Howard E. Morgan

Dependence of Protein Synthesis on Aortic Pressure and Calcium Availability

Increased aortic pressure accelerated protein synthesis in control-beating and arrested-drained hearts supplied with either glucose or pyruvate. Elevation of perfusion pressure from 60 to 120 mm Hg increased oxygen consumption in control-beating but not in arrested-drained preparations. Energy availability, as assessed by adenylate energy charge or creatine phosphate/creatine ratio, or both, was increased in arrested-drained hearts supplied with glucose and perfused at 60 and 120 mm Hg aortic pressure. In control-beating or arrested-drained hearts supplied with pyruvate, energy availability was not improved by elevation of aortic pressure from 60 to 120 mm Hg. An increase of perfusate calcium concentration from 0.5 to 5.0 mM in control-beating Langendorff preparations supplied with glucose and perfused at an aortic pressure of 90 mm Hg doubled oxygen consumption and decreased energy availability, but had no effect on the rate of protein synthesis. In arrested-drained hearts supplied with either glucose or pyruvate and calcium concentrations ranging from 0.5 to 5.0 mM, the rates at 120 mm Hg aortic pressure were 11-25% higher than at 60 mm Hg. These findings provide no evidence to implicate increased oxidative metabolism, energy availability, or extracellular calcium concentration as important factors in the mechanism that accounts for the effects of increased aortic pressure on protein synthesis.

Effects of Cardiac Work and Leucine on Protein Turnover

The purpose of these experiments was to assess effects of cardiac work and leucine in hearts supplied only glucose or substrate and hormone mixtures that simulated plasma. Rates of protein degradation greatly exceeded protein synthesis in Langendorff preparations supplied glucose. This severely negative nitrogen balance was brought closer to zero by provision of more complete substrate mixtures. Cardiac work further improved the nitrogen balance by stimulating protein synthesis in hearts supplied glucose (mixture 1), glucose-insulin-glucagon-lactate-beta-hydroxybutyrate (mixture 2), or palmitate-beta-hydroxybutyrate-glucose (mixture 3) and inhibiting protein degradation in hearts supplied glucose. Cardiac work did not affect the rates of either protein synthesis or degradation in hearts provided insulin-lactate-glucose (mixture 4). The increase in protein synthesis was associated with increased rates of peptide chain initiation. Addition of 1 mM leucine had an additional effect to restore nitrogen balance to zero or to achieve positive balance in working hearts supplied substrate and hormone mixture 2.

Aortic Perfusion Pressure and Protein Synthesis

The effect of increased pressure load on cardiac protein synthesis has been studied in Langendorff preparations and working hearts supplied glucose as substrate. During the second hour of perfusion, elevation of perfusion pressure from 60 to 120 mmHg in Langendorff preparations accelerated protein synthesis by approximately 40% while induction of cardiac work and development of a systolic pressure of 145 mmHg increased synthesis by 22%, as compared to a Langendorff preparation perfused at 60 mmHg. In Langendorff preparations, increased perfusion pressure still accelerated protein synthesis when a drain was placed in the ventricle and intraventricular pressure development was prevented or when the heart was arrested with tetrodotoxin and the ventricle drained. These results suggest that the enhancement of protein synthesis with a pressure load may be induced by passive stretch of the cardiac muscle cell secondary to increased perfusion pressure.

The Control of Protein Turnover in the Isolated Perfused Rat Heart

Publisher Summary This chapter discusses the control of protein turnover in the isolated perfused rat heart. Protein turnover and its hormonal and nonhormonal control was studied in an isolated preparation of perfused rat heart muscle. This preparation permits the control of protein turnover to be examined under well-defined conditions as the level of ventricular pressure development, heart rate, left atrial filling pressure, and coronary flow can be controlled. Levels of protein within cardiac muscle may vary either as a result of changes in the rates of protein synthesis or degradation. In a study described in the chapter, the rates of degradation were estimated by measuring release of phenylalanine from the heart. Net release of phenylalanine would not occur if rates of synthesis and degradation were equal. An approximation of the rate of degradation could be made by measuring the rate of dilution of the specific activity of the free phenylalanine pool. This method underestimated the rate of degradation by the extent to which exchange of phenylalanine across the cell membrane prevented equilibration of the specific activities of the intracellular and extracellular pools of the amino acids.

Stretch, a Common Denominator in Muscle Growth

Increased mechanical work leads to hypertrophy of muscle cells and hyperplasia of non-muscle cells in heart and skeletal muscle. Greater work accelerates energy production via anaerobic glycolysis and oxidative metabolism, elevates oxygen consumption, increases muscle blood flow, and raises protein synthetic rate. The mechanical parameter(s) most closely associated with faster protein synthesis in both heart and skeletal muscle is stretch. Our laboratory has studied the relationship between stretch of the ventricular wall induced by increased aortic pressure and elevated intraventricular pressure and rates of protein synthesis and degradation in heart muscle. The purpose of these studies is to understand the molecular mechanism(s) of cardiac hypertrophy induced by greater afterload or preload.

PEPTIDE-CHAIN INITIATION IN HEART AND SKELETAL MUSCLE

ABSTRACT Rates of peptide-chain initiation were estimated in heart and skeletal muscle by measurements of the rate of protein synthesis and levels of ribosomal subunits. During perfusion of isolated rat heart or hemicorpus with buffer containing glucose and amino acids, polysomes decreased, levels of ribosomal subunits rose, and protein synthesis declined. These findings indicated development of a restraint on peptide-chain initiation in either tissue. In heart muscle, the restraint was relieved by addition of insulin, fatty acids or non-carbohydrate substrates. In skeletal muscle of mixed fiber type, only insulin was effective in relieving the restraint on initiation. In either tissue, activity of eIF-2, assayed by the formation of the ternary complex of met-tRNA f Met , GTP and eIF-2, was unchanged by insulin.

Biochemical Mechanisms of Altered Metabolism in Ischemic Heart

Oxygen deficiency in heart muscle is most commonly induced by reduction in coronary flow, referred to as ischemia. As contrasted to hypoxia or anoxia in which flow of blood with low or zero oxygen tension is maintained, ischemia leads to accumulation of metabolic products that further modify rates of biochemical reactions. After periods of severe ischemia ranging from 30 minutes to 1 hour or more, irreversible damage occurs. Damage of this severity is characterized by disruption of the plasma membrane that is preceded by swelling of both the cell and mitochondria. Concurrently, the myofibrils and intercellular junctions are disrupted, and there is margination of nuclear chromatin. Reperfusion of an irreversibly-injured cell leads to accumulation of Ca++ within the mitochondria and failure to recover contractile activity.

Neutral-alkaline proteases of the rat heart ☆

Abstract Protein degradation in heart may involve proteases active at neutral and alkaline pH and lysosomes containing proteases active at acidic pH. Proteases were assayed by hydrolysis of endogenous protein to free amino acids and [3H] acetylcasein to acid-soluble peptides by homogenates, supernatants, and particulate fractions of hearts from control and 48 80 - treated rats and from heart muscle cells. Homogenates and particulate fractions of 48 80 - treated hearts and muscle cells hydrolyzed endogenous protein and [3H]-acetylcasein (pH 7.8) at only 10% and 60%, respectively, of the rate of homogenates and particulate fractions from control hearts. These findings indicated that a major contributor to the alkaline proteolytic activity of the heart was localized to non-muscle cells, presumably mast cells. In addition, specific activity of the Ca2+-dependent protease was reduced 50% in muscle cell preparations, as compared to control hearts. The majority of the proleolytic activity of heart muscle cells at pH 7.8 was accounted for by a cysteine protease(s) with a molecular weight of approximately 26 000. This activity was present in both the soluble and particulate fractions. These studies demonstrated that a model of the proteolytic pathway involving proteases active at neutral-alkaline pH and lysosomal proteases was feasible in heart muscle cells.