hence, a decrease in blood pressure results in less sodium chloride present at the distal tubule, where the macula densa is located. the macula densa senses this drop in salt concentration and responds through two mechanisms, both of which are mediated by prostaglandin release. first, prostaglandins preferentially vasodilate the renal afferent arteriole, decreasing afferent arteriole resistance and, thus, offsetting the decrease in glomerular hydrostatic pressure caused by the drop in blood pressure. second, prostaglandin activates prostaglandin-sensitive specialized smooth muscle cells of the renal afferent arterioles, juxtaglomerular cells (jg cells), to release renin into the bloodstream. the jg cells can also release renin independently of the macula densa. there are stretch-sensitive baroreceptors lining the arterioles that will release renin if a fall in blood pressure (i.e. decreased stretch of arteriole due to less blood flow) in the arterioles is detected. furthermore, jg cells contain beta-1 adrenergic receptors, and so activation of the sympathetic nervous system will further stimulate renin release.