Effects of angiotensin on the autonomic nervous system

Central angiotensinergic mechanisms associated with hypertension

Following its generation by both systemic and tissue-based renin-angiotensin systems, angiotensin II interacts with specific, G-protein coupled receptors to modulate multiple physiological systems, including the cardiovascular system. Genetic models in which the different components of the renin-angiotensin system have been deleted show large changes in resting blood pressure. Interruption of the generation of angiotensin II, or its interaction with these receptors, decreases blood pressure in hypertensive humans and experimental animal models of hypertension. Whilst the interaction of angiotensin II with the kidney is pivotal in this modulation of blood pressure, an involvement of the system in other tissues is important. Both systemic angiotensins, acting via the blood-brain barrier deficient circumventricular organs, and centrally-generated angiotensin modulate cardiovascular control by regulating fluid and electrolyte ingestion, autonomic activity and neuroendocrine function. This review discusses the pathways in the brain that are involved in this regulation of blood pressure as well as examining the sites in which altered angiotensin function might contribute to the development and maintenance of high blood pressure.

Effect of light exercise on renal hemodynamics in patients with hypertension and chronic renal disease

OBJECTIVE

Increased physical activity is followed by a stimulation of the sympathetic nervous system and this effect is probably more pronounced in patients with chronic renal failure and hypertension than in healthy controls. The role of sustained exercise in hypertensive patients with chronic renal failure, with and without antihypertensive therapy, is unclear, as is hormonal regulation of the renal hemodynamics. We hypothesized that prolonged low-intensity bicycle exercise would have a greater effect in patients with chronic renal failure than in controls, and that antihypertensive treatment would ameliorate these effects.

MATERIAL AND METHODS

Glomerular filtration rate (GFR), effective renal plasma flow (ERPF), mean arterial blood pressure (MAP), norepinephrine (NE) and atrial natriuretic peptide (ANP) were measured in the upright position before and during low-intensity exercise for 2 h in healthy controls (n = 8) and in hypertensive patients with moderately reduced renal function who were not taking antihypertensives (n = 7) or who were receiving treatment with captopril (n = 10), enalapril (n = 6) or verapamil (n = 9).

RESULTS

GFR tended to decrease and ERPF decreased significantly in healthy individuals when exercise duration was prolonged from 1 to 2 h. An earlier decline in GFR and ERPF was seen in the renal failure patients compared with the controls. Filtration fraction (FF) increased during exercise in all groups except the group taking enalapril. MAP increased in the captopril group during exercise but was unchanged in the other groups. Treatment with captopril produced a more pronounced and earlier fall in exercise-induced GFR than in untreated controls, while verapamil treatment completely blunted the decline in GFR, with a concomitant increase in plasma ANP. No significant changes were seen in plasma NE levels, but urinary NE excretion increased in controls and captopril-treated patients during exercise.

CONCLUSIONS

The results suggest that prolonged low-intensity exercise has a substantially greater effect on renal hemodynamics in hypertensive renal failure patients than in healthy controls, with negligible changes in plasma NE levels. Verapamil treatment seems to ameliorate the renal effects of exercise on GFR in these patients, and this may in part be mediated via a stimulatory effect on ANP.

Angiotensin II binding sites in the superior cervical ganglia of spontaneously hypertensive and Wistar-Kyoto rats after preganglionic denervation

Angiotensin II binding was higher in superior cervical ganglia of adult spontaneously hypertensive rats (SHR) when compared to ganglia of Wistar-Kyoto (WKY) rats (571 +/- 29 and 375 +/- 9 fmol/mg protein, SHR and WKY, respectively, P less than 0.05). Unilateral preganglionic denervation reduced binding site density in ganglia of WKY (-39%, P less than 0.05 vs sham operated ganglia in WKY), and the decrease of binding sites was larger in SHR (-59%, P less than 0.01, operated vs sham operated ganglia in SHR). Part of the binding sites in the superior cervical ganglia may be present in or be associated to preganglionic nerves, and the number of these sites is higher in SHR.

Decrease in sympathetic nervous system activity and attenuation in response to stress following renal denervation in the one-kidney one-clip Goldblatt hypertensive rat

We have found that renal denervation in the one-kidney one-clip Goldblatt hypertensive rat results in an attenuation of the hypertension associated with a decrease in sympathetic nervous system activity. To test further the hypothesis that the renal nerves (afferents) contribute to the maintenance of hypertension in this model by modulating sympathetic nervous system activity, plasma norepinephrine and epinephrine changes in response to stress were compared in hypertensive sham-operated (n = 6), renal denervated (n = 6) and control (n = 7) animals. Stress was produced in conscious resting unrestrained animals by administering two brief electrical stimulations to the hindlimb. Resting plasma norepinephrine was significantly higher (P less than 0.01) in hypertensive sham-operated (420 +/- 41 pg/ml) compared to renal denervated (289 +/- 23 pg/ml) or control (296 +/- 25 pg/ml) animals. There was no difference in resting plasma epinephrine among the groups. In response to stress there were significantly greater absolute increases in plasma norepinephrine and epinephrine in hypertensive sham-operated animals compared to renal denervated and control groups, suggesting that the one-kidney one-clip rat exhibits an enhanced sympathoadrenal response to stress. The response to stress in renal denervated rats was the same as that in the control group. Taken together, these data support the concept that the renal afferent nerves modulate sympathetic nervous system activity in the one-kidney one-clip hypertensive rat.

Decrease in peripheral sympathetic nervous system activity following renal denervation or unclipping in the one-kidney one-clip Goldblatt hypertensive rat

Increased sympathetic nervous system activity has been demonstrated in established one-kidney one-clip hypertension in the rat. We have found that renal denervation in this model results in an attenuation of hypertension, unassociated with alterations in sodium or water balance or renin activity. To determine whether the depressor effect of renal denervation is associated with changes in peripheral sympathetic nervous system activity, sham operation (n = 12), renal denervation (n = 13), or unclipping (n = 13) was carried out 2 wk after the onset of one-kidney one-clip hypertension. Normotensive unine-phrectomized age- and sex-matched rats were used as controls (n = 14). Renal denervation resulted in a significant decrease in systolic blood pressure (201+/-7 to 151+/-6 mm Hg), while unclipping lowered systolic blood pressure to normotensive levels (130+/-6 mm Hg). 8 d after operation plasma norepinephrine and mean arterial pressure before and after ganglionic blockade with 30 mg/kg hexamethonium bromide were measured in conscious, unrestrained, resting animals, as indices of peripheral sympathetic nervous system activity. Plasma norepinephrine was significantly higher in hypertensive sham-operated rats (422+/-42 pg/ml) compared with normotensive controls (282+/-25 pg/ml) (P < 0.01). Both renal denervation and unclipping restored plasma norepinephrine to normal levels (273+/-22 and 294+/-24 pg/ml, respectively). Ganglionic blockade in hypertensive sham-operated animals resulted in a significantly greater decrease in mean arterial pressure than occurred in renal denervated, unclipped, or control rats. The data suggest that the depressor effect of renal denervation or unclipping in the one-kidney one-clip hypertensive rat is associated with a decrease in peripheral sympathetic nervous system activity.

Role of the renal nerves in the pathogenesis of one-kidney renal hypertension in the rat

Increased sympathetic nervous system activity has been demonstrated in established one-kidney one clip hypertension in the rat. To determine the importance of the renal nerves in this model of hypertension, renal denervation or sham operation was carried out 2 weeks after clipping. Systolic blood pressure (BP) after clipping the renal artery in 27 uninephrectomized male Charles River rats increased significantly from 125 +/- 3 mm Hg to a stable level of 185 +/-7 mm Hg by 2 weeks, in association with a positive sodium balance. Renal denervation in 13 animals resulted in a significant decrease in BP to 137 +/- 7 mm Hg, while no change in BP was seen after sham operation in 14 animals. There was no difference in mean daily water intake, mean daily sodium intake, mean daily urine volume, or mean fractional urinary sodium excretion between sham-operation and renal-denervated animals during the 2 weeks after operation. Plasma renin activity (PRA) and creatinine clearance were not significantly different at sacrifice 2 weeks after operation. Six of the renal-denervated rats were followed for 11 weeks after surgery. The BP rose again to hypertensive levels (187 +/- 8 mm Hg) by 5 weeks after renal denervation. Repeat renal denervation resulted in a significant decrease to 142 +/- 8 mm Hg. Renal denervation in eight rats with established one-kidney Grollman hypertension (185 +/- 8 mm Hg) also resulted ina significant decrease in systolic BP (143 +/- 8 mm Hg). The data demonstrate the importance of intact renal nerves in the maintenance of hypertension in the one-kidney renal hypertensive rat. The depressor effect of renal denervation is not mediated by alterations in sodium intake or excretion, water intake or excretion, creatinine clearance or PRA.