CONTRIBUTION OF RENAL MEDULLARY MITOCHONDRIAL DENSITY TO URINARY CONCENTRATING ABILITY IN MAMMALS

In mammals, the length of the loops of Henle increases with increasing body size without a concomitant rise in urinary concentrating ability. Because mass-specific metabolic rate falls with increasing body mass, this study sought to determine the extent to which this decline in metabolic rate could explain the low urinary concentrating ability of large mammals with long loops of Henle. Mitochondrial ultrastructural parameters were measured in the medullary thick ascending limbs (mTALs) of a series of nine mammalian genera ranging in body mass from 0.011 kg (bats) to approximately 400 kg (horses). The volume of mitochondria as a percent of mTAL cellular volume declined with increasing body mass (M(b)-0.056). Inner mitochondrial membrane area per volume of mitochondrion also declined with increasing body mass (M(b)-0.034), as did basolateral membrane area per unit mTAL cellular volume (M(b)-0.075). Thus, not only do mitochondria occupy more volume of mTAL cells of smaller mammals, but those mitochondria are also more densely packed with cristae. Inner mitochondrial membrane area per unit volume of mTAL cell cytoplasm scaled as M(b)-0.092. The decline in inner mitochondrial membrane area and basolateral membrane area per volume of mTAL cell may explain at least in part the relationship between body mass and renal concentrating ability in mammals of different sizes.