Calcitriol-dependent, paracellular sodium transport in the embryonic chick intestine

Abstract

1,25-Dihydroxyvitamin D 3 (calcitriol), or vitamin D 3 itself, when added to cultures of 20-day-old embryonic chick small intestine, stimulated sodium (Na +) uptake from the mucosal surface. The calcitriol-mediated increase in Na + uptake appeared to be related to increased tight-junctional or paracellular permeability. Support for this conclusion was, first, that the uptake of other ions, potassium (K +) and rubidium (Rb +), with tight-junctional permeabilities greater than Na +, was also stimulated by calcitriol, and second, perturbation of cellular Na + and K + fluxes by inhibition of Na +/K +-ATPase activity did not affect calcitriol-stimulated Na +, K +, or Rb + transport. Calcitriol stimulation of Na + fluxes across the brush border as an alternate possibility is unlikely for the following reason: the calcium ionophore A23187, while mimicking the stimulatory action of calcitriol on calcium (Ca 2+) uptake, reduced epithelial Na 4 uptake. It is therefore suggested that calcitriol, by virtue of its effect on Ca 2+ transport, reduces rather than stimulates cellular Na + uptake.