AJP - Endocrinology and Metabolism, Vol 240, Issue 4 384-E390, Copyright © 1981 by American Physiological Society
Intestinal phosphate transport and alkaline phosphatase activity in the chick
S. J. Birge and R. C. Avioli
The initial rates of phosphate accumulation by isolated chick intestinal
epithelial cells have been examined. At high concentrations of phosphate
(1.5 mM), phosphate uptake is relatively independent of sodium and
demonstrates a pH optimum of 8.0. At pH 8.0, 56% of the uptake is dependent
on the presence of Ca in the uptake medium compared to 28% at pH 6.8.
Membranes prepared from these same intestinal epithelial cells contain a
Ca-dependent phosphatase that can be distinguished from the more abundant
Mg-dependent alkaline phosphatase. The Ca-dependent phosphatase has a pH
optimum between 8.5 and 9.0 and, compared to the Mg-dependent activity, is
more readily inactivated at 58 degrees C and is relatively resistant to
L-phenylalanine inhibition but more sensitive to
ethane-1-hydroxy-1,1-diphosphonate (EHDP). Both activities are distributed
in a constant proportion between the brush border and basal lateral
membranes and at various segments along the intestine. Vitamin D in vivo
and 25-hydroxyvitamin D [25(OH)D] in vitro stimulated both activities. In
vitro, utilizing the isolated intestinal cells, the stimulation of
phosphate uptake paralleled the increase in Ca-dependent alkaline
phosphatase activity. The role of alkaline phosphatase in intestinal
phosphate transport is discussed.
