Sodium in smooth muscle relaxation

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.