Mucosal calcium uptake by rat cecum: identity with transcellular calcium absorption

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Abstract
Unidirectional intestinal Ca uptake .**GRAPHIC**. at the mucosal surface of rat cecum was investigated in vitro with intact tissue. Uptake is linear for 2-3 min with no indication of rapid Ca binding. Kinetic parameters reveal a maximal velocity of 333 nmol .cntdot. cm-2 h-1 with a half-maximal concentration of 0.98 mM. High-Ca diet decreased .**GRAPHIC**. by more than 60% with respect to both control and low-Ca diets; 1 mM N-ethylmaleimide caused a similar reduction. The activation energy of .**GRAPHIC**. is significantly less than that of transepithelial mucosal-to-serosal Ca absorption. Mucosal uptake was compared with transepithelial Ca fluxes in rat cecum and revealed a 1:1 correlation over a wide range of transport rates. These results are interpreted to implicate a feedback control system between basolateral Ca efflux and brush-border Ca influx.