AJP - Regulatory, Integrative and Comparative Physiology, Vol 252, Issue 4 720-R731, Copyright © 1987 by American Physiological Society
Quantitation of phosphorus excretion in sheep by compartmental analysis
K. M. Schneider, R. C. Boston and D. D. Leaver
The control of phosphorus excretion in sheep has been examined by
constructing a kinetic model that contains a mechanistic set of connections
between blood and gastrointestinal tract. The model was developed using
experimental data from chaff-fed sheep and gives an accurate description of
the absorption and excretion of phosphorus in feces and urine of the
ruminating sheep. Simulation of the response to an intravenous phosphorus
infusion by adding an inflow of 2 g/day of phosphorus into the compartment
describing blood, predicted values for fecal output of phosphorus lower
than found experimentally. However, by alteration of the parameters
describing absorption or salivation, the predictions approached
experimental values. Similarly simulation of the conditions existing when a
liquid diet was infused directly into the abomasum, i.e., a decrease in
salivation rate [L(4.1)] and dietary phosphorus entering compartment 5
(abomasum) instead of compartment 4 (rumen), gave incorrect predictions for
plasma and urinary phosphorus, but when the parameter for urinary
phosphorus was increased the predicted values approached experimental
values. These results indicated the main control site for phosphorus
excretion in the ruminating sheep was the gastrointestinal tract, whereas
for the nonruminating sheep fed the liquid diet, control was exerted by the
kidney. A critical factor in the induction of adaptation of phosphorus
reabsorption by the kidney was the reduction in salivation, and since this
response occurred independently of marked changes in the delivery of
phosphorus to the kidney, a humoral factor may be involved in this
communication between salivary gland and kidney.
