title: Sodium in smooth muscle relaxation.
abstract: The contraction of guinea-pig taenia coli due to high K+ could not be reversed by washing when Na+ was absent from the medium. Reintroduction of Na+ (7 mM or more) caused relaxation. Similar results were obtained with rat uterus. The effect of sodium replacement was not due to change in ionic strength because equal or higher osmoles of choline, Ca++, K+, Mn++, or Mg++ had little or no effect. Persistence of contraction in the Na+-free medium was not due to a "catch state" of the contractile apparatus. Impairment of Ca+ removal from the cytoplasm rather than persistent increase in Ca+ influx seemed to sustain the mechanical response. This was because D600 (a calcium influx blocker) failed to completely relax K+-induced contraction in the absence of Na+ and also because the ability of EGTA to produce relaxation was reduced in the absence of Na+. Measurement of tissue calcium content using the lanthanum method revealed coincident decrease in tissue calcium and tension to control level during Na+-mediated relaxation. The results suggest a role for transmembrane Na+-Ca++ exchange in causing the Na+-mediated relaxation of taenia undergoing Na+-free contracture.
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D002794:Choline / Q000378:metabolism; D003106:Colon / Q000378:metabolism; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007811:Lanthanum / Q000378:metabolism; D008274:Magnesium / Q000378:metabolism; D008345:Manganese / Q000378:metabolism; D009119:Muscle Contraction; D009130:Muscle, Smooth / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism; D012964:Sodium / Q000378:metabolism*
chemical_list: D008345:Manganese; D007811:Lanthanum; D012964:Sodium; D008274:Magnesium; D002794:Choline; D011188:Potassium; D002118:Calcium