Transport of leucine and sodium in central nervous tissue: studies on retina in vitro

In vivo cerebral protein synthesis rates with leucyl-transfer RNA used as a precursor pool: determination of biochemical parameters to structure tracer kinetic models for positron emission tomography

Leucine oxidation and incorporation into proteins were examined in the in vivo rat brain to determine rates and compartmentation of these processes for the purpose of structuring mathematical compartmental models for the noninvasive estimation of in vivo human cerebral protein synthesis rates (CPSR) using positron emission tomography (PET). Leucine specific activity (SA) in arterial plasma and intracellular free amino acids, leucyl-tRNA, alpha-ketoisocaproic acid (KIC), and protein were determined in whole brain of the adult rat during the first 35 min after intravenous bolus injection of L-[1-14C]leucine. Incorporation of leucine into proteins accounted for 90% of total brain radioactivity at 35 min. The lack of [14C]KIC buildup indicates that leucine oxidation in brain is transaminase limited. Characteristic specific activities were maximal between 0 to 2 min after bolus injection with subsequent decline following the pattern: plasma leucine greater than or equal to leucyl-tRNA approximately KIC greater than intracellular leucine. The time integral of leucine SA in plasma was about four times that of tissue leucine and twice those of leucyl-tRNA and KIC, indicating the existence of free leucine, leucyl-tRNA, and KIC tissue compartments, communicating directly with plasma, and separate secondary free leucine, leucyl-tRNA, and KIC tissue compartments originating in unlabeled leucine from proteolysis. Therefore, a relatively simple model configuration based on the key assumptions that (a) protein incorporation and catabolism proceed from a precursor pool communicating with the plasma space, and (b) leucine catabolism is transaminase limited is justified for the in vivo assessment of CPSR from exogenous leucine sources using PET in humans.

Pathophysiology of ischemic cell death: I. Time of onset of irreversible damage; importance of the different components of the ischemic insult

Rabbit retina was used as an example of organized central nervous tissue in in vitro experiments designed to characterize the onset of cell death from ischemia. Retinas were subjected to progressively longer periods of different types of ischemic insult and then given an opportunity to recover before being tested for irreversible damage, using failure to reinstitute protein synthesis as the principal criterion. Anoxia was more damaging than substrate deprivation, but they were synergistic in combination. Restricting the volume of extracellular fluid during the combined deprivation, to simulate complete circulatory arrest in vivo, caused irreversible damage to occur even sooner. The cells were able to recover from 20 min of the complete ischemia, but it took them more than 2 h to do so. After 30 min, there was extensive irreversible damage. Loss of viability was usually associated with failure to reinstitute energy metabolism, as assessed by 2-deoxyglucose uptake. Under some circumstances loss of viability may have been the consequence of the failed energy metabolism. Increasing medium Mg++, prior to ischemia, to levels that greatly reduce energy requirements caused a significant improvement in the recovery of 2-deoxyglucose uptake.

Pathophysiology of ischemic cell death: II. Changes in plasma membrane permeability and cell volume

Isolated rabbit retinas were subjected for various durations to several types of ischemic insult, and then returned to control medium for periods of up to 4 3/4 h before measurements were made of total water, inulin-free water, and plasma membrane permeability as assessed by mannitol penetration into the inulin-free water. Neither anoxia nor substrate deprivation alone, for as long as 50 min, caused significant irreversible swelling, but they were synergistic in combination. Restricting the volume of extracellular fluid during the combined deprivation caused the changes responsible for swelling to occur much sooner. There was a progressive increase in membrane permeability, with a delayed increase in intracellular water beginning about 2 h after the ischemic insult. Cell swelling correlated closely with loss of viability as evidenced by failure to reinstitute protein synthesis, but the swelling appeared to be the consequence rather than the cause of the initial irreversible damage.

In vitro retina as an experimental model of the central nervous system

Methods are described for isolating adult rabbit retinal and maintaining it in a medium designed to resemble CSF. Morphologic, metabolic, nd electrophysiologic measurements obtained on the in vitro retinas showed that they remained in a nearly physiological state for at least 8 h, and even after 2 days in vitro they still exhibited a high level of metabolic activity and electrical responsiveness to light. Physiological activity was modified by photic stimulation, and data are presented to document changes in metabolism in response to the changes in function. The isolated retina appears to offer a number of unusual advantages for studying relationships between function and metabolism in organized mammalian central nervous tissue.

Protein turnover in retina

Rabbit retinas were exposed in vitro to 0.5-h pulses of [3H]leucine or [14C]leucine. Some retinas were harvested promptly after labeling to measure synthesis. These were combined, in double-labeling experiments, with retinas that had been returned to unlabeled medium for a subsequent 1 h or 3.75 h to measure degradation. All of the proteins were solubilized, and separated according to size by gel electrophoresis. The gels were cut into 95 slices, and each slice was differentially counted. The amount of protein in the slice was estimated from the Coomassie blue staining, and its molecular weight from the distribution of molecular weight (MW) standards. Turnover rates of the various sizes of proteins were calculated from these data using certain well-defined assumptions. Retinal protein contained about 32 X 10(3) nmol of polypeptide per g, with a median MW of 27,000. Total synthesis was at the rate of 103 nmol/g of protein/h, with the most rapid synthesis in the 33,000--43,000 MW range, at 2 nmol/g/h for every 1000 increment in MW. Protein renewal averaged 0.52%/h, but varied directly (p < 0.0001) with MW, so that proteins of 10,000 MW were being renewed at about 0.1%/h and proteins of 140,000 MW at about 1.4%/h. Taken together, the measurements of fractional renewal and the measurements of degradation of the newly synthesized proteins demonstrated that each slice contained proteins with markedly different breakdown coefficients, and provided enough information to characterize the proteins in the slice in terms of a fast and slow subgroup. This analysis indicated that: breakdown coefficients varied much more than rates of synthesis and were therefore the prime determinant of the amount of each protein that was present; as MW increased, breakdown coefficients of the long-lived proteins increased (p < 0.0001), accounting in major part for the correlation between size and turnover; most staining bands were due to proteins with peculiarly long lifespans; the proteins with the slowest turnover of all appeared to be histones; there was an unusually rapid synthesis of a 138,000 MW polypeptide with a moderately short half-life (about 3 h).

Choline accumulation by photoreceptor cells of the rabbit retina

Photoreceptor cells of the rabbit retina accumulate choline from the extracellular environment by an overall process that has a high affinity for choline. These cells do not synthesize acetylcholine; instead, the choline taken up is incorporated into phosphorylcholine and eventually phospholipid. A mechanism for efficient choline accumulation is presumably concomitant to the photoreceptor cell's synthesis of large amounts of membrane for outer segment membrane renewal. Its existence in the photoreceptor cell supports previous evidence that high-affinity choline uptake is not confined to neurons that release acetylcholine, but may be present wherever large amounts of choline are required.

Protein synthesis in central nervous tissue: studies on retina in vitro

Rabbit retinas were maintained in vitro in medium that resembled CSF but with leucine varied from 2 to 1000 microM. Both leucine and threonine were isotopically labelled. When leucine in the medium was 100-1000 microM, leucine was incorporated into protein at 2.03 +/- 0.04 (S.E.M.) mumol/g dry wt./h, a turnover per h of 0.55% of the leucine in retinal protein. Incorporation was constant for at least 7 h. It was reduced 34% when the other amino acids were omitted from the medium and 24% when they were increased 15 fold above physiological levels. When medium leucine was reduced to 2 microM with other amino acids constant, 14C-leucine incorporation fell 70% without significant change in 3H-threonine incorporation, indicating a fall in intracellular specific activity of leucine. The intracellular/extracellular concentration ratio of labelled leucine was 4:1 with medium leucine 23 microM. It fell markedly when medium leucine was reduced to 2 microM or increased to 1000 microM. The concentration ratio of labelled threonine was 15:1 with medium leucine at physiological levels but fell to 6:1 when medium leucine was increased to 1000 microM. Decarboxylation removed 1.5% of free intracellular leucine per min and, at physiological concentrations, was 7.7% the rate of protein incorporation. The ratio of protein synthesis/breakdown, estimated from changes in leucine and 7 other essential amino acids in the medium, was nearly unity. The potential of this preparation for study of CNS protein metabolism is discussed.

Functional homogeneity of leucine pool in retina cells

Data on leucine metabolism in isolated rabbit retina are examined for evidence, for or against, a common intracellular pool of free leucine. Data include values for: concentrations, transport rates, degradative metabolism and protein incorporation of labelled leucine measured over a wide range of concentrations; protein incorporation of labelled threonine, measured simultaneously; and an indirect measurement of protein breakdown. The fall in labelled leucine incorporation into protein, when medium leucine was reduced below 100 microM, corresponded closely with the fall in intracellular specific activity predicted from rate of influx of labelled leucine from medium and rate of release of unlabelled leucine from protein breakdown. Protein incorporation of labelled leucine competed with decarboxylation and outward transport and reduced the free intracellular leucine in about the amounts predicted for a common pool. Implications for measurements using labelled amino acid are discussed.