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A juxtaglomerular (JG) cell, located in the kidneys, normally creates, stores, and secretes an enzyme called renin, which helps to regulate potassium and sodium levels in the blood. It is typically a form of smooth muscle cell. Positioned along the edge of small arteries in the juxtaglomerular apparatus, a small structure which controls the function of filtering elements called nephrons, each juxtaglomerular cell can also sense blood pressure. It is usually one of three types of cells in the JG apparatus.
Various types of proteins and their receptors are often found in a juxtaglomerular cell. This kind of cell can also express a protein based on tissue type, or based on whether a particular cell is mature, embryonic, active, or non-active. Sometimes JG cells behave as precursors to other types and, if conditions are right, can turn into components of smooth muscle in the human body.
Also called granular cells, these specialized types can secrete renin when stimulated by certain hormones. Pressure in the kidneys can also trigger secretion, as well as a decrease in the absorption of sodium chloride. A juxtaglomerular cell typically takes on a granulated structure as the enzyme is released, which normally occurs when there is a decrease in blood pressure. If renin is secreted, blood pressure usually increases. Blood pressure is generally regulated by the renin angiotensin aldosterone system, named for the substances which interact in a process that involves the kidneys, adrenal and pituitary glands, lungs, and the liver.
The juxtaglomerular apparatus also includes macula densa cells, which are typically located in a small tube and can sense changes in the concentration of dissolved compounds. These can also sense flow rate inside the small tubules of the kidney. Mesangial cells usually provide connections to small capillaries, and control the constriction and dilation of these small blood vessels by contracting. They generally contain substances called actin and myosin which enable these contractions to occur in response to stimulation by nerve signals.
Research on mice, with a human gene for renin, shows that the enzyme is usually produced and secreted only by a juxtaglomerular cell. If the JG cells are missing or do not work properly, blood pressure tends to be low and kidneys, as monitored in mice, can be smaller than normal as well. These cells, therefore, are generally necessary for the kidneys to function normally. Serious kidney diseases and conditions can result if the activity of the juxtaglomerular cell is abnormal.