Plasma renin activity

Definition

Renin is an enzyme released by the kidneys to help control the body's sodium-potassium balance, fluid volume, and blood pressure . Renin splits angiotensinogen in plasma, forming angiotensin I. This compound is acted upon by plasma-converting enzymes produced in the lungs to form antiotensin II or III. These powerful vasoconstrictors increase blood pressure and stimulate aldosterone release by the adrenal cortex.

Purpose

Plasma renin activity (PRA), also called plasma renin assay, is used to investigate the cause of hypertension . PRA is increased in persons with hypertension of renal origin. It is used to classify persons with essential hypertension. A PRA test, along with a measurement of the plasma aldosterone level, is used for the differential diagnosis of primary and secondary aldosteronism. The latter condition is caused by increased renin release by the kidney(s), and therefore, elevated PRA. Patients with primary hyperaldosteronism (caused by an adrenal tumor that overproduces aldosterone) will have an increased aldosterone level with decreased renin activity.

Precautions

Patients taking diuretics, antihypertensives, vasodilators, oral contraceptives, and licorice should discontinue use of these substances for two to four weeks before the test. It should be noted that renin activity is increased in pregnancy and in diets with reduced salt intake. Also, since renin is affected by body position, as well as by diurnal variation, blood samples should be drawn in the morning, and the position of the patient (sitting or lying down) should be noted.

The nurse or plebotomist performing the venipuncture should follow universal precautions for the prevention of transmission of bloodborne pathogens. Blood should be collected in EDTA in a chilled syringe, and the sample immediately placed on ice. The plasma should be separated from the cells immediately and then frozen until assay.

Description

The kidneys normally release renin in response to decreased blood volume, low plasma sodium, and high plasma potassium levels. The release of renin is the first step in the renin-angiotensin-aldosterone cycle. Renin is produced and secreted by specialized cells called juxta-glomerular cells, located at the junction where the distal tubule meets the afferent and efferent arterioles. These cells secrete renin in response to a decreased flow of blood through the afferent arteriole. Renin is a proteolytic enzyme; it splits angiotensinogen in the plasma forming angiotensin I, which in turn is converted to angiotensin II or III by a converting enzyme produced by the lungs. Angiotensins II and III are powerful blood vessel constrictors. In addition, they stimulate the release of aldosterone from the cortex of the adrenal glands . Aldosterone causes increased sodium reabsorption by the kidneys. As sodium is reabsorbed, the osmotic pressure (osmolality) of the plasma rises, and this rise stimulates osmoreceptors in the central nervous system . These cause secretion of antidiuretic hormone (vasopressin) from the posterior pituitary gland . Vasopressin causes more water to be reabsorbed by the kidney. Reabsorption increases blood volume and restores the blood pressure in the afferent arterioles reducing renin release. Together, angiotensin and aldosterone increase the blood volume, the blood pressure, and the blood sodium to re-establish the body's sodium-potassium and fluid volume balance.

High blood pressure affects about 20 million people in the United States and is a major risk factor for cardiovascular disease and stroke. More than 90% of hypertension is due to essential (primary) hypertension. This form of high blood pressure is genetic and its causes are unknown. Essential hypertension is aggravated by excess sodium intake, and affected individuals vary in their response to treatment. Plasma renin activity can be used to classify persons with essential hypertension into groups (high, normal, or low PRA) that respond differently to treatment. For instance, low PRA-type essential hypertension results from excessive aldosterone secretion (primary aldosteronism), and is effectively treated by diuretic therapy.

Measurement

Renin itself is not actually measured in the PRA test. There are two forms of this test. The classic test is called the plasma renin activity (PRA) test, and the newer form is called the plasma renin concentration (PRC) test. Both tests measure the conversion of angiotensinogen to angiotensin I by renin. The difference between the two is that the former uses endogenous (i.e., the patient's own) plasma angiotensinogen as the substrate, while the latter uses excess angiotensinogen from an exogenous source such as sheep plasma. To perform the activity test, the plasma is thawed and a small measured amount is added to a buffered solution at pH 6 that contains phenylmethyl sulfonyl fluoride to inhibit plasma angiotensinases that also split angiotensinogen. The mixture is incubated for one hour at 37°C and then refrigerated to stop the enzyme activity. Following this, the angiotensin I produced is measured by radioimmunoassay (RIA). To correct for the endogenous angiotensin I present beforehand, an equal amount of the thawed sample is kept at 4°C and then measured for angiotensin I activity by RIA. This value is subtracted from the test result obtained from the 37°C incubation. The plasma renin concentration test is performed as described above, except that the test plasma is first treated with an acid buffer to destroy the endogenous angiotensinogen. A measured volume of the treated plasma is added to a buffered solution containing an excess amount of exogenous angiotensinogen. The amount of angiotensin I produced following the incubation is equivalent to the maximum rate of enzyme activity, and is a more accurate reflection of renin concentration because it is independent of the plasma angiotensinogen

Both the PRA and the PRC are extremely difficult to perform. Not only is renin itself unstable, but the patient's body position and the time the specimen is collected affect the results. Also, the sample must be collected properly: drawn into a chilled syringe and collection tube, placed on ice, and sent to the laboratory immediately. Even when all these procedures are followed, results can vary significantly. An alternative method is the measurement of plasma renin mass by double antibody sandwich immunoassay. This assay uses two monoclonal antibodies, one that binds to the prorenin molecule and a second that binds to renin. This assay detects only active renin because the inactive enzyme is not bound by the second antibody. The mass unit assay is independent of angiotensinogen and therefore not as subject to procedural errors related to temperature inactivation of the enzyme.

Renin stimulation tests

A renin stimulation test is performed to help diagnose and distinguish primary from secondary aldosteronism. The test protocol involves either stimulating salt loss by administration of furosemide, a diuretic; or restricting the patient's salt intake for three to five days. A low sodium level and standing posture stimulate renin release in normal persons, resulting in a two to three fold increase in PRA. Persons with secondary aldosteronism (renin-mediated aldosteronism) typically show a fivefold increase in PRA. Persons with primary aldosteronism show no increase in PRA over the baseline.

One example of a stimulation test is performed as follows. With the patient having been on a low-salt diet and lying down for the test, a blood sample for PRA is obtained. The PRA is repeated with the patient still on the low-salt diet, but now standing upright for two or more hours. In cases of primary hyperaldosteronism, the blood volume is greatly expanded, and a change in position or reduced salt intake does not result in decreased renal blood flow or decreased blood sodium. As a result, renin levels do not increase. However, in secondary hyperaldosteronism, blood sodium levels decrease with a lowered salt intake, and when the patient is standing upright,

KEY TERMS

Aldosteronism —A disorder caused by excessive production of the hormone aldosterone, which is produced by a part of the adrenal glands called the adrenal cortex. Causes include a tumor of the adrenal gland (Conn's syndrome), or a disorder reducing the blood flow through the kidney. This leads to overproduction of renin and angiotensin, and in turn causes excessive aldosterone production. Symptoms include hypertension, impaired kidney function, thirst, and muscle weakness.

Conn's syndrome —A disorder caused by excessive aldosterone secretion by a benign tumor of one of the adrenal glands. This hypersecretion results in malfunction of the body's salt and water balance and subsequently causes hypertension. Symptoms include thirst, muscle weakness, and excessive urination.

the renal blood flow decreases as well. Consequently, renin levels will increase.

Captopril suppression test

The captopril test is a screening test for renovascular hypertension. This is a common form of secondary aldosteronism. For this test, a baseline PRA test is measured; then the patient receives an oral dose of captopril, an angiotensin-converting enzyme (ACE) inhibitor. Blood pressure measurements are taken at this time and again at 60 minutes, when another PRA test is done. Patients with kidney-based hypertension demonstrate greater falls in blood pressure and increases in PRA after captopril administration than do those with essential hypertension. Consequently, the captopril test is an excellent screening procedure to determine the need for a more invasive radiographic evaluation such as renal arteriography.

Preparation

This test requires a blood sample. For the PRA, the patient should maintain a normal diet with a normal amount of sodium (approximately 3 g per day) for three days before the test, unless specified otherwise as for a stimulation test. It is recommended that the patient be fasting (nothing to eat or drink) from midnight on the day of the test.

Aftercare

Discomfort or bruising may occur at the puncture site. Applying pressure to the puncture site until the bleeding stops helps to reduce bruising; warm packs relieve discomfort. Some people feel dizzy or faint after blood has been drawn and should be treated accordingly.

Complications

Other than potential bruising at the puncture site, and/or dizziness, there are no complications associated with this test.

Results

Reference values for the PRA test are laboratory-specific, and depend upon the patient's diet (sodium restricted or normal), the age of the patient, and the patient's posture at the time of the test. Values are also affected if renin has been stimulated or if the patient has received an ACE inhibitor like captopril. A representative normal range for the PRA test in adults on a normal diet is 0.2-3.3 nanograms angioitensin I per mL per hour. For the monoclonal double antibody sandwich assay (direct renin assay), the normal range is 7-76 U/mL for persons not lying down.

Increased PRA levels are seen in up to 15% of persons with essential hypertension (associated with renal injury or vascular disease), malignant hypertension, and kidney-based (renovascular) hypertension. Renin-producing renal and other tumors, while rare, can also cause elevated levels; as can cirrhosis, low blood volume due to hemorrhage, and diminished adrenal function (Addison's disease). Decreased renin levels may indicate increased blood volume due to a high-sodium diet, salt-retaining steroids, primary aldosteronism, or licorice ingestion syndrome. About 25% of persons with essential hypertension will have low renin levels.

Health care team roles

Physicians order PRA tests and interpret the results. A nurse or phlebotomist usually collects the blood and is responsible for icing the sample and transporting to the laboratory. Clinical laboratory scientists/medical technologists perform the renin (and angiotensin I) tests.