Abstract
The fluxes of [7- 14C]nicotinic acid across the chamber-mounted, in vitro bullfrog Rana cateshieana small intestine were measured. The flux of tracer in the direction favoring absorption exceeded the opposite flux to a statistically significant degree ( P < 0.05) at concentrations of 2.0 · 10 −3 M and below. This process saturated at 2.0 · 10 −2 M. mannitol 115 mM added to the mucosal salines abolished solvent drag as measured by 10 mM urea fluxes. The transmural potential difference across this preparation is too small (approx. 3 mV) to cause a significant asymmetry in the transfer of any charged solute. At 2.0 · 10 −4 M nicotinic acid the flux ratio was 25.7 ± 7.3 (mean ± S.E.). Studies of the chemical nature of the flux of tracer at this concentration disclosed that at least 79% of the label traversing the preparation appeared as nicotinic acid. Since [7 14C]nicotinic acid absorption is accomplished by a saturable transport system with, at most, minor metabolism of nicotinic acid, we conclude that nicotinic acid itself is actively transported.
Published Version
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