Biology of Reproduction, Vol 23, 294-304, Copyright © 1980 by Society for the Study of Reproduction

Carnitine Transport into the Perfused Epididymis of the Rat: Regional Differences, Stereospecificity, Stimulation by Choline, and the Effect of Other Luminal Factors

¹ Department of Physiology & Biochemistry, The University of Reading, Whiteknights, Reading RG6 2AJ, England, U. K.

The transfer of carnitine from blood into the perfused, sperm-free lumen of segments of the epididymis has been examined in anesthetized rats. During i.v. infusion of DL-[³ H]-carnitine, radioactive material with chromatographic properties similar to those of free carnitine appeared in perfusates flowing at 1.0-l.3 µl/min through the distal cauda epididymidis within 30 min and slowly attained plateau values at 2-4 h. The entry rate showed a significant (P<0.001) positive and linear correlation with length of the perfused epididymis. Carnitine transport was independent of the tonicity of the perfusing solution (290, 310, or 330 mOSM) and was not altered when the following components of epididymal plasma were added individually to the perfusing solution to produce the same final tonicity (310 mOSM): DL-carnitine (60 mM), L-carnitine (20 mM), phosphocholine (20 mM), inositol (50 mM). Perfusing solutions containing glyceropholsphocholine (20 on 40 mM) significantly increased (P<0.01; P<0.02, respectively) carnitine entry, whereas addition of albumin (40 mg/ml) significantly reduced (P<0.01) carnitine entry. Choline (20 mM) and betaine (20 mM), structural analogues of carnitine, both significantly augmented (P<0.001) carnitine transfer from blood to the distal cauda to 245 and 208%, respectively, whereas glycine and -butyrobetaine at the same concentration were without effect. There were regional differences in the transport indices for carnitine, the rate being greatest in the mid-corpus and intermediate in the proximal cauda, exceeding that in the distal cauda epididymidis by approximately 4:2:1, respectively, at the end of the infusion. Luminal choline (20 mM) significantly increased both the transfer of radioactivity from circulating DL-[³ H]-carnitine into the lumen of the corpus epididymidis and the secretion of carnitine into the perfusate from 200 to 350 pmol/h/cm. In dual isotope infusions of the enantiomers of carnitine, the transport of L-carnitine into the distal cauda epididymidis exceeded that of the D-form 14-fold, and choline (20m mM) in the perfusing solution stimulated the entry of both, but to different extents. A model for the transport of carnitine across the epithelium of the epididymis is proposed.

Note:
ACKNOWLEDGMENTS We are grateful to Dr. W.C.L. Ford and Dr. D. W. Hamilton for helpful comments.

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