4-AP contracts the nonpregnant (NP) mouse myometrium. (A) Representative contractile recording of the response to 4-AP in nonpregnant mouse myometrium. Horizontal component of arrows denotes duration of 4-AP incubation. (B) Pretreatment (15 min) of nonpregnant myometrial tissue strips with 1 μM tetrodotoxin (TTX) failed to alter the contractile response to 4-AP (n = 4). (C) Concentration-response relationships of full-thickness tissue strips (filled circles, n = 5) and isolated, endometrium-free myometrium (open circles, n = 4); data are responses in nonpregnant mice expressed as a mean percentage (± S.E.M.) of the maximal response elicited by 4-AP in each tissue strip, minus the spontaneous activity immediately preceding each addition of 4-AP.