Sperm duct epithelium of brook trout: Na⁺ transport and seminal plasma composition

The transport of Na + by the brook trout sperm duct (ductus deferens) was examined in vitro. Active Na + uptake (averaging 0.6 μequiv.∙cm −2 ∙h −1 ) was not significantly different from transepithelial short-circuit current, indicating that the short-circuit current may be a measure of Na + net transport. Transepithelial conductance was 4.1 ± 0.2 mS∙cm −2 ; an initially high short-circuit current and transepithelial potential (4–7 mV, serosa positive) dropped to near zero in the 1st hour in vitro. Short-circuit current and net Na + transport were stimulated by cyclic adenosine monophosphate and 3-isobuty-1-methylxanthine; transient stimulations of short-circuit current were elicited using epinephrine (lumenally) and by acetylcholine. Brook trout seminal plasma contained high K + (25 mM) and low Na + (119 mM), relative to blood plasma, in part a result of ion transport by the sperm duct epithelium. Seminal plasma composition in turn ensures quiescence of maturing spermatozoa stored in the sperm duct for several weeks prior to spawning.