Systemic and centrally mediated angiotensin II effects in the horse.

The research explores the effect of angiotensin II (AII) when infused intravenously to differentiate equine valvular insufficiency. The study finds that AII infusions can induce clear increases in systemic arterial blood pressure and could potentially aid in identifying mitral and tricuspid valvular dysfunction in horses.

Infusion of Angiotensin II and its Effects

• The study conducted AII infusions at varying doses for 10 minutes and studied their effects. It was observed that the systemic arterial blood pressure showed clear increases with the rise in dosage, indicating a dose-dependent effect of AII infusion. However, the pulmonary arterial blood pressure remained largely unaffected by these infusions.
• The experiment conclusion indicates that this response could be exploited for acoustic differentiation between mitral and tricuspid valvular dysfunction in horses, suggesting that AII infusion could be a valuable diagnostic tool.

Effects on Heart Rate and Thirst

• At certain doses and above (9 pmol kg-1 min-1), the AII infusions caused an increase in heart rate, also known as a chronotropic effect.
• This effect was not strictly dose-dependent, and significant tachyphylaxis (a rapid decrease in response to the drug) was noted.
• Interestingly, the AII infusion also triggered a thirst response in the horses, but only at supraphysiological blood concentration.

Impact on Plasma Aldosterone Concentration

• The researchers also measured the plasma aldosterone concentration (PA) after the AII infusions. It was revealed that the PA values were comparatively high in the morning before the experiment began.
• When multiple AII infusions were administered at 10-minute intervals and increasing dosages, the PA saw an almost fourfold increase, demonstrating cumulative effects.
• Interestingly, the PA pattern revealed that AII-induced secretion of aldosterone continued for a few minutes post-infusion, but the metabolic clearance of the hormone happened within 20 minutes after the infusion.
• In some isolated cases, a decrease in PA was witnessed during the final infusion, indicating aldosterone store depletion due to previous AII administration.

In conclusion, this study revealed promising results regarding the application of AII in differential diagnosis of equine valvular dysfunction, but also highlighted some important considerations regarding its effects on heart rate, thirst response, and plasma aldosterone concentration.