Malnutrition is a major problem in chronic uremia (CRF) in spite of aggressive dialysis and in ARF, intravenous hyperalimentation. We identified an important cause of catabolism in experimental uremia, metabolic acidosis. Clinical studies support this conclusion. Acidosis stimulates degradation of branched-chain amino acids and protein in muscle of CRF rats. Therefore, acidosis also could block the adaptive decrease in protein and amino acid breakdown that normally occur in response to dietary protein restriction, Preliminary data suggest CRF impairs H+ and Na+ transporters and therefore, could increase susceptibility to acidosis. The mechanism for acidosis. The mechanism for acidosis-induced PD in muscle: 1) requires ATP; 2) increases mRNA for ubiquitin (Ub) suggesting activation of the ATP- and Ub- dependent proteinase; and 3) increases mRNA for subunits of this enzyme. Indirect evidence suggests glucocorticoids (GC) are involved in these responses.
Specific Aim 1 : to determine if both acidosis and CRF activate the ATP- and Ub-dependent proteinase. Our hypothesis is that both conditions act through the same catabolic pathway. The effects of ATP depletion on PD in muscles from acidotic and CRF rats will be examined as will the mRNA abundance for Ub, proteinase subunits, heat-shock proteins and Cathepsin B in muscle and other organs. Since acidification also increases PD in BC3H-1 myocytes, we will examine mechanisms for increased mRNA by testing transcriptional regulation & RNA stability.
Specific Aim 2 : to determine if acidosis and CRF increase catabolism in vivo and impair nutritional responses to a low-protein diet. Our hypothesis is that acidosis blocks the adaptive decrease in amino acid and PD occurring in animals fed a low-protein diet. This will be tested in acidotic and CRF rats fed 14 and 9% protein using the L-[1-14C]leucine infusion technique.
Specific Aim 3 : to determine if GC are required for the catabolic response to acidosis. Our hypothesis is that GC is the common catabolic signal. Protein turnover and Northern blots will be measured in muscle of adrenalectomized, acidotic rats and in myocytes.
Specific Aim 4 : to determine if uremia impairs intracellular pH regulation. Our hypothesis is that cell pH falls because of increased cell Na+ (decreasing the driving force for H+ extrusion) and inhibition of Na/H exchange; these changes increase susceptibility to acidosis. Na/H antiporter activity will be measured in hepatocytes of CRF rats. Muscle pH in acidotic and CRF rats will be measured by NMR to assess how it varies in comparison to blood pH.
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