Influence of viscous incubation media on the resistance to diffusion of the intestinal unstirred water layer in vitro.

Everted sacs of rat intestine were incubated in media made viscous with Na-carboxymethylcellulose (CMC). This increased the resistance to diffusion of the mucosal unstirred water layer, as measured by the half-time for the establishment of osmotic streaming potentials. The increase ws not however linearly related to viscosity, but reached a maximum, equivalent to an apparent thickness of 500 mu, at a viscosity of about 100 cP (shear rate = 50 s-1). The inhibition of maltose hydrolysis by CMC conformed to theoretical expectations, and was characterised by an increased apparent Km, with no change in Vmax. This was not true however of galactose transport by everted sacs which showed a decline in maximum transport rate, with no apparent increase in Km. It appears that increased unstirred layer resistance may induce partial anoxia in the everted sac preparation and thus reduce the energy available for active transport. We conclude that the effect of enhanced viscosity is to reduce turbulent flow, and hence stirring, in the intervillous compartment. Such an effect may be brought about by viscous components of the diet and may influence rates of absorption and digestion in the intact intestine in vivo.