Propofol anesthesia does not alter regional rates of cerebral protein synthesis measured with L-[1-(11)C]leucine and PET in healthy male subjects

Abstract

We report regional rates of cerebral protein synthesis (rCPS) in 10 healthy young males, each studied under two conditions: awake and anesthetized with propofol. We used the quantitative L-[1-(11)C]leucine positron emission tomography (PET) method to measure rCPS. The method accounts for the fraction (lambda) of unlabeled leucine in the precursor pool for protein synthesis that is derived from arterial plasma; the remainder comes from proteolysis of tissue proteins. Across 18 regions and whole brain, mean differences in rCPS between studies ranged from -5% to 5% and were within the variability of rCPS in awake studies (coefficient of variation range: 7% to 14%). Similarly, differences in lambda (range: 1% to 4%) were typically within the variability of lambda (coefficient of variation range: 3% to 6%). Intersubject variances and patterns of regional variation were also similar under both conditions. In propofol-anesthetized subjects, rCPS varied regionally from 0.98+/-0.12 to 2.39+/-0.23 nmol g(-1) min(-1) in the corona radiata and in the cerebellum, respectively. Our data indicate that the values, variances, and patterns of regional variation in rCPS and lambda measured by the L-[1-(11)C]leucine PET method are not significantly altered by anesthesia with propofol.

Figure 1. Compartmental Model for the L-[1-¹¹C]Leucine PET Method

(A) The exchangeable leucine pool in the brain (unlabeled-C_E or labeled-C_E*) includes intra- and extra-cellular 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 labeled metabolites in brain are α-ketoisocaproic acid (α-KIC), CO₂, and products of CO₂ fixation. Since 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 (C_E) and protein (P) are assumed to be in a steady state. Labeled leucine is administered as a short infusion at the beginning of the study and is 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, i.e., k₅P* ≈ 0. Labeled CO₂ arises either through catabolism of labeled leucine in brain or through influx from blood following catabolism in other tissues. (B) We assume that fixation of ¹¹CO₂ is negligible during the experimental period (Buxton et al 1987; Siesjo et al 1965), and that diffusible ¹¹CO₂ in brain rapidly equilibrates with the arterial blood (Buxton et al 1987). Under these assumptions, the model for labeled leucine reduces to three tissue compartments, with the concentration in the tissue ¹¹CO₂ compartment known from measurements of ¹¹CO₂ in blood and the equilibrium brain:blood distribution ratio for ¹¹CO₂ (Smith et al 2005). The total concentration of ¹¹C in the field of view of the PET camera (C_T*) includes C_E*, P*, blood in the brain (V_bC_b*, where V_b is the fraction of the volume occupied by blood and C_b* is the concentration of activity in whole blood) and ¹¹CO₂ in brain (V_DC_c*, where C_c* is the ¹¹CO₂ activity in whole blood and V_D is the brain:blood equilibrium distribution volume of ¹¹CO₂), i.e., $${C_T}^{\ast }\left(t\right)\approx (1-V_b)[{C_E}^{\ast }\left(t\right)+P^{\ast }\left(t\right)+V_D{C_c}^{\ast }\left(t\right)]+V_b{C_b}^{\ast }\left(t\right).$$ Time courses of C_E* and P* are given by solution of the system of differential equations: $$\begin{array}{c}\frac{d{C_E}^{\ast }}{dt}=K_1{C_p}^{\ast }\left(t\right)-(k_2+k_3+k_4){C_E}^{\ast }\left(t\right)\hfill \\ \frac{d{P}^{\ast }}{dt}=k_4{C_E}^{\ast }\left(t\right),\hfill \end{array}$$ where C_p* is the plasma [¹¹C]leucine concentration. In both (A) and (B), the tissue is assumed to be homogeneous with respect to concentrations of amino acids, blood flow, rates of transport and metabolism of amino acids, and rates of incorporation into protein.

Figure 1. Compartmental Model for the L-[1-¹¹C]Leucine PET Method

(A) The exchangeable leucine pool in the brain (unlabeled-C_E or labeled-C_E*) includes intra- and extra-cellular 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 labeled metabolites in brain are α-ketoisocaproic acid (α-KIC), CO₂, and products of CO₂ fixation. Since 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 (C_E) and protein (P) are assumed to be in a steady state. Labeled leucine is administered as a short infusion at the beginning of the study and is 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, i.e., k₅P* ≈ 0. Labeled CO₂ arises either through catabolism of labeled leucine in brain or through influx from blood following catabolism in other tissues. (B) We assume that fixation of ¹¹CO₂ is negligible during the experimental period (Buxton et al 1987; Siesjo et al 1965), and that diffusible ¹¹CO₂ in brain rapidly equilibrates with the arterial blood (Buxton et al 1987). Under these assumptions, the model for labeled leucine reduces to three tissue compartments, with the concentration in the tissue ¹¹CO₂ compartment known from measurements of ¹¹CO₂ in blood and the equilibrium brain:blood distribution ratio for ¹¹CO₂ (Smith et al 2005). The total concentration of ¹¹C in the field of view of the PET camera (C_T*) includes C_E*, P*, blood in the brain (V_bC_b*, where V_b is the fraction of the volume occupied by blood and C_b* is the concentration of activity in whole blood) and ¹¹CO₂ in brain (V_DC_c*, where C_c* is the ¹¹CO₂ activity in whole blood and V_D is the brain:blood equilibrium distribution volume of ¹¹CO₂), i.e., $${C_T}^{\ast }\left(t\right)\approx (1-V_b)[{C_E}^{\ast }\left(t\right)+P^{\ast }\left(t\right)+V_D{C_c}^{\ast }\left(t\right)]+V_b{C_b}^{\ast }\left(t\right).$$ Time courses of C_E* and P* are given by solution of the system of differential equations: $$\begin{array}{c}\frac{d{C_E}^{\ast }}{dt}=K_1{C_p}^{\ast }\left(t\right)-(k_2+k_3+k_4){C_E}^{\ast }\left(t\right)\hfill \\ \frac{d{P}^{\ast }}{dt}=k_4{C_E}^{\ast }\left(t\right),\hfill \end{array}$$ where C_p* is the plasma [¹¹C]leucine concentration. In both (A) and (B), the tissue is assumed to be homogeneous with respect to concentrations of amino acids, blood flow, rates of transport and metabolism of amino acids, and rates of incorporation into protein.

Figure 2. Arterial clearance curves from (A) awake and (B) propofol-anesthetized studies in one subject

Closed diamonds (◆) represent plasma [¹¹C]leucine concentration, open squares (□) represent whole blood total ¹¹C concentration, and open triangles (△) represent whole blood ¹¹CO₂ concentration. Expanded abscissa (left inset) shows peak values of plasma [¹¹C]leucine and whole blood total ¹¹C shortly after the end of the 2-min infusion. Expanded ordinate (right inset) shows clearance of plasma [¹¹C]leucine and blood ¹¹CO₂, and slight increases in whole blood total ¹¹C concentration after 40 min. Injected doses were 26 mCi (awake) and 24 mCi (propofol).

Figure 2. Arterial clearance curves from (A) awake and (B) propofol-anesthetized studies in one subject

Closed diamonds (◆) represent plasma [¹¹C]leucine concentration, open squares (□) represent whole blood total ¹¹C concentration, and open triangles (△) represent whole blood ¹¹CO₂ concentration. Expanded abscissa (left inset) shows peak values of plasma [¹¹C]leucine and whole blood total ¹¹C shortly after the end of the 2-min infusion. Expanded ordinate (right inset) shows clearance of plasma [¹¹C]leucine and blood ¹¹CO₂, and slight increases in whole blood total ¹¹C concentration after 40 min. Injected doses were 26 mCi (awake) and 24 mCi (propofol).

Figure 3. Time activity curves from the temporal cortex and corona radiata from typical awake (A and B, respectively) and propofol-anesthetized (C and D, respectively) studies in one subject

Open circles (○) and solid line 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 = 0.041 ml g⁻¹min⁻¹, k₂ + k₃ = 0.090 min⁻¹, k₄ = 0.036 min⁻¹, V_b = 0.066 in the awake study, and K₁ = 0.048 ml g⁻¹min⁻¹, k₂ + k₃ = 0.080 min⁻¹, k₄ = 0.029 min⁻¹, V_b = 0.071 in the propofol anesthesia study. Lambda was 0.71 and 0.73 in the awake and propofol study, respectively; rCPS was 1.83 and 2.02 nmol g⁻¹min⁻¹ in the awake and propofol study, respectively. Parameter estimates in corona radiata were: K₁ = 0.022 ml g⁻¹min⁻¹, k₂ + k₃ = 0.073 min⁻¹, k₄ = 0.026 min⁻¹, V_b = 0.035 in the awake study, and K₁ = 0.027 ml g⁻¹min⁻¹, k₂ + k₃ = 0.073 min⁻¹, k₄ = 0.023 min⁻¹, V_b = 0.041 in the propofol anesthesia study. Lambda was 0.74 and 0.76 in the awake and propofol study, respectively; rCPS was 0.86 and 0.98 nmol g⁻¹min⁻¹ in the awake and propofol study, respectively. Tracer delay was 4 sec and 1 sec in the awake and propofol anesthesia study, respectively. The poor fit, particularly evident in the temporal cortex, is consistent with the effects of tissue heterogeneity (Schmidt et al 1991, 1992). TACs are from the same studies for which the arterial clearance curves are shown in Fig. 2. Note that the ordinate scales for temporal cortex and white matter differ.

Figure 4. MR (left) and L-[1-¹¹C]leucine PET (center and right) images from study of 23-year old male

Awake study is shown in the center column, and propofol anesthetized study is shown at right. Coronal view at the level of the thalamus is shown in the top row, and axial view is on the bottom. [¹¹C]Leucine PET images are color-coded for rCPS; color bar is to the right of the PET images. Slice thickness is 0.94 mm. rCPS was computed from the total activity in each voxel in each 5-min frame of data between 60 and 90 min by use of an alternate equation for rCPS (Eq 3 in Smith et al., 2005) and whole brain rate constants and λ estimated from this subject’s dynamic PET study. For the awake study K₁=0.045 ml g⁻¹min⁻¹, k₂+k₃=0.094 min⁻¹, k₄=0.040 min⁻¹, λ=0.70, and V_b=0.053; for the propofol anesthesia study K₁=0.041 ml g⁻¹min⁻¹, k₂+k₃=0.078 min⁻¹, k₄=0.031 min⁻¹, λ=0.71, and V_b=0.057. rCPS shown is the average of rCPS in the six frames of data. Weighted average whole brain rCPS was 1.90 and 1.91 nmol g⁻¹min⁻¹ in the awake and propofol studies, respectively. PET images have been smoothed with a Gaussian filter (3.9mm FWHM). Bar in the lower right corner is 10 cm and applies to all six images.