Effect of using calcium-free Krebs' solution on basal and A23187-stimulated prostaglandin output from the Day 15 guinea-pig uterus superfused in vitro.

The basal outputs of prostaglandin (PG) F2 alpha and PGE2 from the Day 15 guinea-pig uterus superfused in vitro were unaffected by omitting Ca2+ from the Krebs' solution. In contrast, this omission of Ca2+ reduced the basal output of 6-oxo-PGF1 alpha (which reflects PGI2 production) from the uterus by an average of 50%. Spontaneous and A23187-stimulated contractions of, and the stimulation by A23187 of PGF2 alpha, PGE2 and 6-oxo-PGF1 alpha outputs from the Day 15 guinea-pig uterus were all abolished by superfusing the tissue with Krebs' solution lacking Ca2+. It is concluded that the basal output of 6-oxo-PGF1 alpha, the occurrence of spontaneous contractions, and the effects of A23187 on PG output and contractility of the Day 15 guinea-pig uterus are dependent on extracellular Ca2+. However, the increase in PGF2 alpha output from the guinea-pig uterus on Day 15 compared to days much earlier in the cycle is apparently not dependent upon extracellular Ca2+. The implications of these findings regarding the biochemical mechanisms involved in the increased synthesis of PGF2 alpha (the uterus luteolytic hormone) by the guinea-pig endometrium during the last one-third of the cycle are discussed.