Systemic and centrally mediated angiotensin II effects in the horse

The primary aim of the study was to evaluate the potential value of intravenous (i.v.) infusion of angiotensin II (AII) for phonocardiographic differential diagnosis of equine valvular insufficiency. Ten-minute AII infusions at 4.5-33 pmol kg-1 min-1 induced clear-cut dose-dependent rises in systemic arterial blood pressure (aBP), whereas the pulmonary aBP remained largely unaffected. It implies that i.v. infusion of AII at about 10 pmol kg-1 min-1 could be a valuable tool for the acoustic differentiation between mitral and tricuspid valvular dysfunction in the horse. The infusion at, and above 9 pmol kg-1 min-1 caused increased heart rate. This chronotrophic effect was not strictly dose-dependent and exhibited significant tachyphylaxis. Angiotensin II administration at, or above 9 pmol kg-1 min-1 was needed to induce an urge to drink, suggesting that angiotensin-induced thirst does not appear in the euhydrated horse until the octapeptide reaches supraphysiological blood concentration. Determinations of plasma aldosterone concentration (PA) revealed comparatively high morning control values (269 +/- 46 pmol-1). Three consecutive AII infusions with 10-min intervals and at increasing dosages caused a cumulative, almost fourfold elevation of PA. The PA pattern indicated that AII-induced hypersecretion of aldosterone continued for several minutes after the end of the infusions, but also showed that the metabolic clearance of the hormone took precedency of the secretion within 20 min post-infusion. In two of the horses a fall in PA occurred during a fourth, final infusion, indicating that in these instances the previous AII administration had impoverished the store of aldosterone available for release from the adrenal cortex.