In vitro studies suggest that the female sex steroid hormones [estrogen (E) and progesterone (P)] can affect the myoelectric and mechanical activity of colonic smooth muscle. The present study was designed to examine the influence of the hormones on colonic transit in vivo. Transit was assessed by quantifying the distribution within the colon of a radiolabeled marker (0.5 microCi Na251CrO4), using the geometric center method of analysis. Studies were performed with adult male rats and the following groups of female rats: nonpregnant, ovariectomized, ovariectomy plus hormone pretreatment (100 micrograms X kg-1 X day-1 E + 2.5 mg X kg-1 X day-1 P for 4 days), and pregnant (day 18). Hormone-pretreated animals were studied 24 h following the fourth injection. The data can be summarized as follows. 1) Colonic transit was affected by the timing of the estrus cycle. Rats determined to be in proestrus-estrus had a geometric center value (1.97 +/- 0.50) significantly less than that of metestrus-diestrus animals (4.25 +/- 0.57). 2) Ovariectomy eliminated the biphasic transit pattern observed in estrus-cycling females and resulted in a geometric center value (4.19 +/- 0.17) comparable with that of the metestrus-diestrus animals. 3) E + P pretreatment of ovariectomized rats resulted in a significant decrease in the geometric center (1.94 +/- 0.19) compared with the untreated ovariectomized rats. 4) The geometric center value in pregnant animals (2.22 +/- 0.20) closely resembled the transit data for proestrus-estrus animals and hormone-pretreated animals.(ABSTRACT TRUNCATED AT 250 WORDS)

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