AJP - Renal Physiology, Vol 249, Issue 6 948-F955, Copyright © 1985 by American Physiological Society
Actions of NAD+ on renal brush border transport of phosphate in vivo and in vitro
S. A. Kempson, S. T. Turner, A. N. Yusufi and T. P. Dousa
Previous studies showed that an increase in NAD+ content in renal cortex in
vivo was accompanied by specific inhibition of Na+-dependent inorganic
phosphate (Pi) transport across the renal brush border membrane (BBM).
Further, in vitro addition of NAD+ to isolated renal BBM vesicles
specifically inhibited Na+ gradient-dependent transport of Pi. The present
study examined some aspects of the mechanism of this inhibition by NAD+ in
vitro and in vivo. When NAD+ was increased in vivo by nicotinamide
injection, the apparent Vmax was decreased, but the apparent Km was not
different, indicating apparent noncompetitive inhibition. In the presence
of 0.3 mM NAD+ added in vitro, the apparent Km for Na+-dependent Pi
transport by BBM vesicles was increased, whereas the apparent Vmax was
unchanged, indicating apparent competitive inhibition. These changes in
apparent Km and apparent Vmax were identical when Pi uptake was measured
either at 30-s or at 5-s (the initial rate) incubation times. Inhibition of
Pi transport by BBM vesicles in vitro was due primarily to the action of
intact added NAD+, although there may be some contribution by isotope
dilution due to Pi released from NAD+ by enzymatic hydrolysis. Although in
vitro inhibition of Pi transport by added NAD+ was reversed by washing the
BBM, the inhibition due to increased NAD+ in vivo persisted after extensive
washing of the isolated BBM. The specificity of the inhibitory effect of
NAD+ in vivo was indicated by the finding that changes in renal cortical
content of ATP or Pi, evoked by loading with glycerol or fructose, did not
change BBM transport of Pi.(ABSTRACT TRUNCATED AT 250 WORDS)
