The kinetics of influx of calcium and strontium into rat intestine in vitro.

1. The role of uptake across the brush border in the intestinal absorption of calcium has been studied by examining the kinetics of influx into slices of rat intestine in vitro. Both mucosal and serosal surfaces were exposed to the medium.2. The rate of influx was accurately defined by a two-component expression comprising a saturable (Michaelis-Menten) term and a second term linear with concentration. Influx across the mucosal surface of closed sacs was similar, and the saturable component for slice influx could be ascribed mainly to transport across the mucosal surface. The half-saturation constant for Ca was near 1 mM. This component was predominant at normal luminal concentrations of free Ca in the duodenum of young rats, but less so in jejunum and ileum and in older rats.3. The same kinetic expression applied to Sr influx, with a half-saturation constant of 2-3 mM, and possibly also to Ba with an even higher value.4. The saturable component of Ca influx was greatly reduced by 2,4: dinitrophenol (DNP); influx was also inhibited by iodoacetate, cyanide and at 0 degrees C. Inhibition commenced soon after exposure of the slices. A high concentration of DNP also caused an increase in the linear component of Ca influx.5. The kinetics of Ca influx across the mucosal surface agreed closely with the kinetics of steady-state absorption of Ca either across the whole mucosal epithelium in vivo or across the entire intestinal wall in vitro. This agreement supports the hypothesis that Ca entry across the brush border is the rate-limiting step in absorption; such a hypothesis would allow net Ca translocation while preserving a low intracellular concentration of ionic Ca in the mucosal epithelial cells.