Generality
Renin is a proteolytic enzyme synthesized by the kidney. This protein is released in response to physiological stimuli, such as: decreased blood volume, hypotension and decreased blood sodium and potassium concentrations.
The blood renin assay, together with the aldosterone measurement, is useful in the differential diagnosis of primary and secondary aldosteronism.
What's this
Renin is an enzyme released by the kidney when blood pressure is too low, and the organ is thus in ischemic conditions.
Being a proteolytic enzyme, renin is able to break down peptide bonds present within other protein molecules; more precisely, it acts on angiotensinogen, an alpha2-globulin synthesized by the liver and normally present in the blood.
By the intervention of renin, angiotensinogen is converted into angiotensin I. In turn, this peptide undergoes a further enzymatic conversion giving rise to angiotensin II (thanks to the intervention of the conversion enzyme - ACE).
Angiotensin I and angiotensin II are vasoactive molecules with hypertensive activity. They are therefore able to raise blood pressure, with a hypertensive action that reaches - as far as angiotensin is concerned - a strength about 200 times greater than that of noradrenaline.This result underlies various biological mechanisms, ranging from the increase in cardiac contraction force to the greater release of aldosterone, passing through a marked vasoconstrictor activity at the arteriolar level.
Aldosterone, produced by the adrenal cortex, stimulates sodium reabsorption thereby increasing plasma volume, blood pressure and renal excretion of potassium.
Renin is produced by a group of particular kidney cells belonging to the so-called juxtaglomerular apparatus, therefore located in the walls of the arterioles afferent to the kidney.
Renin is not produced and secreted as such, but in the form of a precursor, pro-renin, which is much more abundant in the bloodstream.
According to the laws of homeostasis, the release of renin is stimulated by conditions of:
- Hypotension (low blood pressure values);
- Hypovolemia (reduced blood volume);
- Hyponatremia (reduced concentrations of sodium in the blood);
- Hyperkalaemia (high concentrations of potassium in the blood).
The release of renin is instead inhibited in the diametrically opposite conditions.
More information on the renin-angiotensin system is presented in this article.
Because it is measured
Plasma renin levels can be evaluated to investigate the origin of particular hypertensive forms, particularly when blood potassium levels are low.
Often, the dosage of renin is combined with that of aldosterone, since - as explained in the previous paragraph - low renin levels associated with high levels of aldosterone, or vice versa, may indicate the presence of a pathological condition (in healthy subjects when increases renin also increases aldosterone, and vice versa).
Aldosterone levels can also be measured in urine collected over 24 hours, thus benefiting from greater standardization (aldosteronemia being influenced by the time of day and the body position assumed - standing or supine).
Normal values
Under normal conditions, the following blood levels of renin are found:
- Orthostatism: 4.4 - 46.1 µIU / mL
- Clinostatism: 2.8 - 39.9 µIU / mL
Note: the reference range of the test may change according to age, gender and equipment used in the testing laboratory. For this reason, it is preferable to consult the ranges reported directly on the report. It should also be remembered that the results of the analyzes must be evaluated as a whole by the general practitioner who knows the patient's medical history.
Renina Alta - Causes
High blood renin can be the consequence of:
- Kidney disease;
- Obstruction of the arteries carrying blood to one or both kidneys (see renal artery stenosis);
- Addison's disease;
- Cirrhosis of the liver;
- Hemorrhage;
- Renal and extrarenal tumors secreting renin;
- Malignant hypertension;
- Congestive heart failure
- Bartter's syndrome (high renin levels without hypertension).
High levels of renin are also recorded in the case of:
- Dehydration;
- Hypokalemia;
- Low sodium diet
- Loss of salts following gastrointestinal diseases (eg diarrhea);
- Pregnancy;
- Taking contraceptive drugs containing estrogen or diuretics.
Low Renin - Causes
Low blood renin can be the result of:
- Conn syndrome;
- Cushing's syndrome;
- Vasopressin (ADH) therapy or sodium-retaining drugs.
How it is measured
To measure renin, the patient must have blood drawn from the arm; sometimes, a sample is collected from a renal or adrenal vein. The test can also be performed on 24-hour urine.
Preparation
Before undergoing the renin test, it is necessary to fast for at least 8 hours, during which a moderate amount of water is allowed.
For the measurement on blood, the doctor may ask the patient to stand upright for at least one "hour (renin in standing position) or supine for at least 2 hours (renin in supine position).
Starting from two - four weeks before the collection, it is necessary to adopt dietary-behavioral measures to improve the diagnostic reliability of the test. In particular, at least two weeks before it is necessary to stop taking anti-hypertensive drugs (diuretics, beta blockers, ACE inhibitors), steroids, progestogens and estrogens.
At the same time, the sources of caffeine and licorice must be removed from one's diet, while the sodium intake must be kept at reasonable levels and as constant as possible. A special diet, low in sodium, can be prescribed in the three days preceding the test, usually performed under fasting conditions for eight hours.
Interpretation of Results
The table below indicates how the changes in renin, aldosterone and cortisol vary according to the different pathologies (source Lab Test Online), indicating the need for a joint dosage in the face of a clinical suspicion.
