Sodium-calcium sites in smooth muscle and their accessibility to lanthanum.

1. The Na, K, Ca, and Mg contents were determined in longitudinal smooth muscles of the guinea-pig ileum incubated in various physiological solutions with or without ouabain or in a solution containing LaCl(3) 10 mM instead of CaCl(2).2. The Na and K contents were dependent upon the K concentration of the physiological solution. A decrease in [K](o) below 6 m-mole/l. increased tissue Na.3. A rise in [Na](i) was followed by an increase of total cell Ca preceding a secondary decrease of total Ca, as well as by an increased (45)Ca content occurring even when total Ca was decreased.4. Muscles washed in the lanthanum solution contained less Ca and Na than those incubated in the physiological solution. The Mg content was not significantly modified in the presence of La.5. In the La solution, the (45)Ca diffusion space was not different from the [(14)C]inulin diffusion space, indicating that there was no Ca entry within the cell nor Ca binding at superficial sites. (45)Ca content of preloaded muscles decreased; this decrease was blocked after 30 min washout in the La solution: this arrest lasted a further 45 min.6. The cell Ca fraction displaced by increased [Na](i) originated from a Ca compartment which was not blocked by La.