Previous studies showed that an increase in NAD+ content in renal cortex in vivo was accompanied by specific inhibition of Na+-dependent inorganic phosphate (P(i)) 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 P(i). 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 V(max) was decreased, but the apparent K(m) was not different, indicating apparent noncompetitive inhibition. In the presence of 0.3 mM NAD+ added in vitro, the apparent K(m) for Na+-dependent P(i) transport by BBM vesicles was increased, whereas the apparent V(max) was unchanged, indicating apparent competitive inhibition. These changes in apparent K(m) and apparent V(max) were identical when P(i) uptake was measured either at 30-s or at 5-s (the initial rate) incubation times. Inhibition of P(i) 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 P(i) released from NAD+ by enzymatic hydrolysis. Although in vitro inhibition of P(i) 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 P(i), evoked by loading with glycerol or fructose, did not change BBM transport of P(i). In summary, inhibition of BBM transport of P(i) by increased NAD+ in vivo differs in kinetics and reversibility compared with the inhibitory effect of NAD+ in vitro, suggesting that different mechanisms may be involved. The characteristics of the inhibitory effect of NAD+ in vivo are analogous to the changes that occur when renal BBM transport of P(i) is modulated by hormonal and metabolic factors.
|Original language||English (US)|
|Journal||American Journal of Physiology - Renal Fluid and Electrolyte Physiology|
|State||Published - 1985|
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