Bicarbonate transport in primary cell cultures of chick proximal tubule
University of Delaware
Bicarbonate (HCO₃ˉ) is an important component of acid/base regulation. Therefore, it is crucial to understand the mechanism of bicarbonate reabsorption in the kidney proximal tubule (PT). In mammals the reabsorption mechanism is well understood, but evidence suggests that birds may use a different mechanism. We hypothesize that in the avian PT HCO3- crosses the luminal membrane in ionic form, rather than as CO2, and that HCO3- ions then leave the basolateral side via an NBC1-like transporter, as in the mammalian PT. The experiments used primary cell cultures of chick PT and electrophysiological studies on these monolayers to measure currents (ISC) associated with ion transport. Monolayers were first stimulated with 1μM forskolin, which activated a chloride secretory current in the avian PT. This was followed by basolateral application of 100 μM DIDS, an inhibitor of NBC1. With bicarbonate in the bathing solution DIDS caused an increase in ISC of 6.25 + 1.565 μAmps/cm2 (n = 6 ), but only 2.30 + 0.58 μAmps/cm2 (n = 5) in the absence of bicarbonate. The increased ISC is consistent with inhibition of electrogenic transport of HCO3- ions via the NBC1 transporter, and the decreased effect of DIDS in the nominal absence of bicarbonate further supports this. Additional studies support both mRNA expression and NBC1 protein expression (western blotting) in chick PT cultures and native tissue. Experiments are underway to test for an alternative apical transporter.