Influence of vascular and lumen flow on sodium movements across anuran intestine in vitro.

There is a positive linear relation between the vascular flow rate and the magnitudes of the unidirectional fluxes of Na in either direction in the steady state across the small intestine of the frog. Both unidirectional Na fluxes increase to the same extent with an increase in vascular flow so there is no apparent effect of flow on net Na movement. Raising the vascular flow rate in individual experiments increases the lumen-blood Na flux proportionately, but reducing the flow from an initially high value reduces the lumen-blood flux only slowly at first then more rapidly. The unidirectional Na fluxes increase linearly as the lumen flow rate is increased. In the colon increasing the vascular flow rate also increases the lumen-blood Na flux but changing the vascular flow rate has little effect on the blood-lumen flux, the net absorption of Na is invariably increased as the vascular flow increases. Using 14C-labelled sucrose as a marker for the extracellular space of the small intestine, it can be shown that the increases in Na flux found with increased vascular flow rates are associated with an increase in this sucrose space. An increase in sucrose space, therefore, seems to enhance the accessibility of the low resistance, paracellular pathways for Na to and from the blood across the epithelium.