Impaired neuronal and vascular responses to angiotensin II in a rabbit congestive heart failure model

Selective AT1 Receptor Antagonism Enhances Sympathetically Mediated Vasoconstriction in Man

The vasoconstrictive action of angiotensin II (AII) is partly, sympathetically mediated and angiotensin-converting enzyme (ACE) inhibitors appear to exert a sympatholytic effect. We examine the effect of an orally administered, selective AT(1) receptor antagonist (losartan 50 mg) on sympathetically mediated vasoconstriction in healthy volunteers in an observer blind crossover study. Seven healthy, normotensive volunteers (21-32 years), were studied on two occasions at the end of each 6-week treatment period (losartan or placebo). Forearm blood flow (FABF) (ml/dl forearm/min) was measured by venous occlusion plethysmography during the application of lower body negative pressure (LBNP) (-20 cm H(2)O) and at the end of each incremental infusion of norepinephrine (60, 120, and 240 pmol/min). Comparison of blood flow changes was by repeated measures analysis of variance; P<0.05 was taken as statistically significant. Losartan did not alter blood pressure compared to placebo. It did significantly enhance LBNP-induced vasoconstriction in both the left arm compared to placebo (-36.6+/-3.4 vs -23.5+/-3.3%; P=0.017) and the right arm compared to placebo (-39.5+/-3.8 vs -21.0+/-3.6%; P=0.005). The FABF response to all doses of infused norepinephrine (60, 120, and 240 pmol/min) was also enhanced by losartan compared to placebo (-35.0+/-2.7 vs -18.2+/-6.0%; -43.6+/-4.3 vs -28.6+/-5.8%, and -53.9+/-3.2 vs -42.5+/-6.8%; P=0.057, respectively. Losartan enhances locally mediated sympathetic vasoconstriction in the forearm circulation of man, probably through its effect on circulating AII concentrations and we postulate that the adrenergic sympathetic constrictor action of AII is not mediated by the AT(1) receptor or is surmountable at this receptor.

AT1-receptor blockade and sympathetic neurotransmission in cardiovascular disease

1. The present survey is dealing with the interactions between the renin-angiotensin-aldosterone system (RAAS) and the sympathetic nervous system (SNS) in various organs and tissues, with an emphasis on the angiotensin AT-receptors located at the sympathetic nerve endings. 2. Angiotensin II, the main effector of the RAAS is known to stimulate sympathetic nerve traffic and its sequelae in numerous organs and tissues, such as the central nervous system, the adrenal medulla, the sympathetic ganglia and the sympathetic nerve endings. These stimulatory effects are mediated by AT(1)-receptors and counteracted by AT(1)-receptor antagonists. 3. Sympatho-inhibition at the level of the sympathetic nerve ending appears to be a class effect of the AT(1)-receptor blockers, mediated by presynaptic AT(1)-receptors. With respect to the ratio pre-/postsynaptic AT(1)-receptor antagonism important quantitative differences between the various compounds were found. 4. Both the pre- and postjunctional receptors at the sympathetic nerve endings belong to the AT(1)-receptor population. However, the presynaptic receptors belong to the AT(1B)-subtype, whereas the postjunctional receptors probably belong to a different AT(1)-receptor subpopulation. 5. Sympatho-inhibition is a class effect of the AT(1)-receptor antagonists. In conditions in which the SNS plays a pathophysiological role, such as hypertension and congestive heart failure, this property may well be of therapeutic relevance.