5-hydroxytryptamine receptors in uterine smooth muscle.

When studying some of the properties of 5-hydroxytryptamine (5HT) receptors in the rat uterine muscle using phenoxybenzamine (PBZ) as an antagonist it was found that the specific receptors for 5HT in the smooth muscle were selectively blocked by PBZ; a period of 20-minute exposure to the antagonist was required for maximal effect. The blockade produced was of long duration and the recovery of response was relatively slow; it was incomplete throughout the 4-hour observation period. A concentration of 1 X 10(-8) g/ml PBZ produced a parallel shift of the dose-response effects while higher concentrations reduced both the slope and maximal response. The reasons for such a shift were discussed. 5HT produced a rapid onset and offset of effect suggesting that the site of 5HT receptor is on the surface of the cell membrane. Moreover, 5HT could protect its own receptor against PBZ blockade.