Q. What triggers renin release? I know that it’s hypo-filtration of the juxtaglomerular apparatus (i.e. reduced fluid flow) but what is the actual trigger for renin release?

A. There are actually three different types of cells in the JGA: granular cells (which secrete renin), macula densa cells (which secrete a vasopressive substance that acts on the afferent glomerular arteriole) and mesangial cells (whose function is a little obscure).

Granular cells (also called juxtaglomerular cells) are little connective tissue cells surrounding glomerular arterioles. They secrete renin in response to three things: 1) beta-adrenergic stimulation, 2) decreased renal perfusion pressure (which is detected by the granular cells themselves) and 3) signals from the macula densa.

Macula densa cells are specialized cells within the wall of the distal tubule. They sense the salt content and the volume of fluid within the tubule (which is related to the glomerular filtration rate). If the salt content is high, or the volume of fluid is low, the macula densa cells do two things: 1) tell the granular cells to release renin, and 2) dilate the afferent arteriole of the glomerulus (by secreting a little vasopressive hormone).

Mesangial cells come in two types: those within the glomerulus (which provide structural support for the capillaries, and also secrete some hormones, like erythropoietin), and those outside the glomerulus (which are considered part of the juxtaglomerular apparatus). The two types are connected by gap junctions. The function of the extraglomerular cells is a bit obscure. They contract when they are stimulated by sympathetic nerves. In addition, they are located in between the macula densa and the afferent arteriole, so maybe they help these two structures communicate.