Posterior pituitary drugs

Posterior pituitary drugs

Posterior pituitary hormones are synthesized in the hypothalamus and stored in the posterior pituitary, which, in turn, secretes the hormones into the blood. These drugs include:

• all forms of antidiuretic hormone (ADH), such as desmopressin acetate and vasopressin
• the oxytocic drug oxytocin.

Pharmacokinetics

Because enzymes in the GI tract can destroy all protein hormones, these drugs can’t be given orally. Posterior pituitary drugs may be given by injection or intranasal spray.

Absorption, distribution, and metabolism

Like other natural hormones, oxytocic drugs are usually absorbed, distributed, and metabolized rapidly. Parenterally administered oxytocin is absorbed rapidly; however, when it’s administered intranasally, absorption is erratic.

Pharmacodynamics

Under neural control, posterior pituitary hormones affect:

• smooth-muscle contraction in the uterus, bladder, and GI tract
• fluid balance through kidney reabsorption of water
• blood pressure through stimulation of the arterial wall muscles.

Going to cAMP

ADH increases cyclic adenosine monophosphate (cAMP), which increases the permeability of the tubular epithelium in the kidneys, promoting reabsorption of water. High dosages of ADH stimulate contraction of blood vessels, increasing the blood pressure.

Desmopressin reduces diuresis and promotes clotting by increasing the plasma level of factor VIII (antihemophilic factor).

Baby talk

In pregnant women, oxytocin may stimulate uterine contractions by increasing the permeability of uterine cell membranes to sodium ions. It also can stimulate lactation through its effect on mammary glands.

Pharmacotherapeutics

ADH is prescribed for hormone replacement therapy in patients with neurogenic diabetes insipidus (an excessive loss of urine caused by a brain lesion or injury that interferes with ADH synthesis or release). However, it doesn’t effectively treat nephrogenic diabetes insipidus (caused by renal tubular resistance to ADH).

The ABC’s of ADH treatment

Desmopressin is the drug of choice for chronic ADH deficiency and is administered intranasally. It’s also indicated for primary nocturnal enuresis. Desmopressin has a long duration of action and relatively few adverse effects.

Short-term ADH treatment is indicated for patients with transient diabetes insipidus after head injury or surgery; therapy may be lifelong for patients with idiopathic hormone deficiencies. Used for short-term therapy, vasopressin elevates blood pressure in patients with hypotension caused by lack of vascular tone. It also relieves postoperative gaseous distention.

They help with deliveries (before, during, and after)

Oxytocics are used to:

• induce labor and complete incomplete abortions
• treat preeclampsia, eclampsia, and premature rupture of membranes
• control bleeding and uterine relaxation after delivery
• hasten uterine shrinking after delivery
• stimulate lactation.

Warning!

Adverse reactions to posterior pituitary drugs

Hypersensitivity reactions are the most common adverse reactions to posterior pituitary drugs.

Natural ADH

Anaphylaxis may occur after injection. Natural antidiuretic hormone (ADH) can also cause:

• ringing in the ears
• anxiety
• hyponatremia (low serum sodium levels)
• proteins in urine
• eclamptic attacks
• pupil dilation
• transient edema.

Synthetic ADH

Adverse reactions to synthetic ADH are rare.

Drug interactions

A variety of drugs can cause interactions with posterior pituitary drugs:

• Alcohol, demeclocycline, epinephrine, and lithium may decrease the ADH activity of desmopressin and vasopressin.
• Chlorpropamide, clofibrate, carbamazepine, and cyclophosphamide increase ADH activity.
• Synergistic effects may occur when barbiturates or cyclopropane anesthetics are used concurrently with ADH, leading to coronary insufficiency or arrhythmias.
• Cyclophosphamide may increase the effect of oxytocin.
• Concurrent use of vasopressors (anesthetics, ephedrine, methoxamine) and oxytocin increases the risk of hypertensive crisis and postpartum rupture of cerebral blood vessels.