The Na(+)-HCO(3)(-) cotransporter NBCn1 (SLC4A7) has multiple variants depending upon splice domains in the cytoplasmic amino- and carboxy-termini of the protein. In this study, we examined the role of the amino-terminal splice domain containing 123 amino acids (cassette II) in the regulation of NBCn1 function and expression. Polymerase chain reaction detected NBCn1 mRNAs containing cassette II in a variety of tissues. Two variants, NBCn1-B containing cassette II and NBCn1-E lacking cassette II, were expressed in Xenopus oocytes and assessed by two-electrode voltage clamp to measure the ionic current mediated by the transporters. The two variants showed similar current-voltage (I-V) relations when measured 3-4 days after RNA injection. Replacment of Cl() with gluconate did not affect the I-V relations. When exposed to solutions containing 20-50 mm Na(+), the current produced by NBCn1-B was slightly more positive than that produced by NBCn1-E. The two currents were similar at 100 mm Na(+). The slope conductances for the two variants were progressively increased at higher Na(+) levels, and the increases were parallel and superimposed. Measured at different time points after RNA injection, NBCn1-B produced lower conductance than NBCn1-E at 24-48 h. Protein expression of NBCn1-B was also low at these time points as determined by immunoblot of oocyte membrane preparation. Expressed in opossum kidney (OK) cells, NBCn1-E caused a 1.5-fold increase in ouabain-sensitive production of p-nitrophenol from p-phenyl phosphate compared with control preparations, whereas NBCn1-B had negligible effect. We conclude that the primary function of cassette II is to reduce NBCn1 protein expression.