2 mmol·kg-1·day-1; N = 5), which still remain hypertensive. After 28 days, nNOS expression was significantly increased by 1.7-fold in the clipped kidneys of 2K1C rats and by 3-fold in the non-clipped kidneys of 2K1C rats compared with sham rats, but was normalized by losartan. With tempol treatment, nNOS expression increased 2-fold in the clipped kidneys and 1.4-fold in the non-clipped kidneys compared with sham rats. The changes in nNOS expression were not followed by changes in the enzyme activity, as measured indirectly by the cGMP method. In conclusion, AT1 receptors and oxidative stress seem to be primary stimuli for increased nNOS expression, but this up-regulation does not result in higher enzyme activity.