Serum angiotensin converting enzyme activity and response to angiotensin I in horses.

This study investigates serum angiotensin converting enzyme (ACE) activity in horses and the blood pressure response following administration of Angiotensin I. The research indicates that the activity of ACE in healthy horses can be reduced through inhibitors such as enalapril, captopril, and enalaprilat, and these inhibitors also dampen the pressor response to Angiotensin I dosing.

Understanding Serum Angiotensin Converting Enzyme Activity

• Serum angiotensin converting enzyme (ACE) is an enzyme that plays a crucial role in the renin-angiotensin system, which regulates blood pressure and fluid balance in the body. High activity levels of this enzyme can lead to elevated blood pressure.
• The study establishes the average ACE activity for healthy horses as 64 +/- 13 mUnits/ml, which serves as the baseline level against which the activity of ACE inhibitors is evaluated.

Sensitivity to ACE Inhibitors

• Enalapril, captopril, and enalaprilat are substances known to inhibit ACE, thereby reducing its activity. In this study, the horses’ serum ACE showed sensitivity to these inhibitors with varying levels of IC50. The IC50 is a measure of the effectiveness of a substance in inhibiting a specific biological or biochemical function. The lower the IC50 value, the less amount of a drug is needed to inhibit half of the enzyme activity, making it more effective.
• Enalaprilat, with an IC50 of just 6 nM (2.1 ng/ml), is the most potent inhibitor of the three substances tested.

Angiotensin I Injection and Blood Pressure Response

• Angiotensin I is a hormone that can increase blood pressure. The intravenous administration of angiotensin I to the healthy horses caused a dose-proportional rise in blood pressure. At the highest dose of 500 ng/kg body weight, the average increased in arterial pressure was 65.6 mmHg over baseline levels.

The Role of Enalaprilat in Attenuating Pressor Response

• The study further established the role of enalaprilat in mitigating the blood pressure response to Angiotensin I. When a single horse was administered enalaprilat prior to an Angiotensin I injection, the response showed apparent attenuation. This supports the notion that ACE inhibitor drugs such as enalaprilat can effectively reduce the blood pressure response, and thus potentially control instances of elevated blood pressure in horses.