Manganese metabolism in rats: an improved methodology for assessing gut endogenous losses.

Manganese homeostasis is believed to be maintained by excretion of excess absorbed manganese through the gut, but the extent of endogenous gut losses of manganese has not been quantitated. We developed a model with rats to quantitate endogenous gut losses of manganese in which the parenterally administered isotope was distributed like fed isotope. Intraportally injected 54Mn complexed to albumin distributed in tissues like the fed isotope, but carrier-free 54Mn injected intraperitoneally, intravenously, or intraportally, or 54Mn complexed to transferrin and injected intraportally did not. Thus, manganese appears to be complexed to albumin or an albumin-like protein when it leaves the intestine. A mathematical model of manganese metabolism in rats fed 54Mn was developed using the SAAM and CONSAM computer programs. It was determined that the liver, not the pancreas, was the major source of endogenous gut losses of manganese. Young, growing rats fed 45 micrograms of Mn/g diet were calculated to absorb 8.2% of their manganese intake and then to lose 37% of the absorbed manganese through gut endogenous losses.