Growing subjects must conserve K. The final regulation of K homeostasis occurs in the CCD which, in the adult, secretes K at high rates. No net K secretion, however, was detected in CCDs isolated from animals ≤2 wks of age and microperfused in vitro (Am. J. Physiol. 266:F57, 1994). K secretion in the fully differentiated CCD results from the active uptake of K into principal cells via basolateral Na-K-ATPase and its passive diffusion through apical K secretory channels into the urinary fluid. ROMK, a cDNA encoding a family of K channels, was recently cloned from rat kidney; these channels are likely candidates for the CCD K secretory channels. To determine whether expression of K secretory channels is developmentally regulated in the CCD, we tested for the presence of ROMK mRNA in single CCDs isolated from maturing (1 to 5 wks) rabbits. RT-PCR was performed on 5-7 mm total length of CCD per sample. For each sample, half of the reverse transcribed cDNA was amplified using ROMK-specific primers and the other half amplified usingβ-actin-specific primers. PCR products were size-fractionated by agarose gel electrophoresis and detected by ethidium bromide staining. An amplification product of expected size for ROMK (303 bp) was obtained in CCDs isolated from rabbits ≥3 wks of age (n=14) but not from those ≤2 wks(n=4), although an actin transcript was detected in all of the latter samples. In contrast, RT-PCR of whole kidney cortex revealed the ROMK amplification product at all ages (n=3 for ≤2 wks; n=9 for ≥3 wks). The identity of this amplification product to ROMK was verified by nucleotide sequencing.

Thus, ROMK mRNA is present in kidney cortex and individual CCDs of rabbits≥3 wks of age. In contrast, ROMK mRNA is detected in whole cortex but not in CCDs of rabbits ≤2 wks of age, implying that K secretory channels are being transcribed early in life in nephron segments other than the CCD. In sum, our results suggest that the K retention characteristic of the neonate is due, at least in part, to a paucity of ROMK secretory channels in the CCD.