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.

Effects of sodium gradient manipulation upon cellular calcium 45Ca fluxes and cellular sodium in the guinea‐pig taenia coli

“…The mean exchangeable Ca content at this time was 304 1 + 5.3 ,umole/kg tissue (n = 290); although this value underestimates the content of the slow component by 27 %, it will be used hereafter as an approximation of cellular Ca content. This value is much higher than those which have been previously obtained for taenia coli (Ma & Bose, 1977;van Breemen, Farinas, Casteels, Gerba, Wuytack & Deth, 1973) using the 'La method', although it is similar to values which have been obtained for vascular smooth muscle using media with lowered temperatures (van Breeman et al 1979;Deth, 1978;Ozaki & Urakawa, 1979;Dutta, Mustafa & Jones, 1980). It is likely that the reduced temperature of EGTA and La media retards the loss of Ca by cells.…”

“…5). In addition, this compound has been widely employed as a Na substitute in previous studies of taenia coli and other types of smooth muscle Reuter et al 1973;Ma & Bose, 1977). It should be pointed out that the low [Cl]0 obtained in sucrose-PSS may decrease the K permeability of the membrane (Casteels, 1970), and that spontaneous action potentials are also abolished upon replacement of NaCl with sucrose (Brading, Bfilbring & Tomita, 1969).…”

Na/Ca exchange and tension development in vascular smooth muscle: effect of amiloride

“…This result can be explained by the of guinea-pig fact that the above conditions do not alter to the same he Na pump. (Ma & Bose, 1977;Van Breemen et al, 1979; )n depolariza -Somlyo et al, 1982;Bolton, 1985 (Ma & Bose, 1977) presumably through the Na/Ca exchange mechanism, and an ATP-driven Ca pump (Blaustein, 1982;Schatzmann, 1985). The inhibition of the Na/Ca exchange mechanism operating in the Ca extrusion mode may result in a decrease of the relaxation rate of the smooth muscle (Ashida & Blaustein, 1987), as we observed in the presence of amiloride.…”

Alterations in [Ca2+]i mediated by sodium‐calcium exchange in smooth muscle cells isolated from the guinea‐pig ureter.

“…or by Na+-K+-ATPase blockade (e.g. Ma & Bose, 1977;Matlib et al 1985;Nabel, Berk, Brock & Smith, 1988). The presence of Na+-Ca2+ exchange in the guinea-pig ureter was established by Aickin and co-workers (Aickin, Brading & Burdyga, 1984;Aickin, 1987) in detailed and elegant investigations of the effects of Na+ substitution and Na+ pump inhibition on tension development and the intracellular Na+ activity.…”