According to the myogenic mechanism of autoregulation, the rise in aerial pressure stretches the afferent aeriole. Stretch of vascular smooth muscle causes opening of membrane Ca2+ channels. This increases intracellular Ca2+, leading to contraction of the smooth muscle. NO and adenosine:are involved in the TG feedback. They are released by macula densa in response to changes in NaCl in tubular fluids. Norepinephrine:acting a1 receptors located mainly in the afferent aeriole, cause vasoconstriction. Dehydration and strong emotional stimuli (fear, pain) activates sympathetic nerves and reduce RBF and GFR. Prostaglandins (PGs): produced locally within the kidneys during pathophysiologic conditions increase RBF without changing the GFR.