Journal of the Renin-Angiotensin-Aldosterone System

Angiotensin II (Ang II) regulates cerebral blood flow by stimulating cerebral vasoconstriction via AT₁receptors. In adult spontaneously hypertensive rats (SHR), the cerebrovascular autoregulatory curve is shifted to the right, in the direction of higher blood pressures, an indication of excessive cerebrovascular vasoconstriction. A restricted capacity to dilate cerebral blood vessels may be responsible for the enhanced vulnerability to cerebrovascular ischaemia during hypertension. We found that chronic treatment with the AT₁-receptor antagonist, candesartan, (0.5 mg/kg/day for 14 days, via osmotic minipumps implanted in the subcutaneous tissue) blocked Ang II binding to AT₁ -receptors in cerebral blood vessels and in brain areas involved in the regulation of cerebrovascular flow, and increased the ratio of lumen-wall area in the middle cerebral artery. Candesartan treatment normalised the lower part of the autoregulatory curve in SHR, and markedly decreased cerebral ischaemia as a consequence of middle cerebral artery occlusion with reperfusion. Protection from ischaemia is related to arterial remodelling, enhanced compensatory vasodilatation in the peripheral area of ischaemia, decreased reduction in cerebral blood flow following the occlusion of a major cerebral blood vessel, and protection from injury in the periphery of the lesion. Our results indicate that pre-treatment with AT₁-antagonists such as candesartan could be of benefit in the prevention and treatment of brain ischaemia.

It has been suggested that tyrosine kinase activity participates in the regulation of signal transduction associated with angiotensin II (Ang II)-induced pharmaco-mechanical coupling in rat aortic smooth muscle. We further tested the effects of genistein, a tyrosine-kinase inhibitor, and its inactive analogue, daidzein, on angiotensin I (Ang I), angiotensin III (Ang III) and angiotensin IV (Ang IV) contractions, as compared with those on Ang II. Genistein partially inhibited Ang II- and Ang I-induced contractions. The genistein-induced inhibition was more evident on Ang III and especially important on Ang IV contractile effects. Thus, Ang IV- and Ang III-induced contractions seem to be more dependent on tyrosine kinase activity than those evoked by Ang II or Ang I. Daidzein did not significantly affect the contractile effects of any of angiotensin peptides tested. These results clearly suggest that the inhibition of the action of angiotensin peptides actions by genistein is mediated by inhibition of endogenous tyrosine kinase activity. Furthermore, our data show that the type and/or intensity of tyrosine kinase activity is differentially associated with the contractile effects of different angiotensin peptides in rat aorta. Nifedipine, a blocker of membrane L-type Ca²⁺ channels, strongly inhibited Ang IV-induced contractions. At the same time, it significantly inhibited Ang III contractile effects as compared with Ang II and Ang I contractions. Meanwhile, we observed a close relationship between calcium influx and tyrosine kinase phosphorylation activity under the stimulatory effects of angiotensin peptides. Furthermore, genistein did not significantly influence the phasic contractions induced by angiotensin peptides in Ca²⁺-free Krebs-Henseleit solution. Thus, it appears that Ca²⁺ influx, rather than the release of Ca²⁺ from IP3-sensitive stores, may play a major role in the contractile effects of angiotensin peptides in rat aorta via tyrosine kinase activation. One argument against a direct action of genistein on the Ca ²⁺ channel itself is that it did not markedly affect the K⁺-induced contraction (depolarisation) in rat aorta. At the same time, a potential role for tyrosine kinase activity in the process of calcium entry is suggested. An elevation of intracellular calcium via tyrosine kinase-mediated processes may mediate the actions of G-protein coupled receptor agonists in smooth muscle, including angiotensin peptides.

Blockade of the renin-angiotensin system with angiotensin-converting enzyme inhibitors (ACE-I) or angiotensin AT₁-receptor antagonists shift the limits of autoregulation of cerebral blood flow (CBF) towards lower blood pressure (BP). The role of AT₂-receptors in the regulation of the cerebral circulation is uncertain. Hence, the present study investigated the effect on CBF autoregulation of blocking of angiotensin AT₂-receptors with PD 123319 in spontaneously hypertensive rats (SHR). Anaesthetised and ventilated SHR were given PD 123319, 0.36 mg/kg/min, intravenously, and compared with a control group. CBF was measured by the intracarotid ^133-xenon injection method and BP was raised by noradrenaline infusion and lowered by controlled haemorrhage in separate groups of rats. The limits of autoregulation were determined by computed least-sum-of-squares analysis. PD 123319 did not influence baseline CBF, but resulted in a minor BP decrease (10 control and 10 treated rats). The lower limit of CBF autoregulation (eight treated and eight control) as well as the upper limit of CBF autoregulation (eight treated and eight control) were not significantly different in PD 123319 and control animals (lower limit treated 102±4 mmHg and control 94±4; NS, and upper limit treated 171±10 mmHg and control 162±7; NS). These findings indicate that acute AT₂-receptor blockade does not influence CBF autoregulation.

Objective The renin-angiotensin system (RAS) plays an important role in the regulation of blood pressure, electrolyte balance and renal function in normal human pregnancy. The present study was designed to assess various components of the RAS and renal function during pregnancy and immediately after pregnancy in the streptozotocin (STZ)-diabetic rat.

Methods Pregnant Wistar rats were allocated to three groups: I- control, non-diabetic rats (n=24), II- STZ-diabetic rats (STZ 55 mg/kg body weight, i.v. on day 10 of pregnancy, n=24), III- diabetic rats, as above, treated with insulin (4 units/day, s.c. n=21). On days 17—18 of pregnancy, or within 24 hours after delivery, the rats were sacrificed and the various components of the RAS were determined.

Results Urinary protein excretion (UP) and creatinine clearance (CCr) were greater in group II, four days after STZ, than in group I (UP: I-7.6±2.8, II-18.6±6.3 mg/24-hour, p<0.001, CCr: I-1.04±0.33, II-2.38±0.7 ml/minute, p<0.001). Mean (±SD) serum angiotensin-converting enzyme (ACE) activity and plasma angiotensin II (Ang II) levels at days 17—18 of pregnancy were greater in the untreated diabetic rats than in control pregnant rats (ACE: 163±18 vs . 111±21 nmol/ml/minute, p<0.001, Ang II: 115±45 vs. 43±10 pg/ml, p<0.005). Postpartum serum ACE activity and plasma Ang II levels were greater in group II (ACE: I-123±14, II-142±24, III-108±21 nmol/ml/minute, p<0.01, Ang II: I-56±38, II-148±62, III-38±17 pg/ml, p<0.001). ACE activity in the lung was greater, whereas the activity in the renal cortex was less, in group II than in group I. Kidney weight in untreated diabetic rats was greater than in the other two groups.

Conclusion Increased serum ACE activity during pregnancy and postpartum in the untreated diabetic rat is associated with enhanced serum Ang II levels, which may contribute to increased protein excretion and renal hypertrophy.

The effects of chronic administration of candesartan, 16 mg once-daily, to normal volunteers on cardiovascular responses to angiotensin II (Ang II) and norepinephrine (NE) were examined.

Fifteen healthy, non-smoking volunteers participated in a randomised, double-blind crossover study of two weeks of candesartan therapy, compared with two weeks of placebo. Blood pressure (BP) responses were measured to increasing infusion rates of intravenous Ang II and NE, along with forearm blood flow (FBF) responses into intra-brachial arterial Ang II, 2 and 24 hours after the last dose of candesartan or placebo. FBF responses to intra-brachial arterial NE were recorded approximately 2 hours following the final dose.

Systolic and diastolic BP responses to intravenous infusions of Ang II during candesartan treatment were completely suppressed and significantly lower than during placebo treatment, 2 hours (candesartan 96±10/55±8 mmHg; placebo 105±5/64±8 mmHg) and 24 hours (candesartan 94±8/54±8 mmHg; placebo 103±7/64±7 mmHg) following the last dose. In contrast, FBF responses to intra-brachial arterial Ang II were significantly suppressed by candesartan compared with placebo in a subgroup of subjects 2 hours following the last dose (n=9), but not 24 hours after the last dose (n=8). FBF responses to NE were also suppressed by candesartan treatment 2 hours following the last dose, while BP responses to intravenous NE were unaltered. Chronic candesartan therapy, 16 mg once-daily effectively suppresses pressor responses to Ang II over the duration of the dosing interval.