The major physiological function of vasopressin is the concentration of urine. The hormone binds to a specific receptor (V2 receptor) on the contraluminal aspect of the tubular cells of the distal collecting duct of the nephron; its gene is on chromosome X, Xq28. Adenyl cyclase is activated to generate cAMP, which in turn stimulates a protein kinase, resulting in rearrangement of intracellular microtubule- and microfilaments. Consequently, luminal tubular pores (water channels) open to allow hypotonic fluid in the nephron to flow across the cell into the hypertonic renal interstitium, thus concentrating the urine As the concentration of plasma vasopressin rises, a reduction in urine flow rate and an increase in urinary osmolality (Fig. 5.13) occur. Maximum antidiuresis occurs at plasma vasopressin concentrations of the order of 4pmol/l, while maximum diuresis in humans occurs with values of 0.5pmol/l or less.
