Contribution of renal medullary mitochondrial density to urinary concentrating ability in mammals.

This study explores the impact of the metabolic rate on the urinary concentration abilities of mammals with different body sizes. The researchers discovered that size and metabolic rate affect the density of mitochondria within the renal medullary cells, possibly influencing renal function.

Objective of the Research

• The researchers of this study aimed to understand why mammals with larger bodies and longer loops of Henle (a part of the nephron in the kidney) do not have a proportionally increased urinary concentrating ability. Their investigation led them to test whether the decline in metabolic rate, which accompanies an increase in body mass, could be responsible for lower urinary concentration in larger mammals.

Methodology of the Research

• The team studied the ultrastructural parameters of mitochondria in the medullary thick ascending limbs (mTALs) of nine different mammalian species. These species ranged significantly in size, from bats with a body mass of 0.011 kg to horses which can weigh approximately 400 kg.
• The parameters measured included the volume of mitochondria as a percentage of mTAL cell volume, the internal mitochondrial membrane area per volume of mitochondrion, and the basolateral membrane area per unit mTAL cellular volume.

Primary Findings

• The research showed that mitochondrial volume as a percent of mTAL cellular volume decreased with increasing body mass. Similarly, the inner mitochondrial membrane area per volume of mitochondrion, as well as the basolateral membrane area per unit mTAL cellular volume, also declined with increasing body mass.
• It was concluded that not only did mitochondria occupy more volume in mTAL cells of smaller mammals, but that the mitochondria of these animals were more densely packed with cristae (inward expansions of the inner mitochondrial membrane, responsible for mitochondrial function).
• The team also noted that the decline in the inner mitochondrial membrane area and basolateral membrane area per volume of mTAL cell could explain, at least partly, the relationship between body mass and renal concentrating ability. This led to the suggestion that lower mitochondrial membrane density in mTAL cells of larger animals might be a factor limiting their urinary concentration capability.