Identification of block 5. Equation 8 is fit to data of Gorman et al. (23, 24) relating interstitial fluid norepinephrine (NE) concentration, N to M. The estimated value of KM is 22.767 μl O2·min−1·g−1·nM−1 for control, 7.072 μl O2·min−1·g−1·nM−1 for experiments with phentolamine, and 1.849 μl O2·min−1·g−1·nM−1 for experiments with phentolamine + propranolol. The estimated value of the y-intercept, M0, is 33.535 μl O2·min−1·g−1.