Kinetics, molecular basis, and differentiation ofl-lactate transport in spermatogenic cells

Abstract

Round spermatid energy metabolism is closely dependent on the presence of l-lactate in the external medium. This l-lactate has been proposed to be supplied by Sertoli cells in the seminiferous tubules. l-Lactate, in conjunction with glucose, modulates intracellular Ca2+concentration in round spermatids and pachytene spermatocytes. In spite of this central role of l-lactate in spermatogenic cell physiology, the mechanism of l-lactate transport, as well as possible differentiation during spermatogenesis, has not been studied in these cells. By measuring radioactive l-lactate transport and intracellular pH (pHi) changes with pHifluorescent probes, we show that these cells transport l-lactate using monocarboxylate-H+transport (MCT) systems. RT-PCR, in situ mRNA hybridization, and immunocyto- and immunohistochemistry data show that pachytene spermatocytes express mainly the MCT1 and MCT4 isoforms of the transporter (intermediate- and low-affinity transporters, respectively), while round spermatids, besides MCT1 and MCT4, also show expression of the MCT2 isoform (high-affinity transporter). These molecular data are consistent with the kinetic data of l-lactate transport in these cells demonstrating at least two transport components for l-lactate. These separate transport components reflect the ability of these cells to switch between the generation of glycolytic l-lactate in the presence of external glucose and the use of l-lactate when this substrate is available in the external environment. The supply of these substrates is regulated by the hormonal control of Sertoli cell glycolytic activity.