1628 CELLULAR GROWTH IN THE MATURING RABBIT KIDNEY CORTICAL COLLECTING TUBULE (CCT)

Abstract

Postnatal development of the proximal nephron is characterized by non-linear growth as a function of time (AJP 245:F391, 1983). We were interested in describing maturation of the CCT, a distal segment whose embryological origin differs from that of the proximal tubule. Mid-CCTs from newborn (NB), 1 month old, and adult rabbits (n=4-7 animals per group) were isolated. Tubules (5-15 per animal) were measured for length and diameter, cell number (acridine orange), dry weight (quartz fiber balance) and K+ content (helium glow photometry). The number of cells/mm tubular length was similar in the NB and adult (534 ± 34 and 551 ± 22). Cell growth was characterized by 50-60% increases (p < 0.05) in volume (1105 ± 85 to 1633 ± 82 μm3/cell), dry weight (169 ± 15 to 260 ± 23 pg/cell), and K+ content (0.106±0.011 to 0.164±0.011 pmol/cell), with changes apparent only after 1 month of age. Cellular accretion of protein and other solids was estimated to account for at least 90% of the observed weight gain. We calculated K+ concentration in both the NB and adult to be approximately 115 mEq/L cell water. Thus, postnatal maturation of the mid-CCT is characterized by surges in cell volume, dry weight, and K+ content after the first month of life. The rise in K+/cell reflects primarily enhanced protein binding rather than addition of free cytosolic K+. The increase in CCT diameter (27.4 ± 0.5 to 33.9 ± 0.4 μm; p < 0.05) results from cell hypertrophy. Since there was no change in the number of cells/mm tubular length, axial growth of the CCT (3-fold) must occur by hyperplasia, probably in the superficial cortex. CCT growth may follow the postnatal surge in SNGFR and the opening of newly functioning superficial nephrons.