Regional rates of cerebral protein synthesis measured with L-[1-11C]leucine and PET in conscious, young adult men: normal values, variability, and reproducibility

Abstract

We report regional rates of cerebral protein synthesis (rCPS) measured with the fully quantitative L-[1-(11)C]leucine positron emission tomography (PET) method. The method accounts for the fraction (lambda) of unlabeled amino acids in the precursor pool for protein synthesis derived from arterial plasma; the remainder (1-lambda) comes from tissue proteolysis. We determined rCPS and lambda in 18 regions and whole brain in 10 healthy men (21 to 24 years). Subjects underwent two 90-min dynamic PET studies with arterial blood sampling at least 2 weeks apart. Rates of cerebral protein synthesis varied regionally and ranged from 0.97+/-0.70 to 2.25+/-0.20 nmol/g per min. Values of rCPS were in good agreement between the two PET studies. Mean differences in rCPS between studies ranged from 9% in cortical regions to 15% in white matter. The lambda value was comparatively more uniform across regions, ranging from 0.63+/-0.03 to 0.79+/-0.02. Mean differences in lambda between studies were 2% to 8%. Intersubject variability in rCPS was on average 6% in cortical areas, 9% in subcortical regions, and 12% in white matter; intersubject variability in lambda was 2% to 8%. Our data indicate that in human subjects low variance and highly reproducible measures of rCPS can be made with the L-[1-(11)C]leucine PET method.

Figure 1

Compartmental model for the l-[1-¹¹C]leucine PET method. The exchangeable (unlabeled-C_E or labeled-$C_E^{\ast }$) leucine pool in the brain includes intra- and extracellular free leucine and intracellular tRNA-bound leucine. K₁ and k₂ are the rate constants for carrier-mediated transport of leucine from plasma to tissue and back, respectively. k₃ is the rate constant for the first two steps in leucine catabolism, transamination, and decarboxylation. Following an injection of leucine labeled on the carboxyl carbon the only possible labeled metabolites in brain are α-KIC, CO₂, and products of CO₂ fixation. Because there is very little labeled α-KIC in brain (Keen et al, 1989), this pool is not explicitly represented in the model and k₃ combines the transamination and decarboxylation reactions. k₄ and k₅ are the rate constants for leucine incorporation into protein and for the release of free leucine from proteolysis, respectively. Unlabeled leucine and protein are assumed to be in steady state, so that k₄C_E = k₅P. Labeled leucine is administered as a short infusion at the beginning of the study and is, therefore, not in steady state. Because of the long half-life of protein in brain (Lajtha et al, 1976), it is assumed that there is no significant breakdown of labeled product (P*) during the experimental interval, that is, k₅P* = 0. Labeled CO₂ arises either through catabolism of labeled leucine in brain or through influx from blood (C_c*) after catabolism in other tissue. We assume that fixation of ¹¹CO₂ is negligible during the experimental period (Buxton et al, 1987; Siesjo et al, 1964), and that diffusible ¹¹CO₂ in brain rapidly equilibrates with the arterial blood (Buxton et al, 1987).

Figure 2

Arterial clearance curves from one study. Open diamonds (◇) represent plasma [¹¹C]leucine concentration, solid squares (■) represent whole blood total ¹¹C concentration, and open triangles (△) represent whole blood ¹¹CO₂ concentration. (Inset A) Expanded abscissa shows peak values of plasma [¹¹C]leucine and whole blood total ¹¹C shortly after the end of the 2-min infusion. (Inset B) Expanded ordinate shows clearance of plasma [¹¹C]leucine and blood ¹¹CO₂, and slight increases in whole blood total ¹¹C concentration after 40 mins.

Figure 3

Time courses of activity in (A) temporal cortex and (B) corona radiata from one study. Open circles (○) and solid lines represent the measured and fitted total activity in the ROI, respectively. As indicated in the figure, dashed and dotted lines represent model estimates of total ¹¹C in blood in the brain, and concentrations of labeled protein, leucine, and ¹¹CO₂ in tissue. Parameter estimates in temporal cortex were K₁ = 0.048 mL/g per min, k₂ + k₃ = 0.071 min^(-1)), k₄ = 0.041 min^(-1), V_b = 0.078. The λ-value was 0.63 and rCPS was 2.00 nmol/g per min. Parameter estimates in corona radiata were K₁ = 0.028 mL/g per min, k₂ + k₃ = 0.065 min^(-1), k₄ = 0.033 min^(-1), V_b = 0.039. The λ-value was 0.67 and rCPS was 1.02 nmol/g per min. Tracer delay was 13 secs as estimated from the whole brain time-activity curve. Data are from the same study for which the arterial clearance curves are shown in Figure 2.

Figure 4

l-[1-¹¹C]Leucine PET (upper) and MR (lower) images from study of a 21-year old man. Sagittal view at the midline is shown on the left, coronal view at the level of the thalamus is shown in the center, and axial view is on the right. [¹¹C]Leucine PET images are color-coded for rCPS; color bar is to the right of the PET images. Slice thickness is 1.0 mm. rCPS was computed from the total activity in each voxel in each 5-min frame of data between 60 and 90 mins by use of an alternate equation for rCPS (equation 3 in Smith et al, 2005) and whole brain rate constants and λ estimated from this subject’s dynamic PET study, that is, K₁ = 0.040 mL/g per min, k₂ + k₃ = 0.106 min^(-1), k₄ = 0.035 min^(-1), λ = 0.75, V_b = 0.066. Rate of cerebral protein synthesis shown is the average of rCPS in the six frames of data. Bar in the upper left MR image is 10 cm and applies to all six images.