Transgenic mice for studies of the renin-angiotensin system in hypertension.

Hypertension is a polygenic and multi-factorial disorder that is extremely prevalent in western societies, and thus has received a great deal of attention by the research community. The renin-angiotensin system has a strong impact on the control of blood pressure both in the short- and long-term, making it one of the most extensively studied physiological systems. Nevertheless, despite decades of research, the specific mechanisms implicated in its action on blood pressure and electrolyte balance, as well as its integration with other cardiovascular pathways remains incomplete. The production of transgenic models either over-expressing or knocking-out specific components of the renin-angiotensin system has given us a better understanding of its role in the pathogenesis of hypertension. Moreover, our attention has recently been refocused on local tissue renin-angiotensin systems and their physiological effect on blood pressure and end-organ damage. Herein, we will review studies using genetic manipulation of animals to determine the role of the endocrine and tissue renin-angiotensin system in hypertension. We will also discuss some untraditional approaches to target the renin-angiotensin system in the kidney.

Paradoxical regulation of short promoter human renin transgene by angiotensin ii.

We previously reported the generation of transgenic mice containing the entire human renin gene with a 900-bp promoter. To determine whether all the required elements for angiotensin II-mediated suppression of human renin are present in these mice, angiotensin II was chronically infused by means of osmotic minipump at both low and high doses, 200 and 1000 ng/kg per minute, respectively. Blood pressure was measured by tail-cuff, and kidney renin mRNA levels were quantitated using ribonuclease protection assays. Blood pressure was unchanged in mice receiving either vehicle or low-dose angiotensin II infusion but was increased by approximately 40 mm Hg with the higher dose of angiotensin II. Mouse renin mRNA decreased by >60% during both pressor and nonpressor angiotensin II infusion. Human renin mRNA was not suppressed by nonpressor angiotensin II and was paradoxically increased 1.9-fold by pressor angiotensin II. The lack of upregulation during nonpressor angiotensin II suggested that the increase might be pressure-mediated. To test this, the angiotensin II-induced increase in blood pressure was prevented by coadministration of the vasodilator, hydralazine (15 mg/kg per day). Hydralazine alone decreased blood pressure (-27+/-3 mm Hg) and increased mouse renin mRNA 2.4-fold. Human renin mRNA was unresponsive to this vasodilator-induced fall in pressure and despite the normalization of blood pressure by hydralazine, high-dose angiotensin II still caused a 2.1-fold increase in human renin mRNA. Thus, the first 900 bp of the human renin promoter does not contain all the elements required for appropriate angiotensin II-mediated suppression of human renin mRNA.

Is insulin resistance linked to hypertension?

1. The volume of work reporting insulin resistance in multiple forms of chronic hypertension has generated tremendous interest in whether this abnormality is an important factor in causing hypertension. Insulin resistance, however, is an imprecise term used interchangeably to describe widely disparate types of impairment in insulin action throughout the body and the type of insulin resistance has major ramifications regarding its potential for inducing long-term increases in blood pressure (BP). 2. Hepatic insulin resistance (impaired insulin-mediated suppression of hepatic glucose output) is the primary cause of fasting hyperinsulinaemia and is a cardinal feature of obesity hypertension. Evidence from chronic insulin infusion studies in rats suggests hyperinsulinaemia can increase BP under some conditions; however, conflicting evidence in humans and dogs leaves in question whether hyperinsulinaemia is a factor in hypertension induced by obesity. 3. Peripheral insulin resistance (impaired insulin-mediated glucose uptake, primarily of an acute glucose load in skeletal muscle) also present in obesity hypertension, but now reported in lean essential hypertension as well, is linked most notably to impaired insulin-mediated skeletal muscle vasodilation. This derangement has also been proposed as a mechanism through which insulin resistance can cause hypertension. 4. The present review will discuss the lack of experimental or theoretical support for that hypothesis and will suggest that a direct link between insulin resistance and BP control may not be the best way to envision a role for insulin resistance in cardiovascular morbidity and mortality.

Maintenance of baseline angiotensin II potentiates insulin hypertension in rats.

Chronic insulin infusion in rats increases mean arterial pressure (MAP) by a mechanism dependent on angiotensin II (Ang II). However, the fact that plasma renin activity (PRA) decreases with insulin infusion suggests that Ang II sensitivity is increased and that the parallel reduction in Ang II may partly counteract any hypertensive action of insulin. This study tested that hypothesis by clamping Ang II at baseline levels during chronic insulin infusion. Sprague-Dawley rats were instrumented with artery and vein catheters, and MAP was measured 24 hours per day. In seven angiotensin clamped rats (AC rats), renin-angiotensin II system activity was clamped at normal levels throughout the study by continuous intravenous infusion of the angiotensin-converting enzyme inhibitor benazepril at 5 mg/kg per day (which decreased MAP by 18+/-2 mm Hg) together with intravenous Ang II at 5 ng/kg per minute. Control MAP in AC rats after clamping averaged 99+/-1 mm Hg, which was not different from the 101+/-2 mm Hg measured before clamping Ang II levels. Control MAP in the 8 vehicle-infused rats averaged 105+/-2 mm Hg. A 7-day infusion of insulin (1.5 mU/kg per minute IV) plus glucose (20 mg/kg per minute IV) increased MAP in both groups of rats; however, the increase in MAP was significantly greater in AC rats (12+/-1 versus 5+/-1 mm Hg). This enhanced hypertensive response to insulin in AC rats was associated with a greater increase in renal vascular resistance (153+/-10% versus 119+/-6% of control) and a significant increase in renal formation of thromboxane (149+/-11% of control). Thus, decreased Ang II during insulin infusion limits the renal vasoconstrictor and hypertensive actions of insulin, and this may be caused, at least in part, by attenuation of renal thromboxane production.

Inhibition of thromboxane synthesis attenuates insulin hypertension in rats.

Chronic insulin infusion in rats increases mean arterial pressure (MAP) and reduces glomerular filtration rate (GFR), but the mechanisms for these actions are not known. This study tested whether thromboxane synthesis inhibition (TSI) would attenuate the renal and blood pressure responses to sustained hyperinsulinemia. Male Sprague-Dawley rats were instrumented with arterial and venous catheters, and MAP was measured 24 h/day. After 4 days of baseline measurements, endogenous synthesis of thromboxane was suppressed in 7 rats by infusing the thromboxane synthetase inhibitor, U63557A, intravenously (30 microg/kg/min) for the remainder of the experiment; 7 other rats received vehicle. Baseline MAP was not significantly different between vehicle and TSI rats (96 +/- 1 v 99 +/- 1 mm Hg). After 3 days of U63557A or vehicle, a 5-day control period was started, followed by a 7-day infusion of insulin (1.5 mU/kg/min, intravenously). Glucose (22 mg/kg/min, intravenously) was infused along with insulin to prevent hypoglycemia. In the control period, MAP was not different between vehicle and TSI rats (99 +/- 2 v 100 +/- 1 mm Hg), but MAP increased throughout the 7-day infusion period only in the vehicle rats with an average increase in blood pressure of 7 +/- 2 mm Hg. In the control period, GFR was lower in vehicle rats compared with TSI rats (2.5 +/- 0.1 v 3.1 +/- 0.2 mL/min, P = .06), and the decrease to 81% +/- 4% and 91% +/- 6% of control, respectively, during insulin was significant only in the vehicle rats. All variables returned toward control during a 6-day recovery period. These results suggest that full expression of hypertension and renal vasoconstriction during hyperinsulinemia in rats is dependent on a normal ability to synthesize thromboxane.

Insulin-induced hypertension in rats depends on an intact renin-angiotensin system.

This study tested the dependence of insulin-induced hypertension in rats on a functional renin-angiotensin system. Rats were instrumented with chronic artery and vein catheters and housed in metabolic cages. After acclimation, 10 rats began receiving the angiotensin-converting enzyme inhibitor (ACEI) benazepril at 1.8 mg.kg-1.d-1 via a continuous intravenous infusion that was maintained throughout the study; 8 control rats received vehicle. Four days after starting ACEI or vehicle, all rats entered a 5-day control period that was followed by a 7-day insulin infusion at 1.5 mU.kg-1.min-1. Glucose was coinfused at 22 mg.kg-1.min-1 to prevent hypoglycemia. Insulin infusion in control rats increased mean arterial pressure (MAP; measured 24 h/d) from an average of 101 +/- 1 to 113 +/- 2 mm Hg on day 1; MAP averaged 110 +/- 1 mm Hg for the 7-day infusion period. Glomerular filtration rate decreased, although not significantly, from 2.7 +/- 0.1 to 2.1 +/- 0.2 mL/min on day 3. Chronic ACEI decreased baseline MAP from an average of 97 +/- 1 to 79 +/- 1 mm Hg and markedly attenuated the increase in MAP during insulin. MAP averaged 81 +/- 1 mm Hg for the 7-day period and increased significantly, to 85 +/- 2 mm Hg, only on day 3. Likewise, the tendency for glomerular filtration rate to decrease was blunted. These results indicate that insulin-induced hypertension in rats depends on angiotensin II and suggest that a reduction in glomerular filtration rate contributes to the shift in pressure natriuresis.

Thromboxane is required for full expression of angiotensin hypertension in rats.

Recent studies suggest that thromboxane (TX) mediates a significant component of angiotensin II (ANG II)-induced hypertension. However, there is little information to support the hypothesis that this relationship is important during chronic, physiological increases in ANG II, particularly while controlling for variation in endogenous ANG II levels induced by TX inhibition. This study tested that hypothesis in 27 chronically instrumented rats. After baseline measurements, suppression of endogenous TX was induced and maintained throughout the study in 13 rats by i.v. infusion of the TX synthesis inhibitor (TSI) U63557A: the other 14 rats received vehicle. Baseline mean arterial pressure (MAP) was not different between groups and was unchanged by TSI or vehicle. Continuous inhibition of ANG II production was then initiated in both groups of rats by i.v. infusion of the angiotensin-converting enzyme inhibitor (ACEI) benazepril. ACEI reduced blood pressure similarly in vehicle and TSI rats, from 105 +/- 2 to 91 +/- 2 mm Hg and 103 +/- 1 to 89 +/- 1 mm Hg, respectively. ANG II was then infused at 5 ng.kg-1.min-1 i.v. for 7 days in six rats from each group to restore ANG II activity to baseline levels. This dose increased MAP to 103 +/- 2 and 101 +/- 1 mm Hg in vehicle and TSI rats, respectively, values not different from pre-ACEI levels. Seven TSI rats and eight vehicle rats received a higher dose of ANG II (20 ng.kg-1.min-1 i.v.). After 7 days, MAP was higher in vehicle than in TSI rats (143 +/- 5 versus 120 +/- 4 mm Hg). These results suggest that endogenous TX is an important determinant of MAP in ANG II hypertension but may have a diminished role in blood pressure regulation when ANG II is at normal and subnormal levels.

Chronic adrenergic receptor blockade does not prevent hyperinsulinemia-induced hypertension in rats.

Increased adrenergic activity has been suggested to mediate the hypertension associated with hyperinsulinemia. This study tested whether combined alpha1- and beta-adrenergic receptor blockade would prevent insulin-induced hypertension when euglycemia was maintained by continuous intravenous glucose infusion. Sprague-Dawley rats (n = 16) were instrumented with artery and vein catheters and placed in metabolic cages. Propranolol and prazosin (10 mg/kg/day each) were infused continuously intravenously in 9 rats and 7 other rats received vehicle. Mean arterial pressure (MAP) and heart rate (HR) were measured 24 h per day using computerized methods. After a control period, a 7-day intravenous infusion of insulin (1.5 microU/kg/min) was begun and glucose was coadministered intravenously at 23 mg/kg/min to prevent hypoglycemia. The MAP averaged 93 +/- 1 mm Hg in the blockade rats during the control period, which was significantly lower than the 98 +/- 1 mm Hg in the normal rats. During insulin infusion, MAP increased similarly in both groups, with a 10 +/- 2 mm Hg and 11 +/- 1 mm Hg increase in normal and blockade rats, respectively, by day 7. The HR also increased in both groups: from 417 +/- 8 beats/ min to 426 +/- 13 beats/min (P = NS) in normal rats and from 379 +/- 10 beats/min to 419 +/- 10 beats/min (P < .05) in blockade rats. Control sodium excretion averaged 2.5 +/- 0.1 mEq/day in both groups and no significant change in sodium balance was measured in either group. All variables returned toward control after stopping insulin. These results suggest that increased adrenergic activity is not required for chronic hyperinsulinemia to raise blood pressure in rats.

Cardiac output and renal function during insulin hypertension in Sprague-Dawley rats.

Hyperinsulinemia has been reported to cause hypertension in rats; however, the renal and hemodynamic mechanisms are not known. In this study, changes in renal function, cardiac output (CO), and total peripheral resistance (TPR) were measured during chronic insulin infusion in eight rats (approximately 350 g). After a 4-day control period, a 7-day insulin infusion was begun (1.5 mU.kg-1.min-1 iv), together with glucose (22 mg.kg-1.min-1 iv) to prevent hypoglycemia. Mean arterial pressure (MAP), CO, TPR, and heart rate were measured 24 h/day. MAP increased from 92 +/- 1 to 100 +/- 2 mmHg on day 1 and was 108 +/- 4 mmHg by day 7 of insulin. CO tended to decrease during insulin infusion, although not significantly, averaging 94 +/- 4% of the control value of 121 +/- 7 ml/min. Heart rate did not change significantly from the control value of 384 +/- 8 beats/min. TPR increased significantly to 122 +/- 11% of control by day 7. In five rats, glomerular filtration rate and effective renal plasma flow decreased to 73 +/- 4 and 66 +/- 5% of control, respectively, during insulin. Urinary sodium excretion averaged 2.6 +/- 0.1 and 2.7 +/- 0.1 meq/day during the control and insulin-infusion periods, respectively. These results indicate that insulin hypertension in rats is initiated by an increase in TPR rather than by increased CO. Also, the fact that sodium balance was maintained at elevated arterial pressure suggests that the ability of the kidneys to excrete sodium was impaired chronically during insulin infusion.

High-fructose diet does not raise 24-hour mean arterial pressure in rats.

High intakes of the simple sugars--glucose, sucrose, and fructose--have been reported to raise significantly systolic pressure in rats. It is not clear, however, if under those conditions the acute measurement of blood pressure, especially with the tail-cuff technique, represents accurately the effect of the diet on mean arterial pressure throughout the day. In this study, 15 Sprague-Dawley rats (approximately 325 g) were chronically instrumented with arterial and venous catheters and placed on a diet containing 61% vegetable starch and 5% dextrose; seven rats remained on this diet throughout the study. After 4 days of control measurements, eight rats were switched to a diet that substituted 66% fructose for the vegetable starch and dextrose. Mean arterial pressure (MAP) was measured 20 h/day by computerized methods. MAP during the 4 control days averaged 100 +/- 3 and 105 +/- 3 mm Hg in low-fructose (LF) and high-fructose (HF) diet rats, respectively. Switching to the HF diet caused no change in MAP, and after 11 days MAP averaged 104 +/- 2 and 108 +/- 3 mm Hg in the LF and HF rats, respectively. In addition, the variability of MAP over the 20-h period each day was not altered by the HF diet, and raising sodium intake fourfold caused a similar rise in MAP in both groups. There also were no significant changes in plasma glucose or insulin concentrations. Thus, a change in dietary simple sugar content from 5% dextrose to 66% fructose did not change MAP or alter blood pressure variability or sodium sensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)