Anomalous calcium secretion in rat ileum: role of paracellular pathway

The mechanism of apparent calcium secretion by unstripped rat ileum in vitro has been investigated using mannitol and polyethylene glycol (mol wt 900) as markers for extracellular transepithelial flux. In the absence of electrochemical gradients between the mucosal and serosal bathing media in a modified Ussing chamber, net fluxes of both mannitol and polyethylene glycol were observed from serosa to mucosa in the presence of 11 mM D-glucose. Mucosal-to-serosal calcium flux reveals a significant cellular component, but serosal-to-mucosal calcium permeability is a linear function of mannitol permeability, suggesting an exclusively extracellular route. For the mucosal-to-serosal fluxes, inhibition of calcium flux by 1 mM N-ethylmaleimide results in a calcium-to-mannitol permeability ratio indistinguishable from that measured for serosal-to-mucosal flux. This evidence suggests that the apparent calcium secretion observed at 10 mM medium calcium is not the result of a cellular secretory mechanism. It is proposed that a hydrostatic driving force generated internal to the tissue but external to the cells results in net calcium secretion at calcium conentrations that saturate the cellular absorptive mechanism.