Failure of Progestogens to prolong Pregnancy in the Guinea Pig

Abstract

PROGESTERONE treatment in cases of threatened abortion and premature labour has been of little therapeutic value^1–3. These rather unexpected results, in view of the now classical work of Corner and Allen^4,5, have brought into question the plausibility of the “progesterone theory for the maintenance of pregnancy” when applied to the human being⁶. Recent development of the concept of the local action of progesterone^7–10, however, has provided grounds for considering pregnancy maintenance in terms of a ‘basic mechanism’¹¹, which allows for species variation with regard to ovariectomy and response to oxytocin and progesterone treatmen. This theory states that the significant physiological level of progesterone in the uterus is a function of placental progesterone which diffuses directly to the myometriu. Under these conditions the systemic progesterone-level assumes a proportionately less important role in pregnancy maintenance. According to this theory, then, the local concentration gradient of progesterone in the myometrium surrounding the placenta results in asynchronous activity⁹ and a general lowering of excitability, conduction and pharmacological reactivity¹², hence the local progesterone ‘block’ to the onset of parturition⁷.