Renal tubular differentiation in mouse and mouse metanephric culture

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Abstract
Sodium-potassium adenosine triphosphatase (Na-K-ATPase) activity was measured by microassay, and the surface density of basolateral membranes was measured morphometrically in postglomerular segments of single tubules isolated from normally developing, intact mouse kidneys and from transfilter metanephric cultures. Proximal tubule Na-K-ATPase activity was 1092±480 pmol/mm per hour in newborn mice, increasing to 2462±258 in 1-week-old and 3470–578 pmol/mm per hour in adult mice. The Na-K-ATPase activity in newborn mice was approximately one-third of the activity in adult mice. Tubular Na-K-ATPase in transfilter metanephric culture was 972±536 pmol/mm per hour, a mean value almost identical to that in newborn mice. The surface density of basolateral cell membranes was 1.36±0.60 μm2/μm3 in newborn mice and 1.34±0.45 μm2/μm3 in 1-week-old mice, increasing to 2.70±0.98 μm2/μm3 in 4-week-old mice and 2.89±0.51 μm2/μm3 in adult mice. The surface density of tubular basolateral cell membranes in transfilter metanephric culture was 1.13±0.51 μm2/μm3, not significantly different from the surface density in newborn mice. The calculated mean surface area of basolateral membranes per unit tubular length was greater in cultures than in newborns, however, because total epithelial volume per unit length was significantly larger in the cultured tubules. Membrane surface area in intact mice increased with age, the surface area per unit length of tubule in adults being 4.6 times the area in newborn animals. The ratio of enzyme activity to membrane surface area more than doubled in the 1st week of life without any increase in the density or surface area of basolateral membranes. The ratio fell thereafter, as membrane area increased with maturation, to a value in the adult animal three-fourths of that in the newborn. The early postnatal increase in enzyme activity, beginning almost immediately after birth, possibly relates to an increased density of enzyme sites on the membrane. The postnatal spurt in enzyme activity, without a corresponding increase in membrane area, suggests that tubules have considerable functional reserve in generating a complement of sodium pumps. The studies of proximal tubules grown in metanephric culture show that the basolateral membranes and the sodium pump are initiated independently of renal blood flow and tubular fluid flow, presumably as inherent characteristics. The functional data confirm the similarity, already shown by morphometric studies, between natural tubules and those grown in culture.