MESSENGER-RNA ENCODING 4 ISOFORMS OF THE PLASMA-MEMBRANE CALCIUM-PUMPAND THEIR VARIANTS IN RAT-KIDNEY AND NEPHRON SEGMENTS

To survey the presence of the four different isoforms of the plasma me mbrane calcium pump (PMCA) and their alternative splicing variants in the rat kidney, three major zones (cortex, outer medulla, and inner me dulla) were macrodissected and probed for the presence of mRNA encodin g these isoforms and their variants at the splicing site C by using re verse transcription-polymerase chain reaction (RT-PCR). Both the corte x and the outer medulla showed PMCA 1b, 2b, 3(a and c), and 4b. Semiqu antitative comparisons indicated that isoform 2b is more abundant in t he cortex than in the outer medulla and conversely, that isoform 3 (a and c) is more abundant in the outer medulla than in the cortex. The i nner medulla showed only mRNA for isoforms 1b and 4b. The nephron segm ents present in the cortex and outer medulla were microdissected and a nalyzed by RT-PCR. Isoforms 1b, 2b, and 4b were found in all nephron s egments but were found more frequently in tubular segments with high r ates of Ca2+ reabsorption, suggesting that these isoforms may be invol ved in transepithelial transport. On the other hand, mRNA encoding iso form 3 (a and c) was most abundant in descending thin limb of Henle bu t was detected also in glomeruli and cortical thin ascending limb. Its distinct localization suggests that this isoform might have another f unction, such as in intracellular signalling.