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-independent
HCO
3 transport
1 Renal Unit,
2 Program in Membrane Biology, and
4 Department of Pathology,
The lumen of the
epididymis is the site where spermatozoa undergo their final maturation
and acquire the capacity to become motile. An acidic luminal fluid is
required for the maintenance of sperm quiescence and for the prevention
of premature activation of acrosomal enzymes during their storage in
the cauda epididymis and vas deferens. We have previously demonstrated
that a vacuolar H+-ATPase
[proton pump (PP)] is present in the apical pole of apical and narrow cells in the caput epididymis and of clear cells in the
corpus and cauda epididymis and that this PP is responsible for the
majority of proton secretion in the proximal vas deferens. We now show
that PP-rich cells in the vas deferens express a high level of carbonic
anhydrase type II (CAII) and that acetazolamide markedly inhibits the
rate of proton secretion by 46.2 ± 6.1%. The rate of
acidification was independent of
Cl
and was strongly
inhibited by SITS under both normal and
Cl
-free conditions (50.6 ± 5.0 and 57.5 ± 6.0%, respectively). In the presence of
Cl
,
diphenylamine-2-carboxylate (DPC) had no effect, whereas SITS inhibited
proton secretion by 63.7 ± 11.3% when applied together with DPC. In Cl
-free
solution, DPC markedly inhibited proton efflux by 45.1 ± 7.6%,
SITS produced an additional inhibition of 18.2 ± 6.6%, and bafilomycin had no additive effect. In conclusion, we propose that CAII
plays a major role in proton secretion by the proximal vas deferens.
Acidification does not require the presence of
Cl
, but DPC-sensitive
Cl
channels might
contribute to basolateral extrusion of
HCO
3 under
Cl
-free conditions. The
inhibition by SITS observed under both normal and
Cl
-free conditions
indicates that a
Cl
/HCO
3
exchanger is not involved and that an alternative
HCO
3 transporter participates in proton secretion in the proximal vas deferens.
carbonic anhydrase II; self-referencing proton-selective electrode; immunocytochemistry; hydrogen-adenosine 5'-triphosphatase
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