Different effects of papaverine, isoprenaline and caffeine on rat uterine contractility in Ca-free medium

The inhibitory effects of papaverine, isoprenaline and caffeine on contractility in rat isolated uterus were studied in Ca-free medium. Uterine contractions were elicited by oxytocin or vanadate and the magnitude of the contractile response was not similar for the two spasmogens, indicating that they act under different intracellular stores. Oxytocin- or vanadate-induced contractile responses were completely relaxed by papaverine, indicating a mechanism less specific than isoprenaline or caffeine. Both of them relax completely the oxytocin-induced contraction and only partially the vanadate-induced contraction. After caffeine treatment, the recovery of the contractile response to oxytocin or vanadate was total, but after isoprenaline treatment the contractile response could not be restored. This suggests that different mechanisms of relaxation of uterine smooth muscle operate for isoprenaline and caffeine.