Renal cortical basolateral Na+/HCO3- cotransporter: II. Detection of conformational changes with fluorescein isothiocyanate labeling

The sodium bicarbonate cotransporter: structure, function, and regulation

The role of the Na(+)-coupled HCO(3)(-) transporter (NBC) family is indispensable in acid-base homeostasis. Almost all tissues express a member of the NBC family. NBC has been studied extensively in the kidney and plays a role in proximal tubule HCO(3)(-) reabsorption. Although the exact function of this transporter family on other tissues is not very clear, the ubiquitous expression of NBC family suggests a role in cell pH regulation. Altered NBC activity caused by mutations of the gene responsible for NBC protein expression results in pathophysiologic conditions. Mutations of NBC resulting in important clinical disorders have been reported extensively on one member of the NBC family, the kidney NBC (NBC1). These mutations have led to several structural studies to understand the mechanism of the abnormal NBC1 activity.

Extracellular HCO(3)(-) dependence of electrogenic Na/HCO(3) cotransporters cloned from salamander and rat kidney

We studied the extracellular [HCOabstract (3) (-)] dependence of two renal clones of the electrogenic Na/HCO(3) cotransporter (NBC) heterologously expressed in Xenopus oocytes. We used microelectrodes to measure the change in membrane potential (DeltaV(m)) elicited by the NBC cloned from the kidney of the salamander Ambystoma tigrinum (akNBC) and by the NBC cloned from the kidney of rat (rkNBC). We used a two-electrode voltage clamp to measure the change in current (DeltaI) elicited by rkNBC. Briefly exposing an NBC-expressing oocyte to HCOabstract (3 )(-)/CO(2) (0.33-99 mM HCOabstract (3)(-), pH(o) 7.5) elicited an immediate, DIDS (4, 4-diisothiocyanatostilbene-2,2-disulfonic acid)-sensitive and Na(+)-dependent hyperpolarization (or outward current). In DeltaV(m) experiments, the apparent K(m ) for HCOabstract (3)(-) of akNBC (10. 6 mM) and rkNBC (10.8 mM) were similar. However, under voltage-clamp conditions, the apparent K(m) for HCOabstract (3)(-) of rkNBC was less (6.5 mM). Because it has been reported that SOabstract (3)(=)/HSO abstract (3)(-) stimulates Na/HCO(3 ) cotransport in renal membrane vesicles (a result that supports the existence of a COabstract (3)(=) binding site with which SOabstract (3)(=) interacts), we examined the effect of SOabstract (3)(=)/HSO abstract (3)(-) on rkNBC. In voltage-clamp studies, we found that neither 33 mM SOabstract (4)(=) nor 33 mM SOabstract (3) (=)/HSOabstract (3)(-) substantially affects the apparent K(m) for HCO abstract (3)(-). We also used microelectrodes to monitor intracellular pH (pH(i)) while exposing rkNBC-expressing oocytes to 3.3 mM HCOabstract (3 )(-)/0.5% CO(2). We found that SO abstract (3)(=)/HSOabstract (3 )(-) did not significantly affect the DIDS-sensitive component of the pH(i) recovery from the initial CO(2 )-induced acidification. We also monitored the rkNBC current while simultaneously varying [CO(2)](o), pH(o), and [COabstract (3)(=)](o) at a fixed [HCOabstract (3)(-)](o) of 33 mM. A Michaelis-Menten equation poorly fitted the data expressed as current versus [COabstract (3)(=)](o ). However, a pH titration curve nicely fitted the data expressed as current versus pH(o). Thus, rkNBC expressed in Xenopus oocytes does not appear to interact with SOabstract (3 )(=), HSOabstract (3)(-), or COabstract (3)(=).

Renal cortical basolateral Na+/HCO3- cotransporter. III. Evidence for a regulatory protein in the inhibitory effect of protein kinase A

The activity of the Na-H antiporter is inhibited by cyclic AMP-dependent protein kinase A (cAMP-PKA). The inhibitory effect of PKA on the Na-H antiporter is mediated through a regulatory protein that can be dissociated from the antiporter by limited protein digestion. PKA also inhibits the activity of the Na+/HCO3- cotransporter. We investigated whether the activity of Na+/HCO3- cotransporter and the effect of PKA on this transporter may also be regulated by limited protein digestion. In rabbit renal cortical basolateral membranes (BLM) and in solubilized BLM reconstituted in liposomes (proteoliposomes), trypsin (100 micrograms) increased 22Na uptake in the presence of HCO3 but not in the presence of gluconate, indicating that trypsin does not alter diffusive 22Na uptake but directly stimulates the Na+/HCO3- cotransporter activity. In proteoliposomes phosphorylated with ATP, the catalytic subunit (CSU) of cAMP-PKA decreased the activity of the Na+/HCO3- cotransporter (expressed as nanomoles/mg protein/3s) from 23 +/- 10 to 14 +/- 6 (P < 0.01). In the presence of trypsin, the inhibitory effect of CSU of cAMP-PKA on the activity of Na+/HCO3- cotransporter was blunted. To identify a fraction that was responsible for the inhibitory effect of the CSU on the Na+/HCO3- cotransporter activity, solubilized proteins were separated by size exclusion chromatography. The effect of CSU of cAMP-PKA on the Na+/HCO3- cotransporter activity was assayed in proteoliposomes digested with trypsin with the addition of a fraction containing the 42 kDa protein (fraction S+) or without the 42 kDa protein (fraction S-).(ABSTRACT TRUNCATED AT 250 WORDS)