[Hormones of the posterior region of the hypophyseal gland]. 
Arginine vasopressin is the main hormone involved in the regulation of body fluid osmolality.  The hormone is released by the posterior pituitary whenever water deprivation causes an increased plasma osmolality or whenever the cardiovascular system is challenged by hypovolaemia and/or hypotension.  The main site of action of this hormone is the renal collecting duct, but vasopressin is also a potent vasopressor and neurotransmitter, it has a role in the secretion of corticotrophin, in the regulation of the cardiovascular system, temperature and other visceral functions.  Vasopressin also promotes the release of coagulation factors by vascular endothelium and increases platelet aggregability.  In addition to its classical contractile effect on uterine myometrial and mammary glandular myoepithelial cells, oxytocin acts as neurotransmitter, stimulates endometrial prostaglandin production, pituitary prolactin secretion, luteolysis, sperm transport and natriuresis, and may play a role in immune function.  Sensorial stimuli arising from the cervix and vagina as well as stimulation of the breast can induce secretion of oxytocin from the posterior pituitary.  There are many vasopressin and oxytocin analogues (agonists and antagonists) that are synthetized with the goal of increasing duration of action and selectivity for the receptor subtypes, while non-peptide antagonists are orally active.  The oxytocin and the vasopressin V1a, V1b and V2 receptors have recently been cloned and shown to form a sub-family within the large superfamily of G-protein-linked receptors.  Renal V2 receptors mediate vasopressin-induced water reabsorption via induction of intracellular cAMP production in collecting duct cells.  Most remaining actions of vasopressin on blood vessel constriction, liver glycogenolysis, platelet adhesion, adrenal angiotensin II secretion and certain brain functions are mediated via V1a-type receptors that are coupled to a Gq/11 protein.  V1 receptor activation leads to stimulation of phospholipases C, D and A2, and an increase in intracellular calcium.  Vasopressin stimulates pituitary corticotrophin release via a third vasopressin receptor type (V1b) which is present in corticotrophs.  Oxytocin induces myometrial contraction, endometrial prostaglandin F2a production, mammary gland milk ejection, renal natriuresis and specific sexual, affilitative and maternal behaviours via oxytocin receptors which are also coupled to a G1/11 protein.  Although only one oxytocin receptor type has been cloned so far, recent binding studies indicate that uterine endometrial oxytocin receptors may constitute a distinct receptor subtype.  Expression of oxytocin receptors have relevant up- and down-regulation by oestrogens and progesterone.