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Journal of Physiology (2002), 542.2, p. 335
© Copyright 2002 The Physiological Society
DOI: 10.1113/jphysiol.2002.020511
Email: patrickwong{at}cuhk.edu.hk
As early as 1966, Voglmayr et al. (1966) reported that a significant amount of fluid could be collected from a catheter implanted into the rete testis of conscious rams. The fluid was plasma-like in ionic content and originated from the seminiferous tubules by active secretion. The Sertoli cells were later identified as the source of the fluid. While the physiological role of the fluid - to carry the newly shed immature spermatozoa to the epididymis - was not hard to envision, it was not until 5 years later that micropuncture and micro-analytical techniques were applied to study the ionic mechanisms of fluid secretion by the seminiferous tubules.
Using a variation of the micropuncture technique, Tuck et al. (1970) found that the fluid secreted into oil columns, called the primary fluid (PF), contained a high K+ concentration but low Na+ and Cl- concentrations (Na+/K+/Cl-: 38/ 112/ 62 mequiv l-1). The HCO3- concentration was estimated to be ~88 mequiv l-1 (by inference from the difference between cations and Cl-). These values were significantly different from another fluid, called the free flow fluid (FFF), collected spontaneously from the tubule without prior injection of oil (Na+/K+/Cl-: 108/45/120 mequiv l-1). The authors also measured the transtubular potential (Vt) and found that the values differed under the two conditions. In the oil-filled segment of the tubule, Vt had a value of +1 mV whereas in the native tubule without oil injection, Vt was -7.4 mV (lumen negative). Based on these findings, Tuck et al. (1970) suggested that the seminiferous epithelium normally secretes a K+-rich solution which is mixed in the tubule with a Na+-rich low K+-containing fluid secreted by the rete testis. Since this time, little work has been done to prove or disprove this hypothesis which has prevailed and remained unchallenged to date.
Fisher (2002) in this issue of The Journal of Physiology re-examined the transtubular potentials in the rat seminiferous tubules microperfused with Ringer solution (ionic content resembling that of the FFF) at varying perfusion rates. The results show that Vt has a value of -5 mV (lumen negative) when the tubule was perfused at the slow rate of 0.14 µl min-1. Similar values were obtained when electrical measurements were made by alternative non-perfusion electrophysiological methods. At the end of these experiments, normal morphology of the seminiferous epithelium was found to be preserved in the tubules. However, when perfused at a high rate (> 0.42 µl min-1), Vt fell towards zero. Microscopic examination of these tubules revealed damage and stripping off of the seminiferous epithelium from the myoid cell layer. These results show that the seminiferous epithelium is very fragile and its integrity is vulnerable to mechanical insults associated with experimental manipulations. These results have therefore shaken the tenability of the admixture hypothesis of Tuck et al. (1970). It transpires that the low Vt and the associated high K+ but low Na+ and Cl- of the PF could have been the erroneous measurements caused by damage of the seminiferous epithelium by high hydrostatic pressure generated during oil injection. The Vt of -7 mV measured under free flow conditions (Tuck et al. 1970) is consistent with the values recorded in the slowly perfused tubules with the epithelium intact during perfusion (Fisher, 2002). These values presumably represent the more normal values prevailing in the intact tubules in vivo.
The paper by Fisher can explain the rather 'suspiciously' high K+ in the primary secreted fluid. Although epithelia which secrete K+-rich fluid do exist, a good example of which is the secretion of endolymph by the stria vascularis of the cochlea, the general belief is that most mammalian epithelia secrete an isotonic NaCl fluid based on the secretory model of Silva et al. (1977). According to this model, secondary active chloride transport is the driving force of fluid secretion. Using an equivalent circuit model, Cook & Young (1989) showed that location of 20 % of the total cell K+ conductance into the apical membrane of cells secreting NaCl increases secretory rate. Most mammalian secretory epithelial cells secrete a basically isotonic NaCl fluid with K+ concentration a few times higher than the blood concentration. Most of these cells possess apical K+ channels which could be responsible for the relatively high K+ in their secretions. Hence, secretion of a near-isotonic NaCl fluid with elevated K+ concentration by the modified Silva model of secretion could account for the ionic content of the FFF with Na+/K+/Cl-: 108/45/ 120 mequiv l-1. The germ cells in the seminiferous epithelium may also have contributed to the high K+ in the seminiferous tubular fluid. It has been shown that germ cells express K+ channels in their membrane. During spermiogenesis, round spermatids undergo cell volume contraction by losing intracellular fluid. This process would add K+ into the lumen of the tubule. Seminiferous tubules devoid of germ cells have been known to secrete a fluid rich in Na+ instead of K+.
Although Tuck et al. (1970) predicted a high HCO3- concentration (88 mequiv l-1) in the PF, later micropuncture work failed to confirm this presumption. Carbonic anhydrase, an enzyme required for the generation and transport of HCO3- is not present in the seminiferous tubules albeit in the interstitial cells, arguing against HCO3- being the major secreted anion. Using cultured rat Sertoli cell epithelia, Ko et al. (1998) showed that apical ATP stimulates an inward short circuit current (Isc) carried by Cl-. The ATP-induced Isc is not sensitive to acetazolamide, an inhibitor of carbonic anhydrase, or baliofomycin, an inhibitor of H+-ATPase, but is sensitive to apical N-phenylanthranilic acid (DPC) or DIDS, blockers of Cl- channels. Removal of extracellular HCO3- also has little effect on the agonist-simulated Isc whereas removal of Cl- abolishes the Isc response. Their results show that Sertoli cells secrete Cl- rather than HCO3-.
The work of Fisher (2002) has therefore thrown new light on the secretion of seminiferous tubular fluid and helped resolve many long-standing controversies. The paper has laid down a solid framework upon which further work can be built to unravel the secretory mechanisms of the seminiferous tubules.
REFERENCES
COOK, D.I. & YOUNG, J.A. (1989). Journal of Membrane Biology 110, 139-146.
[Medline]
FISHER, D. (2002). Journal of Physiology 542, 445-452.
[Abstract/Full Text]
KO, W.H., CHAN, H.C., CHEW, S.B. & WONG, P.Y.D. (1998). Journal of Physiology 512, 471-480.
[Abstract/Full Text]
SILVA, P., STOFF, J., FIELD, M., FINE, L., FORREST, J.N. & EPSTEIN, F.H. (1977). American Journal of Physiology 233, F298-306.
[Medline]
TUCK, R.R., SETCHELL, B.P., WAITES, G.M.H. & YOUNG, J.A. (1970). Pflugers Archiv 318, 225-243.
[Medline]
VOGLMAYR, J.K., WAITES, G.M.H. & SETCHELL, B.P. (1966). Nature 210, 861-863.
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