A transcervical biopsy technique was used for the repeated collection of samples of bovine endometrium from individual cows to monitor the development of oxytocin receptors. The collection of biopsies from three cyclic cows at two-day intervals between days 13 and 17 after oestrus did not result in any shortening of the length of the cycle (mean +/- sem 22.0 +/- 0.5 days). Furthermore, the assay of serial blood samples from three cows around the time of biopsy on days 15 and 17 demonstrated that the technique did not induce any release of prostaglandin F2 alpha (PGF2 alpha). Oxytocin receptors were undetectable (< 20 fmol/mg protein) on days 13 and 15 of the cycle, increased to a mean of 121 +/- 16 fmol/mg protein on day 17 (before luteolysis), and were highest in samples collected at oestrus (498 +/- 84 fmol/mg protein). To determine whether a single biopsy of endometrium would give a representative measure of the concentration of uterine oxytocin receptors, the concentrations were determined in samples of caruncular and intercaruncular endometrium collected from the upper, middle and lower proportions of uteri from three oestrous animals at slaughter. The concentrations were similar in both types of tissue (caruncular 730 +/- 15 fmol/mg protein, intercaruncular 727 +/- 48 fmol/mg protein) and there were no differences between the three regions of the uterus.
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