VASOPRESSIN: The action of
vasopressin (AVP) is not limited to regulating water excretion but also plays an essential role in regulating the corticotropic axis during stress. PHYSIOLOGY:
Vasopressin is synthesized in the hypothalamus and stored in the posterior pituitary. It acts on 3 types of receptors (RV). RV1a are vascular receptors mediating the
vasoconstrictor and glycogenolytic effects of the
hormone. Anterior pituitary RV1b or V3 mediate stimulating effects on the corticotropic axis. Renal RV2 regulate water and
urea excretion. Hypothetical extrarenal RV2 would be responsible for the
vasodilator and procoagulant effects of the
hormone. MODIFICATIONS IN
ENDOCRINE DISEASES: Pituitary or adrenal hypocorticism syndromes include
hyponatremia with secondary plasma hypoosmolality and reduced glomerular filtration due to the direct effect of
glucocorticoids and also the effect of vasopressinism. Certain endogenous hypercorticisms appear to be related to an overexpression of RV: in
ACTH-independent Cushingís syndrome, adrenal overexpression of eutopic RV1a, and in
ACTH-dependent Cushingís syndrome, pituitary overexpression of eutopic RV1b or ectopic RV2. In addition, inappropriate secretion of
antidiuretic hormone is frequent after transphenoidal surgery, particularly for corticotropic
adenomas. DYNAMIC TESTS: The physiological response of
ACTH and also AVP to
corticotropin releasing hormone (CRH) in the petrous sinus, the unusual responses of certain corticotropic
tumors to
dDAVP, or certain forms of
ACTH-independent
hypercortisolism to
lysine vasopressin (LVP) suggest excessive or ectopic expression of RV in corticotropic or adrenal cells: tumorgenesis of these cells could also depend, at least partially, on AVP.