PATHWAYS OF PHOSPHATE-TRANSPORT IN CHICK JEJUNUM - INFLUENCE OF VITAMIN-D AND EXTRACELLULAR-SODIUM

The effect of vitamin D3 on intestinal phosphate (Pi) absorption was studied in everted sacs prepared from jejunum of either vitamin D-deficient (-D) or vitamin D-replete (+D) chicks. Vitamin D3 stimulates the maximal velocity (Vmax) of a mucosal active Pi transport mechanism from 125 to 314 nmol·min-1·g-1 tissue. Km of this process remains virtually unchanged (-D: 0.15 mmol·l-1; + D: 0.18 mmol·l-1). Active Pi entry into the epithelium depends on extracellular Na+. Reduction of buffer Na+ reduces Vmax in the + D group to 182 nmol·min-1·g-1 tissue but has no significant effect in the -D animals (Vmax=105 nmol·min-1·g-1 tissue). In this group, the predominant effect of Na+ substitution is a shift of Km to 1.13 mmol·l-1, while Km in the +D group is changed only to 0.53 mmol·l-1. Transeptithelial Pi transport in the + D group involves the mucosal phosphate pump and hence an intracellular pathway, proceeding at a rate of 48 nmol·min-1·g-1 tissue. This is in contrast to -D Pi transfer (8 nmol·l-1·g-1 tissue) which is by a diffusional, Na+-insensitive, and presumably paracellular pathway. Transepithelial calcium transport (-D: 3.3 nmol·min-1·g-1; + D: 7.6 nmol·min-1·g-1 tissue) does not require the presence of extracellular Na+ and apparently involves pathways different from those of the Pi absorptive system. © 1979 Springer-Verlag.