Dissociation between the circulating renin-angiotensin system and angiotensin II receptors in central losartan-induced hypertension

Intracranial Pressure During the Development of Renovascular Hypertension

[Figure: see text].

Central losartan administration increases cardiac workload during aerobic exercise

To assess the effects of central administration of losartan, an antagonist of angiotensin II AT₁ receptors, on cardiovascular function during aerobic exercise, heart rate, systolic and diastolic arterial pressures and rate pressure product of Wistar rats were measured as cardiac workload indexes. The animals ran on a treadmill until fatigue after an intracerebroventricular injection of losartan or saline. Pulsatile arterial pressure was recorded by a catheter implanted into the ascending aorta, from which were derived cardiovascular parameters to estimate the cardiac workload. Total exercise time and exercise workload were determined as performance indexes. The rats showed a more intense increase in heart rate after 8 min of exercise and sustained until fatigue (P < .05). Furthermore, the rats injected with losartan had a higher increase of both systolic and diastolic arterial pressures as well as rate pressure product from approximately 6 min of exercise until fatigued (P < .05). In addition, a 22% reduction in exercise time was found in losartan-rats (P < .01). This ergolytic effect induced by losartan was strongly inversely correlated with rate-pressure product during aerobic exercise (r = 0.78, P ≤ .01). The data shows that central administration of losartan augments the cardiac workload during aerobic exercise, which courses in parallel with the reduced exercise performance.

Effects of intracerebroventricular losartan on angiotensin II-mediated pressor responses and c-fos expression in near-term ovine fetus

The renin-angiotensin system plays an important role in cardiovascular control. Intracerebroventricular (i.c.v.) angiotensin (ANG) II causes a reliable pressor response in the fetus at 90% gestation. To determine the roles of brain AT1 and AT2 receptors in this response, the effects of the central AT1 and AT2 receptor antagonists losartan and PD123319 were investigated in chronically prepared near-term ovine fetuses. Losartan at 0.5 mg/kg (i.c.v.) abolished central ANG II-induced pressor responses. High-dose losartan (5 mg/kg, i.c.v.) showed a potentiation of the pressor response to i.c.v. ANG II, accompanied by bradycardia. Associated with the pressor responses, c-fos expression in the cardiovascular controlling areas was significantly different between the low and high doses of losartan. These areas included the subfornical organ, median preoptic nucleus, organum vasculosum of the lamina terminalis, and paraventricular nuclei in the forebrain, and the tractus solitarius nuclei, lateral parabrachial nuclei in the hindbrain. Low-dose losartan markedly reduced c-fos in these areas after i.c.v. ANG II, while the high-dose losartan together with ANG II elicited a much stronger FOS-immunoreactivity in these areas than that induced by i.c.v. ANG II alone. This is a novel finding, that c-fos expression in the brain can be both activated and inhibited under the same condition. Central ANG II-induced fetal pressor responses were not altered by PD123319 (0.8 mg/kg). These results indicate that i.c.v. losartan at a high and a low dose has strikingly different effects on central ANG II-induced pressor responses in fetuses at late gestation, and that the AT1 mechanism plays an important role in fetal cardiovascular regulation.

Induced fetal depressor or pressor responses associated with c-fos by intravenous or intracerebroventricular losartan

Previous fetal studies have indicated depressor responses of intravenous (i.v.) administration of angiotensin antagonists. However, little is known of central effects of angiotensin blockers on fetal cardiovascular controlling. The cardiovascular effects of central administration of the angiotensin-1 (AT(1)) and angiotensin-2 (AT(2)) receptor antagonists, losartan and PD123319, were investigated in the chronically catheterized near-term ovine fetuses. Intravenous losartan produced within 1.5 min a significant and persistent depressor response [maximum Delta mean arterial pressure (MAP)=9 mm Hg] without altering fetal heart rate. Intracerebroventricular (i.c.v.) administration of losartan (1-5 mg/kg) increased fetal arterial pressures (Delta MAP=9-14 mm Hg). Central application of losartan (1 mg/kg) also increased fetal heart rate (maximum Delta heart rate=33 beat per minute). Losartan increased c-fos expression in the median preoptic nucleus and paraventricular nuclei in the forebrain, and the tractus solitarius nuclei, the lateral parabrachial nuclei, and the ventrolateral medullabrain. These brain sectors are with abundant AT(1) receptors and have been demonstrated in the involvement in cardiovascular regulation. In contrast, intracerebroventricular injection of the AT(2) receptor antagonist PD123319 had no effect on fetal arterial pressure and heart rate. The results demonstrate strikingly functional differences of losartan on the fetal cardiovascular regulation in central and peripheral sides.