ADH acts on -

Ans. is 'c' i.e.. Collecting ductAnti diuretic hormone (ADH)o ADH (vasopressin) is a nonapeptide secreted by posterior pitutary (neurohypophysis) along with oxytocin,o It is synthesized in hypothalamus (Supraoptic and paraventricular area) nerve cell bodies as large precursor peptide along with its binding protein neurophysin and is transported down the axons to nerve endings in the median eminence and pars nervosa.o The two main physiological stimuli for ADH release are rise in plasma osmolarity and contraction of ECF volume.o Other factors which can influence ADH secretion are:Inhibit ADH secretion: Cold environemnt. ethyl alcoholStimulate ADH secretion: Pain, exercise, stress, sleep and drugs like morphine ADH (Vasopressin) receptorsV1 receptorsV2 receptorsV1aV1b (also called V3)o Kidney - | water permeabilityo Blood vessels - vasodilatationo Release of vWF & Factor VIII from endotheliumo Blood vessels - vasoconstrictiono Visceral smooth muscle -contractiono Liver - glycogeno lysiso Platelet - aggregationAnterior pituitary- release of ACTHo V2 receptors are more sensitive to ADH than V1 receptors.o Mechanism of action in kidney - Activation of V, receptors increase cAMP formation intracellularly - activation of cAMP dependent protein kinase A - Exocytosis of aquaporin -2 water channel to apical membrane - | water permeability. V2 receptor activation also increases permeability of CD to urea by stimulating the urea transporter.The characteristic feature of cells lining the collecting duct is their responsivenessto Antidiuretic hormone (ADH).If ADH is presentIf ADH is absentCollecting duct cells become permeable to water.Concentrated urine is passed.Collecting duct cells are impermeable to water.Hypotonic fluid which enters the collecting duct is passed as such (Dilute urine).