Disappearance of 4-AP responses in pregnant mouse myometrium. Representative contractile recordings are from nonpregnant (panel A) and term-pregnant (P20) mouse myometrium (panel B). Horizontal component of arrows denotes duration of each drug incubation. All 3 recordings in each panel were obtained from the same tissue strip and are representative of results obtained from 5 nonpregnant and 5 term-pregnant mice. Panel A: Contractile responses to maximal concentrations of 4-AP, acetylcholine (Ach), and oxytocin (OT) in nonpregnant mouse myometrium. Panel B: Contractile responses to Ach and OT are shown in contrast to the absence of a contractile response to 300 μM 4-AP in term-pregnant mouse myometrium.