Extreme blood pressure polymorphisms influence the bicarbonate‐independent conductance of NBCn1 but not its Na/HCO 3 cotransport activity

Genome‐wide association studies identified SLC4A7 as a locus associated with extreme blood pressure levels (Ehret G.B. et al. Nature. 2011 vol. 478 p.103). The product of this gene, the electroneutral Na + ‐coupled HCO 3 − cotransporter NBCn1 is an important mediator of acid–base movement wherever it is expressed, which includes epithelial tissues, neurons and vascular smooth muscle. In addition to Na/HCO 3 cotransport, NBCn1 mediates a steady‐state conductance ( G n1 ) that is independent of HCO 3 − transport. Exome sequencing of SLC4A7 was conducted among African and European American individuals from the Atherosclerosis Risk in Communities (ARIC) study. Individuals with whole exome sequence data were selected from the extremes (above upper quartile and below lower quartile) of the systolic and diastolic blood pressure distributions. In total, 42 single‐nucleotide polymorphisms (SNPs) associated with extremes of blood pressure were found within SLC4A7 . Six SNPs located in the NBCn1 coding sequence (these result in the E252D, L261M, E326K, T563A, N756S, and P896S missense mutations) were selected for insertion into the human NBCn1‐B variant to test their function when expressed in Xenopus oocytes. All six NBCn1‐B mutants exhibited a near‐normal Na/HCO 3 cotransport activity as determined by the rate of intracellular pH (pH i ) recovery from a CO 2 ‐induced acidification. However, when the G n1 is normalized to cotransport activity to control for surface presentation, five of the mutants displayed a 12–35% reduction in the measured G n1 . The sixth mutation, P896S, which resides in the fourth extracellular loop (EL4) of NBCn1, caused a reduction in G n1 of 84% on the wild‐type E326 background and 76% on the extreme blood‐pressure–associated K326 background. We conclude from this work that the polymorphisms we tested have little, if any effect on the Na/HCO 3 cotransport rate, but can have substantial effects on the G n1 . Thus, these polymorphisms would not be expected to have a major, direct effect on pH i , but would be expected to affect cell membrane voltage ( V m ). In particular, reduced G n1 would be expected to hyperpolarize cells (i.e., shift V m toward more negative voltages). Such shifts in V m would be expected to secondarily affect a wide range of ion channels and electrogenic solute carriers in the SLC superfamily. Support or Funding Information R01‐HL090969, R01‐DK30344, R01‐DK81567