Neutral amino acid absorption by the rat epididymis

The technique of stopped-flow/split-drop microperfusion was used to study the absorption of the neutral amino acid alpha-aminoisobutyric acid (AIB) from different epididymal regions of the rat. Absorption of AIB from the lumen of the caput, corpus, and cauda was saturable and time-dependent. The apparent Km values for each of the regions studied were similar (approximately 6 mM), whereas the Vmax values were progressively higher from caput, corpus, and cauda, respectively. Absorption of AIB from the lumina of the caput, corpus, and cauda epididymidis was linear over 60 min. The absorption of AIB from the lumen of the caput was sodium-dependent and inhibitable by 2-methyl-alpha-aminoisobutyric acid (MeAIB), a specific inhibitor of neutral amino acid transport. Similarly, absorption of AIB from the lumen of the corpus epididymidis was sodium-dependent; however, uptake was not significantly reduced in the presence of MeAIB. Absorption of AIB from the lumen of the cauda epididymidis was neither sodium-dependent nor inhibitable by MeAIB. It is suggested that neutral amino acid absorption involves different transport carriers in different epididymal regions. These findings also support our previous observations that there exists a selective permeability barrier from lumen to blood along the epididymal duct.