The role of Na+-H+ exchange in fluid and solute transport in the rat efferent ducts

In vivo microperfusion techniques were used to investigate the role of Na+- H+ exchange in the efferent ducts of the rat. Individual efferent ducts wer e perfused with a Krebs-Ringer bicarbonate solution (KRB) containing 0, 1, 3, 5 or 7.5 mM amiloride. Concentrations of 1-5 mM amiloride inhibited flui d reabsorption from the efferent ducts in a linear dose-dependent manner wi th an apparent K-m of 3 mM. Inhibition was maximal at 5 mM with reabsorptio n reduced by about 70 %. The effects of amiloride were completely reversibl e and there was little effect of amiloride on luminal osmolality and concen trations of Na+, Cl- or K+. It is concluded that Na+-H+ exchange is one of the principal mechanisms responsible for fluid and electrolyte reabsorption in the efferent ducts and offers a means by which the efferent ducts are a ble to achieve flow-dependent, autoregulated fluid reabsorption.