RELATIONSHIP BETWEEN BODY-MASS, TISSUE METABOLIC-RATE, AND SODIUM-PUMP ACTIVITY IN MAMMALIAN LIVER AND KIDNEY

The allometric relationship between body mass and tissue metabolism wa s examined in liver and kidney cortex slices from mouse, rat, rabbit, sheep, and cattle, representing an similar to 12,000-fold difference i n body mass and an 11-fold difference in mass-specific basal metabolic rate. Larger mammals have lower tissue metabolic rates (TMR; mu mol O -2 . g wet wt(-1). min(-1)) at 37 degrees C, yielding the equations TM R = 3.6 M(-0.21) for liver slices and TMR = 3.2 M(-0.11) for kidney co rtex slices, where M is body mass in grams. This appears to be an intr insic property of the tissue and is not due to differences in extracel lular space or tissue protein content, because these are relatively co nstant in all mammalian species examined. The allometric relationships remain when tissue metabolism is expressed relative to ''active cell mass'' in tissue slices. Potassium uptake rate (KUR; mu mol K+. g wet wt(-1). min(-1)) was also measured (as Rb-86(+) uptake) and was also l ower in larger mammals, yielding the equations KUR = 1.2 M(-0.14) in, liver slices and KUR = 3.4 M(-0.13) for kidney cortex slices. The ener getic costs of sodium pump activity were estimated to be <10% of TMR f or liver and kidney cortex from all five mammalian species.