Different mechanisms of adaptive increase in Na+-Pi cotransport across renal brush-border membrane

Abstract

We explored the biochemical mechanism by which thyroid hormone (T3) and low-phosphate diet (LPD) cause an adaptive increase in Na+-Pi cotransport across renal brush-border membrane (BBM). The rate of Na+-Pi cotransport was determined by 32Pi uptake by BBM vesicles (BBMV), and the number of Na+-Pi symporters was assessed by binding of [14C]phosphonoformic acid (PFA) on BBMV. In BBMV of both T3-treated rats and LPD-fed rats, the Na+ gradient-dependent 32Pi uptake increased (Vmax increased; Km Pi was not changed). The Na+-dependent [14C]PFA binding on BBMV increased (higher Vmax, no change in Km PFA) in response to T3, but it remained unchanged in rats fed LPD. Both the increase of Na+-Pi cotransport and of Na+-dependent [14C]PFA binding in response to T3 were blocked by actinomycin D or cycloheximide. Addition of benzyl alcohol to BBMV in vitro increased Na+-Pi cotransport, but [14C]PFA binding did not change; the [3H]phlorizin binding and cotransports of other solutes decreased or did not change. The exposure of BBMV to cholesterol decreased Na+-Pi cotransport without changing [14C]PFA binding. We suggest that the adaptive increase of Na+-Pi cotransport elicited by T3 is due to an increase in number of Na+-Pi cotransporters in BBM. In contrast, in response to LPD the number of Na+-Pi cotransporters is unchanged, and the increased Na+-Pi cotransport is due to faster translocation of Na+ with Pi due to enhanced fluidity of BBM.