Uterine motility depends on the formation of the contractile protein actomyosin under the influence of the Ca²⁺ -dependent phosphorylating enzyme myosin light-chain kinase. Oxytocin promotes contractions by increasing the intracellular Ca²⁺. Oxytocin has specific receptors in the myometrium and the receptor concentration increases greatly during pregnancy, reaching a maximum in early labor at term. The response to a given dose of oxytocin is very individualized and depends on the sensitivity of the uterus, which is determined by the oxytocin receptor concentration. However, the physician should be aware of the fact that oxytocin even in its pure form has inherent pressor and antidiuretic properties which may become manifest when large doses are administered. These properties are thought to be due to the fact that oxytocin and vasopressin differ in regard to only two of the eight amino acids (see PRECAUTIONS section).

Oxytocin is distributed throughout the extracellular fluid. Small amounts of the drug probably reach the fetal circulation. Oxytocin has a plasma half-life of about 1 to 6 minutes which is decreased in late pregnancy and during lactation. Following intravenous administration of oxytocin, uterine response occurs almost immediately and subsides within 1 hour. Following intramuscular injection of the drug, uterine response occurs within 3 to 5 minutes and persists for 2 Oxytocin has a plasma half-life of about 1 to 6 minutes which is decreased in late pregnancy and during lactation. Following intravenous administration of oxytocin, uterine response occurs almost immediately and subsides within 1 hour. Following intramuscular injection of the drug, uterine response occurs within 3 to 5 minutes and persists for 2 to 3 hours. Its rapid removal from plasma is accomplished largely by the kidney and the liver. Only small amounts are excreted in urine unchanged.

REFERENCES

1. Seitchik J, Castillo M: Oxytocin augmentation of dysfunctional labor. I. Clinical data. Am J Obstet Gynecol 1982; 144:899-905.

2. Seitchik J, Castillo M: Oxytocin augmentation of dysfunctional labor. II. Muciparous patients. Am J Obstet Gynecol 1983; 145:777-780.

3. Fuchs A, Goeschen K, Husslein R et al: Oxytocin and the initiation of human parturition. III. Plasma concentrations of oxytocin and 13, 14-dihydro-15-keto-prostaglandin F2a in spontaneous and oxytocininduced labor at term. Am J Obstet Gynecol 1983; 145:497-502.

4. Seitchik J, Amico J, et al: Oxytocin augmentation of dysfunctional labor. IV. Oxytocin pharmacokinetics. Am J Obstet Gynecol 1984; 150:225-228.

5. American College of Obstetricians and Gynecologists: ACOG Technical Bulletin Number 110-November 1987: Induction and augmentation of labor.

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