N tau; -methylhistidine excretion is a valid index of myofibrillar protein breakdown in the guineapig (Cavia porcellus)

The recovery of radioactivity in the urine of guineapigs following a bolus intravenous dose of chromatographically pure 14 C-N τ -methylhistidine was measured in order to test whether the excretion of N τ -methylhistidine (N τ -MH) is a valid index of myofibrillar protein breakdown in these animals. Four male and four female guineapigs were dosed and after 7 days, 91.65 ± 2.82% and 3.58 ± 0.91% of injected radioactivity was recovered in the excreta and tissues, respectively. The average total recovery of 95.2 ± 3.0% was not significantly different from 100%. Male guineapigs excreted the radioactivity more slowly than females (70% of the dose excreted within 74 h vs 39 h, respectively) but cumulative excretion at 7 days was the same for each sex. Chromatographic analysis of the urine showed almost all of the radioactivity to be associated with a single peak corresponding to N τ -MH, indicating a lack of significant metabolism. These data show that although the clearance of 14 C-N τ -MH is slower than in rats or humans the urinary excretion of N τ -MH is a valid index for myofibrillar protein degradation in the guineapig.