Calcium dependence of basal electrolyte transport in rabbit ileum

The effect on active electrolyte transport of altering the calcium concentration in the fluid bathing the mucosal and serosal surfaces of rabbit ileum was studied in vitro using the Ussing chamber voltage-clamp technique. Calcium-deprived (no-calcium) bathing solutions simultaneously placed on both the mucosal and the serosal surfaces decreased short-circuit current and potential difference, increased conductance, equally increased net sodium and chloride absorption, and decreased the residual ion flux. There was no effect on glucose-dependent sodium absorption. These changes were constant for 150 min and, except for conductance, were reversed by replacing the bathing solutions with standard Ringer-HCO₃ containing 1.2 mM calcium. These changes in transport were associated with a decrease in total ileal calcium content as measured by atomic absorption spectrometry. Adding the calcium channel blocker verapamil (10^-4 M) to the solutions bathing the serosal plus mucosal surfaces of rabbit ileum or to the serosal surface alone duplicated these effects. The effect of the calcium-deprived mucosal plus serosal bathing solutions on sodium transport was prevented by removing chloride from the bathing solutions; similarly, the effect on chloride transport was abolished by removing sodium from the bathing solutions. These studies suggest that lowering intracellular calcium is associated with stimulation of active ileal sodium and chloride absorption. This information must be combined with the previous demonstration that increasing intracellular calcium with agents such as serotonin inhibits sodium and chloride absorption. Together, these data suggest that calcium may be a physiological regulator of active sodium and chloride transport in the ileum.