Physiological Role of the Neurohypophysis
The hormones of the posterior pituitary are not synthesized within the gland itself. The cell bodies of the neurons producing these peptides are located in the supraoptic and paraventricular nuclei of the hypothalamus. The substances travel down axons forming the hypothalamo-hypophysial tract into the neurohypophysis. There, they are stored in enlarged terminals and secreted directly into the capillary network upon stimulation.
The two main hormones of this system are oxytocin and vasopressin. Structurally, they are cyclic nonapeptides (containing 9 amino acid residues), whose conformation is maintained by a disulfide bridge between cysteine residues.
Oxytocin and Demoxytocin
Oxytocin (Oxytocinum) is a synthetic analogue of the natural hormone. Its molecule consists of a pentapeptide ring and a three-amino-acid side chain.
The drug exhibits a pronounced uterotonic effect: it stimulates specific myometrial receptors, increasing the tone and strength of uterine contractions. The density of these receptors increases as pregnancy progresses, peaking before labor and in the postpartum period. The drug also has a lactogenic action—it contracts the myoepithelial cells of the mammary gland alveoli, forcing milk into the ducts. It is administered intravenously or intramuscularly.
Demoxytocin (Demoxytocinum) is a modified version with a deaminated cysteine at position 1. This modification makes the molecule resistant to peptidases, allowing the drug to be administered orally or buccally (transbuccally). The main clinical difference is the complete absence of a vasopressin-like (antidiuretic) effect, allowing it to be used safely in patients with arterial hypertension.
Desmopressin and Vasopressin Receptors
Desmopressin (Desmopressinum) is a synthetic substitute for vasopressin. It differs from the natural hormone in two details: the absence of an amino group at the N-terminus and the substitution of L-leucine with D-arginine. This increases resistance to enzymes and alters receptor selectivity. The effect lasts 8–20 hours.
The action is mediated through two types of receptors:
- V₂-receptors (renal): located in the distal segments of the nephron. Through adenylate cyclase activation, they trigger the synthesis of aquaporin proteins. These "water pores" are inserted into the cell membrane, enhancing water reabsorption (antidiuretic action).
- V₁a-receptors (vascular): located in arterial smooth muscle. They are coupled to phospholipase C, increase intracellular calcium levels, and cause vasoconstriction, raising blood pressure.
- Furthermore, stimulation of V₂-receptors increases clotting factor VIII activity, while V₁ receptors stimulate platelet aggregation. The drug is administered parenterally or intranasally because it is completely degraded in the GI tract.
Animal-Derived Preparations
Historically, the first group of drugs consisted of extracts from the posterior pituitary of cattle and pigs. Today they are used less frequently, but they remain an important part of the classification:
- Adiurecrin (Adiurekrinum): a dry lyophilisate containing vasopressin. It is used as nasal drops for diabetes insipidus and nocturnal enuresis. It acts for up to 6 hours.
- Pituitrin and Hypofysin: combined agents containing both oxytocin and vasopressin. Hypofysin features a higher degree of purification and a reduced vasopressin content. In obstetrics, they are used to stimulate labor and stop postpartum hemorrhage.