Christensen, "Role of amino acid transport and countertransport
in nutrition and metabolism," Physiological Reviews, vol.
Alberti, "Measures of tubular function in normoalbuminuric insulin-dependent diabetic patients and their relationship with sodium lithium countertransport
activity," Nephron, vol.
stimulates PAH uptake and net secretion in isolated rabbit renal tubules.
NHE3 is a member of NHE ([Na.sup.+]/[H.sup.+] exchanger) family, which mediates [Na.sup.+] absorption by an electroneutral countertransport
of [H.sup.+] for [Na.sup.+] across the plasma membrane (Hayashi et al., 2002).
Depending on the tubular segment, the movement of [Na.sup.+] down this concentration gradient into the cell may be as a sodium ion alone, in cotransport with other organic substances such as amino acids and glucose or ions such as chloride, or in a countertransport mechanism with ions such as hydrogen.
The luminal entry of sodium requires various carriers that transport sodium coupled or in countertransport with other solutes across the luminal membrane.
In the proximal tubule, [Na.sup.+]' also crosses the luminal membrane in countertransport with hydrogen ion ([H.sup.+]), thereby exchanging electropositive ions.
Increase in [Na.sup.+]/[Li.sup.+] countertransport across the red cell membrane has been suggested as such an early marker [1-4], although this was not uniformly confirmed [5-7].
Until recently the [Na.sup.+]/[Li.sup.+] countertransport activity was measured as the [Li.sup.+] efflux rate in medium containing 150 mmol/L [Na.sup.+] after subtraction of the efflux rate in [Na.sup.+]-free medium.
This could lead to pH changes in the erythrocyte, which could influence [Na.sup.+]/[Li.sup.+] countertransport. Elving et al., however, reported that after the washing cycles the intracellular HC[O.sub.3.sup-] and [Cl.sup.-] concentrations were identical in cells loaded with either [Li.sub.2]C[O.sub.3] or LiCl .
reported that changes in [K.sub.0.5] for [Na.sup.+] rather than in [V.sub.max] are the explanation for increased [V.sub.150] [Na.sup.+]/[Li.sup.+] countertransport in insulin-dependent diabetes mellitus patients with nephropathy .