Endocrine effects of vasopressin in critically ill patients

Vasopressin, also called antidiuretic hormone, is a 9 amino-acid peptide, synthesized from a precursor containing neurophysin II, by neurones from the supra-optic and peri-ventricular nuclei, and then stored in the posterior hypophysis. Vasopressin regulates plasmatic osmolality and volaemia via V2 receptors at the levels of the kidney, and vascular smooth muscle tone via V1a arterial receptors. Both its synthesis and release from hypophysis vesicles depend on variations in plasma osmolality, volaemia, and arterial blood pressure. In addition, vasopressin interacts with the main hormonal systems involved in the response to stress, including the hypothalamic-pituitary adrenal axis, other anterior pituitary hormones, mainly prolactin and growth hormone, the renin-angiotensin system, and regulates insulin synthesis and glucose metabolism. Interestingly, during critical illness, exogenous administration of vasopressin showed little effects on the circulating levels of these various hormones, except an increase in prolactin. The absence of endocrine effects of vasopressin during critical illness is unclear and may relate to an already maximal hormonal stimulation or to down-regulation of vasopressin receptors.