Uptake and release of free <scp>l</scp> -carnitine by boar epididymal spermatozoa <i>in vitro</i> and subsequent acetylation rate

Abstract

In the male reproductive tract, very high concentrations (mmol l-1) of free L-carnitine and acetyl-L-carnitine are found in the epididymides, seminal plasma and spermatozoa. It has been reported that the uptake of free L-carnitine by spermatozoa might be related to the epididymal maturation of the sperm membrane, since a greater uptake was found by caput than by cauda spermatozoa in vitro. However, the free L-carnitine concentrations estimated inside the gametes were never greater than those of the surrounding medium. In this study, we investigated the mechanism of transport of free L-carnitine and its ester acetyl-L-carnitine, through the plasma membrane of mature and immature epididymal boar spermatozoa. In vitro, we found a passive diffusion of both compounds to the spermatozoa, whatever the maturation stage. The spermatozoa might progress in the epididymal lumen and accumulate high amounts of free L-carnitine. The active uptake of free L-carnitine occurs only across epididymal mucosa. These results are in agreement with those reported on cells of other organs that exchange pharmacological free L-carnitine concentrations (mmol l-1) by a passive mechanism through the plasma membrane. The acetylation of high amounts of free L-carnitine inside the spermatozoa was found only in caudal spermatozoa. This result suggests that oxidative metabolism (producing acetyl CoA) might be more active in mature cells. The acetyl-L-carnitine added to the incubation medium of boar spermatozoa was hydrolysed. Enzymatic activity of the sperm membrane is low and this may partially explain the low concentrations of acetyl-L-carnitine found in the caudal epididymal plasma.