Effect of acetylcholine and secretin on medullary collecting duct function in the rat.

Microcatheterization was used to study the effect of renal arterial infusion of acetylcholine or secretin on medullary collecting duct function in anaesthetized rats. Acetylcholine infusion was associated with natriuresis and increased sodium delivery to, and decreased reabsorption in, the collecting duct. No changes from control function were found with secretin. Renal blood flow was increased with acetylcholine (+82%, p < 0.001), but unchanged with secretin (+15%, nonsignificant). We conclude that acetylcholine natriuresis is due to inhibition of tubular reabsorption of sodium in the medullary collecting duct, as well as in upstream nephron segments. While the latter may be hemodynamically mediated, the former indicates a direct transport effect of the hormone in the terminal nephron segment.