he changes of contractile responses produced by the repeated aministration of oxytocin to uterus were examined.
Uterus isolated from non-pregnant rat mounted in an organ bath(5㎖ in volume) through which medium flowed at a constant rate of 12 ㎖ min^-1. The isometric contractions were divided into three components: contractile frequency, trough tension and peak tension. After equilibration, the contractile responses during which the isolated uterus was exposed by flowing medium containing 2nM oxytocin for minutes were called as first response. After the bath medium was changed to oxytocin-free medium for 5 and 15 minutes by flowing, the strip was re-exposed to same concentration of xoytocin by flowing. The contratile reponses of the strip re-exposed to oxytocin were called as second response.
During exposure to oxytocin (2 and 20 nM), contractile frequency and trough tension were gradually decreased, but peak tension was not. The contractile frequency and trough tension in the second response to 2nM oxytocin after 5 minutes in oxytocin-free medium were decreased in comparision with those in the first response. After 15 minutes in oxytocin-free medium, the contractile frequency and trough tension in the second response were similar to those in the first response. Peak tension in the second response was similar to that in the first response without relation to the exposed time in oxytocin-free medium.
These results indicate that contractile frequency and trough tension of uterine contraction are desensitized by re-exposure to oxytocin, but peak tension is not.
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