Oxytocin-induced changes in single cell K+ currents and smooth muscle contraction of guinea-pig gastric antrum.

To study the effects of oxytocin on both spontaneous phasic contractions and K+ outward currents (IK) of the so-called 'non-target' smooth muscle cells, physiological concentrations of oxytocin ranging between 10(-12) mol/l and 10(-8) mol/l were applied to smooth muscle preparations and single voltage-clamped cells isolated from the circular layer of the guinea-pig gastric antrum. Oxytocin (10(-12) mol/l to 10(-8) mol/l) suppressed, in a dose-dependent manner, the tetrodotoxin- and atropine-resistant spontaneous phasic contractions and shifted rightward the dose-response curves of 10(-7) mol/l charybdotoxin and 10(-3) mol/l BaCl2. In cells with preloaded intracellular Ca2+ stores, oxytocin (10(-12) mol/l to 10(-9) mol/l) caused a dose-dependent activation of the charybdotoxin-blockable non-inactivating component of IK (IK(sl)) of single voltage-clamped cells, which was accompanied by hyperpolarization of the cell membranes. 8Lys-vasopressin and 8arg-vasopressin failed to mimic the effects of oxytocin on both contraction and K+ currents. Further, the oxytocin-induced activation of IK(sl) was effectively antagonized by 5 x 10(-8) mol/l U-73122 or 5 x 10(-6) mol/l 2-nitro-4-carboxyphenyl N,N-diphenylcarbamate (inhibitors of the cell membrane phospholipase C), as well as by intracellularly applied heparin (selective inhibitor of inositol-1,4,5-trisphosphate (IP3)-induced Ca2+ release channels). In cells incubated in the absence of Ca2+ entry throughout the study, oxytocin (10(-9) mol/l) caused a slight and transient increase of IK(sl) amplitudes. Neither ryanodine (10(-6) mol/l) nor cyclopiazonic acid (10(-6) mol/l) were able to restore the IK-activating effect of oxytocin in these cells. The data obtained suggest (i) that selective oxytocin receptors are present on the membranes of guinea-pig antral smooth muscle cells, (ii) that the oxytocin-related relaxation may result from the activation of Ca(2+)-sensitive K+ conductivity via activation of IP3-induced release of Ca2+ from the submembrane located cisternae of the sarcoplasmic reticulum Ca2+ stores and (iii) in turn, this evokes a non-inactivating component of IK, hyperpolarizing the cell membrane.