Release and Effects of Oxytocin on Estradiol and Progesterone Secretion in Porcine Corpora Lutea as Measured by anin VivoMicrodialysis System*

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Abstract
Individual corpora lutea (CL) of Gottinger miniature pigs were implanted with an in vivo microdialysis system. This system functions like an artificial capillary, allowing diffusion of intraluteally secreted substances into the lumen of the dialysis system and administration of hormones into individual CL and simultaneous measurement of the response. After surgery the sows are fully awake and unrestrained. In the present study the in vivo release rates and secretion dynamics of progesterone (P) and oxytocin (OXT) were investigated. The dialysis system was implanted at day 2–4 of the estrous cycle, and dialysis experiments were performed throughout the next 3 days. Fractions were collected at 30 min intervals, and the concentrations of P and OXT were measured by RIA. Three major observations were made: Spontaneous intraluteal secretion of P and OXT occurred in a pulsatile manner. OXT secretion episodes in individual CL often coincide, indicating a simultaneous release from many CL of one ovary but also from the CL located in the contralateral ovary. OXT episodes also often coincide with P pulses; statistical evaluation revealed a significant correlation between P and OXT secretion. Intraluteal application of OXT stimulated luteal P and estradiol (E2) release in a dose-dependent manner. E2 added to the perfusates was also stimulatory to P release. The stimulation of P release by OXT could be antagonized by prior treatment of the CL with tamoxifen. We demonstrate for the first time in vivo the secretion of OXT from porcine CL. The microdialysis system enabled us to collect samples at the site of steroid and peptide release, i.e. within the intact luteal tissue. Our results suggest a stimulatory effect of OXT on P release from young and middle-aged CL and are in marked contrast to the previously demonstrated inhibitory effect of OXT on P release when luteal cells were cultured in vitro. A possible explanation for this apparent discrepancy is that OXT stimulates intraluteal release of E2, which is a powerful P releasing hormone, overcoming the direct inhibitory effect of OXT. This suggestion is substantiated by the observation that E2, when added to the perfusion medium, indeed stimulated P release. (Endocrinology126: 2350–2358, 1990)