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Transport of carnitine and acetylcarnitine by carnitine/organic cation transporter (OCTN) 2 and OCTN3 into epididymal spermatozoa

Division of Pharmaceutical Sciences, Department of Molecular Biopharmaceutics, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan, ¹ Faculty of Pharmaceutical Sciences, Tokyo University of Science, 2641 Yamasaki, Noda, Chiba 278-8510, Japan, ² Department of Histology and Embryology, Graduate School of Medical Science, Kanazawa University, Takara-machi, Kanazawa 920-8640, Japan and ³ Chugai Pharmaceutical Co. Ltd, Shizuoka, Japan

Correspondence should be addressed to A Tsuji; Email: tsuji{at}kenroku.kanazawa-u.ac.jp

Carnitine and acetylcarnitine are important for the acquisition of motility and maturation of spermatozoa in the epididymis. In this study, we examined the involvement of carnitine/organic cation transporter (OCTN) in carnitine and acetylcarnitine transport in epididymal spermatozoa of mice. Uptake of both compounds by epididymal spermatozoa was time-dependent and partially Na⁺-dependent. Kinetic analyses revealed the presence of a high-affinity transport system in the spermatozoa, with K_m values of 23.6 and 6.57 µM for carnitine and acetylcarnitine respectively in the presence of Na⁺. Expression of OCTN2 and OCTN3 in epididymal spermatozoa was confirmed by immunofluorescence analysis. The involvement of these two transporters in carnitine and acetylcarnitine transport was supported by a selective inhibition study. We conclude that both Na⁺-dependent and -independent carnitine transporters, OCTN2 and OCTN3, mediate the supply of carnitine and acetylcarnitine to epididymal spermatozoa in mice.