Renin is usually measured as the plasma renin activity ("PRA"). Patients with Bartter syndrome may also have elevated renin and aldosterone levels. People often have high blood pressure with a low renin and low aldosterone blood level. Renin and aldosterone can be tested in the blood. Renin inhibitors can therefore effectively reduce hypertension. Hence, a deficiency in vitamin D leads to an increase in renin secretion. The kidneys secrete a variety of hormones, including erythropoietin, calcitriol, and renin. Bisoprolol inhibits renin secretion by about 65% and tachycardia by about 30%. Pepstatin was the first synthetic renin inhibitor. Normally perfused kidneys produce a proportionate amount of a substance called renin. This factoring indicates secondary hypoaldosteronism (sodium low, potassium and renin enzyme will be low). This is measured by the plasma renin activity (PRA). Hopes of breakthrough appeared in 1982 when development of the second generation renin inhibitors began. Mineralocorticoid is only added in cases where the plasma renin activity is high. Angiotensinogen is also known as renin substrate.