Na+ and Cl- transport by the urinary bladder of the freshwater rainbow trout (Oncorhynchus mykiss)
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Freshwater (FW) rainbow trout (Oncorhynchus mykiss) urinary bladders mounted in vitro under symmetrical saline conditions displayed electroneutral active absorption of Na(+) and Cl(-) from the mucosal side; the transepithelial potential (V(t)) was 0.1 mV, and the short-circuit current was less than 1 microA cm(-2). Removal of Na(+) from mucosal saline decreased Cl(-) absorption by 56% and removal of Cl(-) decreased Na(+) absorption by 69%. However, active net absorption of both Na(+) and Cl(-) was not abolished when Cl(-) or Na(+) was replaced with an impermeant ion (gluconate or choline, respectively). Under physiological conditions with artificial urine (¿Na(+) = 2.12 mM, ¿Cl(-) = 3.51 mM) bathing the mucosal surface and saline bathing the serosal surface, transepithelial potential (V(t)) increased to a serosal positive approximately +7.6 mV. Unidirectional influx rates of both Na(+) and Cl(-) were 10-20-fold lower but active absorption of both ions still occurred according to the Ussing flux ratio criterion. Replacement of Na(+) with choline, or Cl(-) with gluconate, in the mucosal artificial urine yielded no change in unidirectional influx of Cl(-) or Na(+), respectively. However, kinetic analyses indicated a decrease in maximum Na(+) transport rate (J(max)) of 66% with no change in affinity (K(m)) in the low Cl(-) mucosal solution relative to the control solution. Similarly, there was a 79% decrease in J(max) values for Cl(-), again with no change in K(m), in the low-Na(+) mucosal bathing. The mucosal addition of DIDS, amiloride or bumetanide (10(-4) M) had no effect on either Na(+) or Cl(-) transport, under either symmetrical saline or artificial urine/saline conditions. Addition of the three drugs simultaneously (10(-4) M), or chlorothiazide (10(-3) M), under symmetrical saline conditions also had no effect on Na(+) or Cl(-) transport rates. Cyanide (10(-3) M) addition to mucosal artificial urine caused a slowly developing decrease of Na(+) influx to 59% and Cl(-) influx to 50% in the period after drug addition. Na(+) and Cl(-) reabsorption appears to be a partially coupled process in the urinary bladder of O. mykiss; transport mechanisms are both dependent upon and independent of the other ion.
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