AJP - Renal Physiology, Vol 249, Issue 6 903-F911, Copyright © 1985 by American Physiological Society
Transport of N1-methylnicotinamide across brush border membrane vesicles from rabbit kidney
S. H. Wright
A preparation of isolated brush border membranes from the rabbit renal
cortex was used to examine the characteristics of N1-methylnicotinamide
(NMN) transport in the kidney. Transport was independent of the presence of
Na+ under Na+ equilibrium conditions. However, outwardly directed Na+
gradients stimulated NMN uptake, whereas inwardly directed Na+ gradients
inhibited NMN uptake. Transport appeared to involve two parallel processes:
one saturable with a Jmax of 5 nmol X mg protein-1 X min-1 and an apparent
Kt of 0.6 mM, and a second that behaved like passive diffusion.
Countertransport of NMN was observed when vesicles were preloaded with
either NMN or another organic cation, tetraethylammonium (TEA). TEA and
several structural analogues, as well as a wide variety of other organic
cations and bases, were effective inhibitors of NMN uptake, though
nicotinamide and p-aminohippuric acid did not interact with the uptake
process. Outwardly directed proton gradients (pH 6.0 in, 7.6 out)
stimulated transport, suggesting that NMN uptake may involve a
countertransport of H+. The electrical potential difference across the
vesicle membrane was manipulated using gradients of the permeant organic
ion thiocyanate (SCN-); an outwardly directed gradient of SCN- (i.e., a
depolarizing condition) stimulated uptake and produced a transient
accumulation of NMN above that noted at equilibrium, whereas an inwardly
directed SCN- gradient inhibited uptake of NMN. The data can be explained
by postulating the presence of an electrogenic NMN+-H+ antiporter in the
rabbit luminal membrane that could play a role in organic cation secretion.
