Direct and indirect effects of angiotensin II on venous tone in conscious rats

Effect of aliskiren addition to amlodipine on ankle edema in hypertensive patients: a three-way crossover study

OBJECTIVE

The aim of this study was to assess the effect of aliskiren and amlopidine on ankle-foot volume (AFV) and pretibial subcutaneous tissue pressure (PSTP).

RESEARCH DESIGN AND METHODS

After 4-week placebo, 120 outpatients with grade 1 - 2 hypertension were randomized to amlodipine 10 mg or aliskiren 300 mg or their combination for 8 weeks in three crossover periods. At the end of each treatment, blood pressure, AFV, PSTP, plasma renin activity (PRA) and norepinephrine were assessed.

RESULTS

Both monotherapies similarly reduced systolic blood pressure (SBP; p < 0.001) and diastolic blood pressure (DBP; p < 0.001), but the reduction was greater with amlodipine/aliskiren combination (SBP: - 24.6 mmHg, p < 0.001 vs monotherapy; DBP: -20.9 mmHg, p < 0.01 vs monotherapy). Amlodipine increased both AFV (+ 28.4%, p < 0.01) and PSTP (+ 80.4%, p < 0.01), while the combination produced a less marked increase in AFV (+ 6.6%, p < 0.01 vs amlodipine) and PSTP (+ 20.1%, p < 0.01 vs amlodipine). Plasma norepinephrine increased with amlodipine (+ 53.5%, p < 0.01) and this increase was not reduced by aliskiren addition. PRA was unaffected by amlodipine, while it was reduced by both aliskiren monotherapy (- 77.7%, p < 0.01) and aliskiren/amlodipine combination (- 75.7%, p < 0.01).

CONCLUSIONS

Direct renin inhibition by aliskiren partially counteracts the microcirculatory changes responsible for calcium-channel-induced edema formation, possibly through preferential vasodilation of venous capacitance vessels.

Involvement of the AT1 receptor in the venoconstriction induced by angiotensin II in both the inferior vena cava and femoral vein

Although angiotensin II-induced venoconstriction has been demonstrated in the rat vena cava and femoral vein, the angiotensin II receptor subtypes (AT(1) or AT(2)) that mediate this phenomenon have not been precisely characterized. Therefore, the present study aimed to characterize the pharmacological receptors involved in the angiotensin II-induced constriction of rat venae cavae and femoral veins, as well as the opposing effects exerted by locally produced prostanoids and NO upon induction of these vasomotor responses. The obtained results suggest that both AT(1) and AT(2) angiotensin II receptors are expressed in both veins. Angiotensin II concentration-response curves were shifted toward the right by losartan but not by PD 123319 in both the vena cava and femoral vein. Moreover, it was observed that both 10(-5)M indomethacin and 10(-4)M L-NAME improve the angiotensin II responses in the vena cava and femoral vein. In conclusion, in the rat vena cava and femoral vein, angiotensin II stimulates AT(1) but not AT(2) to induce venoconstriction, which is blunted by vasodilator prostanoids and NO.

Effect of valsartan or olmesartan addition to amlodipine on ankle edema in hypertensive patients

INTRODUCTION

The objective of this study was to compare the effect on ankle edema of adding valsartan (V) or olmesartan (O) to amlodipine (A) in the treatment of hypertension.

METHODS

After a 4-week placebo period, 74 adult outpatients with essential hypertension (diastolic blood pressure [DBP] >90 and <110 mmHg, and systolic blood pressure [SBP] >140 mmHg) were treated with A 10 mg once daily for 4 weeks. Thereafter, nonresponder patients (DBP >90 mmHg and/or SBP >140 mmHg; n=51) were randomized to receive additional V 160 mg once daily or O 20 mg once daily for 8 weeks in two crossover periods, each separated by a 4-week placebo period. Clinic blood pressure (BP), heart rate, and ankle/foot volume (AFV) were evaluated and blood samples were drawn to evaluate plasma norepinephrine (NE) levels.

RESULTS

Both V/A and O/A induced a greater SBP/DBP reduction than A monotherapy (-26.4/-20.8 mmHg and -24.4/-19.1 mmHg, respectively; all P<0.001 vs. baseline and P<0.01 vs. A). A monotherapy increased AFV by 24%, P<0.001 vs. baseline, while the addition of either V or A reduced such increases. However, with V/A the AFV increase (+9.7%, P<0.05 vs. baseline, P<0.01 vs. A) was lower than with O/A (+16.7%, P<0.01 vs. baseline, P<0.05 vs. A); the difference between the two combinations was significant. Plasma NE levels were significantly increased by A (+44.6%) and values did not change with the addition of V (+35.2%) or O (+33.7%). Plasma active renin (PAR) was unchanged by A but increased by V/A (+214.4%, P<0.05 vs. baseline) and further by O/A (+325.6%, P<0.01 vs. baseline; difference between the 2 combinations: P<0.05). An inverse correlation was found between the AFV decrease and PAR increase (r=-0.31, P<0.05).

CONCLUSION

Adding V or O to A reduced ankle edema, but this effect was more pronounced with V. The greater degree of renin-angiotensin system activation observed with Ocould be related to such a difference.

Circulating angiotensin II and dietary salt: converging signals for neurogenic hypertension

Circulating angiotensin II (Ang II) combined with high salt intake increases sympathetic nerve activity (SNA) in some forms of hypertension. Ang II-induced increases in SNA are modest, delayed, and specific to certain vascular beds. The brain targets for circulating Ang II are neurons in the area postrema (AP), subfornical organ (SFO), and possibly other circumventricular organs. Ang II signaling is integrated with sodium-sensitive neurons in the SFO and/or organum vasculosum of the lamina terminalis (OVLT) and drives sympathetic premotor neurons in the rostral ventrolateral medulla (RVLM) via the paraventricular nucleus (PVN). It is likely that, over time, new patterns of gene expression emerge within neurons of the SFO-PVN-RVLM pathway that transform their signaling properties. This transformation is critical in maintaining increased SNA. Identification of a novel gene supporting this process may provide new targets for treatment of neurogenic hypertension.

The venous circulation: a piscine perspective

Vascular capacitance describes the pressure-volume relationship of the circulatory system. The venous vasculature, which is the main capacitive region in the circulation, is actively controlled by various neurohumoral systems. In terrestrial animals, vascular capacitance control is crucial to prevent orthostatic blood pooling in dependent limbs, while in aquatic animals like fish, the effects of gravity are cancelled out by hydrostatic forces making orthostatic blood pooling an unlikely concern for these animals. Nevertheless, changes in venous capacitance have important implications on cardiovascular homeostasis in fish since it affects venous return and cardiac filling pressure (i.e. central venous blood pressure), which in turn may affect cardiac output. The mean circulatory filling pressure is used to estimate vascular capacitance. In unanaesthetized animals, it is measured as the central venous plateau pressure during a transient stoppage of cardiac output. So far, most studies of venous function in fish have addressed the situation in teleosts (notably the rainbow trout, Oncorhynchus mykiss), while any information on elasmobranchs, cyclostomes and air-breathing fishes is more limited. This review describes venous haemodynamic concepts and neurohumoral control systems in fish. Particular emphasis is placed on venous responses to natural cardiovascular challenges such as exercise, environmental hypoxia and temperature changes.

Splanchnic Circulation Is a Critical Neural Target in Angiotensin II Salt Hypertension in Rats

Chronic angiotensin II (Ang II) infusion, in rats fed high salt, engages the sympathetic nervous system to increase venomotor tone. The splanchnic sympathetic nervous system is the most important regulator of venous tone, indicating that splanchnic sympathetic nervous system activity may be increased in Ang II salt hypertension. We hypothesized that celiac ganglionectomy (CGx), to selectively disrupt sympathetic innervation to the splanchnic circulation, would attenuate arterial pressure (AP), and venous tone increases in Ang II salt hypertension. Rats fed 2% or 0.4% NaCl were instrumented to allow AP measurement by radiotelemetry at the same time as surgical CGx or sham operation. Ang II was delivered by minipump (150 ng/kg per minute) for 14 days. CGx reduced AP independent of salt diet during control. CGx markedly attenuated Ang II hypertension in rats on 2% NaCl but had little effect in rats fed 0.4% NaCl. To test the possibility that CGx exerted its effects via renal denervation, rats were subjected to the same protocol but received selective bilateral renal denervation. Renal denervation decreased AP during control but had no protective effect on Ang II hypertension and actually tended to exacerbate the pressor response. Finally, separate groups of rats underwent CGx or sham operation and were instrumented to allow repeated measures of mean circulatory filling pressure, an index of venous tone. In addition to attenuating Ang II salt hypertension, CGx completely prevented Ang II salt-induced increases in mean circulatory filling pressure and substantially attenuated depressor responses to acute ganglion blockade. We conclude that, in the presence of high salt, Ang II activates the splanchnic sympathetic nervous system to increase venomotor tone and AP.

Chronic low-dose angiotensin II infusion increases venomotor tone by neurogenic mechanisms

The purpose of this study was to identify changes in venomotor tone in the chronic low-dose angiotensin II (Ang II) model of hypertension and to establish the contribution of sympathetic nerve activation to these venomotor tone changes. Male Sprague-Dawley rats were acclimatized to a 0.4% or 2.0% NaCl diet for 7 days and then catheterized to allow chronic and repeated measures of arterial pressure, central venous pressure, and mean circulatory filling pressure (MCFP), an index of venous smooth muscle tone, in conscious undisturbed rats. After 4 days of recovery and a 3-day control period, an Ang II or physiological saline-filled osmotic minipump was implanted subcutaneously to deliver Ang II (150 ng/kg per minute) or vehicle control for 14 days. MCFP was measured in duplicate before and after acute ganglionic blockade with hexamethonium (30 mg/kg i.v.) on control day 2 and Ang II infusion on days 1, 3, 7, and 14. Blood volume was also measured on these days and was unchanged for the duration of the study in all of the groups. Arterial pressure was increased for the duration of Ang II infusion in rats on both 0.4% and 2% NaCl diets, but the increase was significantly greater in the 2% NaCl group and completely abolished by hexamethonium. MCFP was significantly increased for the entire Ang II infusion period only in rats fed 2% NaCl, and this increase was completely abolished by hexamethonium. We conclude that the combination of chronic low-dose Ang II infusion and high dietary salt intake engages the sympathetic nervous system to increase venomotor tone.

Venous responses during exercise in rainbow trout, Oncorhynchus mykiss: α-adrenergic control and the antihypotensive function of the renin–angiotensin system

The role of the alpha-adrenergic system in the control of cardiac preload (central venous blood pressure; P(ven)) and venous capacitance during exercise was investigated in rainbow trout (Oncorhynchus mykiss). In addition, the antihypotensive effect of the renin-angiotesin system (RAS) was investigated during exercise after alpha-adrenoceptor blockade. Fish were subjected to a 20-min exercise challenge at 0.66 body lengths s(-1) (BL s(-1)) while P(ven), dorsal aortic blood pressure (P(da)) and relative cardiac output (Q) was recorded continuously. Heart rate (f(H)), cardiac stroke volume (SV) and total systemic resistance (R(sys)) were derived from these variables. The mean circulatory filling pressure (MCFP) was measured at rest and at the end of the exercise challenge, to investigate potential exercise-mediated changes in venous capacitance. The protocol was repeated after alpha-adrenoceptor blockade with prazosin (1 mg kg(-1)M(b)) and again after additional blockade of angiotensin converting enzyme (ACE) with enalapril (1 mg kg(-1)M(b)). In untreated fish, exercise was associated with a rapid (within approx. 1-2 min) and sustained increase in Q and P(ven) associated with a significant increase in MCFP (0.17+/-0.02 kPa at rest to 0.27+/-0.02 kPa at the end of exercise). Prazosin treatment did not block the exercise-mediated increase in MCFP (0.25+/-0.04 kPa to 0.33+/-0.04 kPa at the end of exercise), but delayed the other cardiovascular responses to swimming such that Q and P(ven) did not increase significantly until around 10-13 min of exercise, suggesting that an endogenous humoral control mechanism had been activated. Subsequent enalapril treatment revealed that these delayed responses were in fact due to activation of the RAS, because resting P(da) and R(sys) were decreased further and essentially all cardiovascular changes during exercise were abolished. This study shows that the alpha-adrenergic system normally plays an important role in the control of venous function during exercise in rainbow trout. It is also the first study to suggest that the RAS may be an important modulator of venous pressure and capacitance in fish.

Increase by NG-nitro-L-arginine methyl ester (L-NAME) of resistance to venous return in rats

1. The effects of the nitric oxide (NO) synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME), on mean circulatory filling pressure (MCFP), total peripheral resistance (TPR), cardiac output (CO) and resistance to venous return (Rv) were studied in rats. 2. In conscious, unrestrained rats, L-NAME (0.5-16 mg kg-1) dose-dependently increased mean arterial pressure (MAP) but not MCFP, an inverse index of venous compliance, either in the absence or presence of the ganglionic blocker mecamylamine (10 mg kg-1). 3. In pentobarbitone-anaesthetized rats, L-NAME (2, 4, 8 mg kg-1) increased MAP and reduced CO in a dose-related manner but did not change MCFP, TPR (+84, +140 and +192%) as well as Rv (+62, +72, +110%) were dose-dependently increased by L-NAME. 4. Our results show that L-NAME reduces CO by increasing arterial as well as venous resistances. L-NAME does not affect MCFP.