Renal actions of angiotensin II in renovascular hypertension

Renovascular hypertension: structural changes in the renal vasculature

Experimental narrowing of the main renal artery to produce hypertension increases the aorta-glomerular capillary pressure difference and vascular resistance. This article examines the hypothesis that hypertension also may be caused by structural changes that narrow intrarenal blood vessels, similarly increasing preglomerular vascular resistance and the aortic-glomerular capillary pressure gradient. There is evidence of both wall hypertrophy and lumen narrowing of the preglomerular arteries in spontaneously hypertensive rats, with increased preglomerular resistance and aortic-glomerular capillary pressure difference. We have also attempted to induce structural changes in renal-preglomerular vessels experimentally by infusing angiotensin II at low doses (0.5 to 4.5 ng/kg per minute) into the renal artery of Sprague-Dawley rats and greyhound dogs for up to 4 weeks. This angiotensin II infusion produced apparent dose-related effects on preglomerular vessel structure and hypertension. The possibility that hypertension may be induced by structural changes in preglomerular resistance vessel walls, by simulation of the hemodynamic effects of main renal artery stenosis, deserves further investigation.

Chronic intrarenal infusion of low-dose angiotensin II in dogs increases arterial pressure without impairment of renal function

1. To determine whether chronic angiotensin II (AngII) infusion into the renal artery, at a dose which increases systemic arterial pressure, reduces glomerular filtration rate (GFR) and renal blood flow, AngII was infused at 0.5 ng/kg per min into the renal artery or intravenously in chronically instrumented dogs for 1 month. 2. Mean arterial pressure (MAP) rose significantly (P < 0.05) during the infusion of AngII into the renal artery (+7 +/- 2 mmHg on days 26-30). There were no significant changes in GFR or renal blood flow. When the same dose of AngII was infused intravenously, MAP did not change significantly (-2 +/- 2 mmHg) and there were no significant changes in GFR or in renal blood flow. 3. We conclude that AngII infused into the renal artery for 1 month, at a dose which was initially subpressor, causes a rise in arterial pressure that is not associated with impairment of renal function.

Low dose angiotensin II infusions into the renal artery induce chronic hypertension in conscious dogs

Angiotensin II was infused at 0.5 ng/kg/min either directly into the left renal artery (n = 5) or intravenously (n = 4) for 28 days in conscious dogs. Renal artery infusion of angiotensin II had no significant effect on mean arterial pressure after 1 and 24 h, but pressure had increased by 12 +/- 2, 12 +/- 4, 8 +/- 3 and 13 +/- 3 mmHg on days 7, 14, 21 and 28, respectively, during infusion. Renal blood flow decreased significantly at 24 hours (p = 0.02) but was not significantly reduced subsequently. Over days 7-28, central venous pressure and haematocrit rose significantly but body weight did not change significantly. During intravenous infusion of angiotensin II, arterial pressure increased (5 +/- 4, 7 +/- 5 and 5 +/- 3 mmHg on days 7, 14 and 21, respectively), body weight rose and haematocrit fell significantly, but central venous pressure did not change. Thus, angiotensin II infused into the renal artery, at a dose which had no initial pressor effect, produced chronic, stable hypertension, with equivocal evidence of renal fluid retention. We conclude that elevated levels of angiotensin II in the kidney alone can cause chronic hypertension.

Renal venous wedge pressure in renal wrap hypertension in rabbits

1. Renal cellophane wrapping to produce hypertension causes thickening of the capsule of the kidney. To determine whether this compresses the kidney, deep renal vein wedge pressure was measured as an estimate of tissue pressure in anaesthetized rabbits 1 month after cellophane wrapping (n = 5) or a sham operation (n = 3). 2. Renal vein wedge pressure was 18.3 +/- 2.0 mmHg in hypertensive rabbits and 8.4 +/- 1.1 mmHg in the sham-operated rabbits. 3. Arterial pressure was raised or lowered with angiotensin II or glyceryl trinitrate, respectively. Arterial and wedge pressures were approximately linearly related and, at any given arterial pressure, wedge pressure was approximately 8 mmHg higher in the cellophane-wrapped kidney than in the kidney of the sham-operated group. 4. These results, showing that renal wedge pressure is elevated in renal wrap rabbits, indicate that the kidneys are compressed, probably by the thickened renal capsule. This may explain the increased renal vascular resistance seen in this form of hypertension.

Renal effects of atrial natriuretic peptide in conscious rabbits with renal wrap hypertension

Atrial natriuretic peptide (ANP, 2 micrograms/min) was infused intravenously into rabbits four weeks after renal wrap or sham operation. Mean arterial pressure (MAP) averaged 132 +/- 4 mmHg in the renal wrapped rabbits and 89 +/- 3 mmHg in the sham rabbits, and glomerular filtration rate (GFR) was significantly lower in the hypertensive rabbits (6.2 +/- 1.0 ml/min) than in sham rabbits (8.9 +/- 0.7 ml/min). In sham rabbits, ANP caused a significant diuresis, natriuresis and increase in GFR. Enalapril pretreatment blunted these responses. In the hypertensive rabbits, ANP reduced mean arterial pressure but did not cause significant diuresis or natriuresis or change in GFR. Enalapril pretreatment did not significantly alter this response to ANP. In separate experiments, nitroprusside was infused to lower arterial pressure in hypertensive rabbits by a similar amount to that achieved with ANP and this reduced GFR, sodium and urine excretion rates. Thus ANP maintained GFR and sodium excretion in hypertensive rabbits compared to an equihypotensive dose of nitroprusside. In summary, ANP did not cause natriuresis or diuresis in renal wrapped kidneys at a dose which was effective in normal kidneys, but did maintain GFR, sodium and water excretion rates, compared to an equally hypotensive dose of nitroprusside.