Unique localization of mRNA encoding plasma membrane Ca²⁺ pump isoform 3 in rat thin descending loop of Henle

In our previous study of plasma membrane Ca²⁺ pump (PMCA) isozymes in the rat kidney, we found that the mRNA coding isoform PMCA3 was detected primarily in the outer medullary zone of rat kidney tissue. We now investigated the location of the mRNAs coding for isoforms PMCA3 and PMCA4 of Ca²⁺ pump in the nephron segments that are present in outer medullary parenchyma using the method of reverse transcription and polymerase chain reaction. In mRNA extracted from whole dissected outer medulla we detected mRNA encoding three splice variants (a, b, and c) of isoform PMCA3; isoform PMCA4 was found in the outer medulla almost exclusively as variant b. Analysis of mRNA from microdissected tubule segments show that proximal straight tubules (PST), medullary thick ascending limbs, outer medullary collecting ducts, and descending thin limb of Henle's loop (DTL) all contained mRNA for isoform 4b. In contrast, the mRNA encoding isoform 3 was detected exclusively in DTL and not in other nephron segments. The unique presence of isoform 3 in DTL is rather surprising, since the specific role of this nephron segment in vectorial Ca²⁺ transport or in intracellular Ca²⁺ signaling is not known. The data suggest that isoform 3 in cells of the DTL may have a hitherto unrecognized specific role in Ca²⁺ signaling or transport of Ca²⁺, which is distinct from the role of the isoforms of the PMCA in all other nephron segments.