Physiological role of the neurohypophysis
Posterior pituitary hormones 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. Through axons forming the hypothalamo-neurohypophysial tract, the substances travel down to the neurohypophysis. There they are stored in swollen axon terminals and secreted directly into the capillary network when needed.
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 effect, contracting myoepithelial cells of the mammary alveoli and expressing milk into the ducts. It is administered intravenously or intramuscularly.
Demoxytocin (Demoxytocinum) is a modified version with deaminated cysteine at position 1. This modification makes the molecule resistant to peptidases, allowing the drug to be administered orally or buccally (cheek pouch). The main clinical difference is the complete absence of a vasopressin-like (antidiuretic) effect, making it safe to use 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 enzymatic stability 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 parts of the nephron. Through adenylate cyclase activation, they trigger the synthesis of aquaporin proteins. These "water pores" are embedded into the cell membrane, enhancing water reabsorption (antidiuretic effect).
- V₁a receptors (vascular): located in arterial smooth muscle. They are coupled to phospholipase C, increase intracellular calcium levels, and cause vasoconstriction, thereby raising blood pressure.
Additionally, stimulation of V₂ receptors increases the activity of blood clotting factor VIII, while V₁ receptors increase platelet aggregation. The drug is administered parenterally or intranasally, as it is completely degraded in the gastrointestinal tract.
Animal-derived preparations
Historically, the first group of drugs consisted of extracts from the posterior pituitary of cattle and swine. Today they are used less frequently, but they remain an important part of the classification:
- Adiurekrin (Adiurekrinum): a dry lyophilisate containing vasopressin. Used as nasal drops for diabetes insipidus and nocturnal enuresis. Acts for up to 6 hours.
- Pituitrin and Hyphotocin: combination drugs containing both oxytocin and vasopressin. Hyphotocin features a higher degree of purification and a reduced vasopressin content. In obstetrics, they are used to induce labor and arrest postpartum hemorrhage.
Briefly: answers to topical questions
What are the indications for the clinical use of oxytocin?
- Labor induction (Oxytocinum) — used only when the cervix is fully dilated or in combination with cervical-relaxing agents.
- Management of postpartum hemorrhage (Oxytocinum) — for uterine atony and subinvolution.
- Lactation stimulation (Oxytocinum) — to facilitate milk ejection in the early postpartum period.
To avoid confusing vasopressin receptors, use this visual association: V1 — vessels (the number 1 resembles a straight blood vessel, blood pressure regulation), V2 — kidneys (a person has two kidneys, regulation of H₂O reabsorption).
Class and exam questions
Why is oxytocin used only when the cervix is dilated?
Oxytocin powerfully stimulates myometrial contractions, but it does not relax the cervix at all. Using it in the presence of incomplete cervical dilation without antispasmodics carries a high risk of trauma and uterine rupture.
What is the advantage of demoxytocin over conventional oxytocin?
Due to molecular deamination, demoxytocin is not degraded by enzymes and can be administered buccally. Furthermore, it lacks the antidiuretic side effect, making it permissible in patients with elevated blood pressure.
Why is desmopressin prescribed for hemophilia?
The drug stimulates V₂ receptors, leading to an increase in blood clotting factor VIII activity. Simultaneously, via V₁ receptors, it enhances platelet aggregation, providing a comprehensive hemostatic effect.
What else to study on this topic
- Biochemical mechanisms: phospholipase C and adenylate cyclase signaling pathways
- The role of aquaporin proteins in the regulation of water and electrolyte balance
- Structure-function analysis: how amino acid substitution alters peptide pharmacokinetics
- Complications of therapy: risk of water intoxication during prolonged oxytocin infusion
- Differential approach to selecting uterotonics in obstetrical practice
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