C. Louise Milligan

Metabolic recovery from exhaustive exercise in rainbow trout

Abstract Exercise to exhaustion results in severe metabolic, acid-base, and endocrine disturbances to fish. Recovery metabolism from this type of activity may place limits on performance, since the time requirements for restoration of high energy stores (e.g. glycogen, high energy phosphates) will ultimately determine the frequency of maximal performance. This article reviews the types of metabolic, acid-base, and endocrine disturbances associated with exhaustive exercise in rainbow trout, the mechanism(s) of recovery, and their regulation. In particular, the roles of catecholamines and cortisol in regulating recovery metabolism are explored.

Renal responses of trout to chronic respiratory and metabolic acidoses and metabolic alkalosis

Exposure to hyperoxia (500-600 torr) or low pH (4.5) for 72 h or NaHCO3 infusion for 48 h were used to create chronic respiratory (RA) or metabolic acidosis (MA) or metabolic alkalosis in freshwater rainbow trout. During alkalosis, urine pH increased, and [titratable acidity (TA) -[Formula: see text]] and net H+ excretion became negative (net base excretion) with unchanged [Formula: see text] efflux. During RA, urine pH did not change, but net H+ excretion increased as a result of a modest rise in [Formula: see text] and substantial elevation in [TA -[Formula: see text]] efflux accompanied by a large increase in inorganic phosphate excretion. However, during MA, urine pH fell, and net H+excretion was 3.3-fold greater than during RA, reflecting a similar increase in [TA -[Formula: see text]] and a smaller elevation in phosphate but a sevenfold greater increase in[Formula: see text] efflux. In urine samples of the same pH, [TA - [Formula: see text]] was greater during RA (reflecting phosphate secretion), and[Formula: see text] was greater during MA (reflecting renal ammoniagenesis). Renal activities of potential ammoniagenic enzymes (phosphate-dependent glutaminase, glutamate dehydrogenase, α-ketoglutarate dehydrogenase, alanine aminotransferase, phospho enolpyruvate carboxykinase) and plasma levels of cortisol, phosphate, ammonia, and most amino acids (including glutamine and alanine) increased during MA but not during RA, when only alanine aminotransferase increased. The differential responses to RA vs. MA parallel those in mammals; in fish they may be keyed to activation of phosphate secretion by RA and cortisol mobilization by MA.Exposure to hyperoxia (500-600 torr) or low pH (4.5) for 72 h or NaHCO(3) infusion for 48 h were used to create chronic respiratory (RA) or metabolic acidosis (MA) or metabolic alkalosis in freshwater rainbow trout. During alkalosis, urine pH increased, and [titratable acidity (TA) - HCO(-)(3)] and net H(+) excretion became negative (net base excretion) with unchanged NH(+)(4) efflux. During RA, urine pH did not change, but net H(+) excretion increased as a result of a modest rise in NH(+)(4) and substantial elevation in [TA - HCO(-)(3)] efflux accompanied by a large increase in inorganic phosphate excretion. However, during MA, urine pH fell, and net H(+) excretion was 3.3-fold greater than during RA, reflecting a similar increase in [TA - HCO(-)(3)] and a smaller elevation in phosphate but a sevenfold greater increase in NH(+)(4) efflux. In urine samples of the same pH, [TA - HCO(-)(3)] was greater during RA (reflecting phosphate secretion), and [NH(+)(4)] was greater during MA (reflecting renal ammoniagenesis). Renal activities of potential ammoniagenic enzymes (phosphate-dependent glutaminase, glutamate dehydrogenase, alpha-ketoglutarate dehydrogenase, alanine aminotransferase, phosphoenolpyruvate carboxykinase) and plasma levels of cortisol, phosphate, ammonia, and most amino acids (including glutamine and alanine) increased during MA but not during RA, when only alanine aminotransferase increased. The differential responses to RA vs. MA parallel those in mammals; in fish they may be keyed to activation of phosphate secretion by RA and cortisol mobilization by MA.

Muscle and Liver Intracellular Acid-Base and Metabolite Status after Strenuous Activity in the Inactive, Benthic Starry Flounder Platichthys stellatus

In addition to an extracellular acidosis in which blood metabolic acid load greatly exceeded lactate load, exhaustive activity in starry flounder resulted in an intracellular acidosis of largely metabolic origin in the white muscle, with intracellular pH dropping from 7.56 to 7.27, as measured by DMO distribution. An accumulation of lactate and depletion of glycogen in addition to a shift of fluid from the extracellular to intracellular space were associated with the postexercise acidosis. Pyruvate levels increased in blood and later in muscle; the relative rise in pyruvate was greater than that in lactate so the lactate:pyruvate ratio declined. The muscle intracellular acidosis was corrected sooner than the extracellular acidosis (4–8 h vs. 8–12 h). The restoration of muscle pHi was associated with an increase in pyruvate, a restoration of glycogen stores, and clearance of the lactate load. It is suggested that both lactate and acidic equivalents (H⁺) were cleared from the muscle via in situ oxidation and/or glyconeogenesis and that the rapid correction of the intracellular acidosis through efflux of part of the H⁺ load facilitated metabolic recovery. The liver showed a progressive alkalinization after exercise. This alkalinization was of metabolic origin and not associated with lactate accumulation. Except for a short-lived depression 0.5 h after exercise, red cell intracellular pH remained virtually constant.

The Effect of Cortisol on Recovery from Exhaustive Exercise in Rainbow Trout (Oncorhynchus mykiss): Potential Mechanisms of Action

The effects of cortisol on metabolic recovery from exhaustive exercise in rainbow trout (Oncorhynchus mykiss) and potential mechanisms of action were investigated. When the postexercise rise in cortisol is prevented in fish by blocking either cortisol synthesis with metyrapone or cortisol release with dexamethasone, there is a faster recovery of blood and muscle metabolites and acid-base status in those fish than in control fish. To investigate whether preventing the rise in plasma cortisol is responsible for these effects, two experiments were done. Cortisol infused intofish treated with metyrapone returned the rate of recovery to that of control fish. Treatment with 11-deoxycortisol or deoxycorticosterone, intermediates in the cortisol biosynthetic pathway, the levels of which are possibly increased by metyrapone treatment, did not increase the rate of recovery; indeed, plasma cortisol was elevated and recovery prolonged in fish treated with 11-deoxycortisol. These experiments indicate that preventing the postexercise rise in plasma cortisol is associated with decreasing the time required for metabolic and acid-base recovery. The mechanism of cortisol action is not alteration of net acid excretion at the gills or mediated by some action at the RU486-sensitive cortisol receptor. It is suggested that cortisol may play an adaptive role in recovery from exhaustive exercise by providing lactate as a postexercise aerobic fuel.

Effects of Strenuous Activity on Intracellular and Extracellular Acid-Base Status and H⁺ Exchange with the Environment in the Inactive, Benthic Starry Flounder Platichthys stellatus

Exhaustive activity in starry flounder resulted in an acidosis in the whole-body extracellular fluid (ECF) and intracellular fluid (ICF) compartments. In the ECF, the acidosis consisted of a short-lived respiratory component (increase in CO₂ tension [Pco₂]) followed by a longer-lived metabolic component (decrease in [

Branchial Acid and Ammonia Fluxes in Response to Alkalosis and Acidosis in Two Marine Teleosts: Coho Salmon (Oncorhynchus kisutch) and Starry Flounder (Platichthys stellatus)

HCO_{3}^{-}

Fuel use during glycogenesis in rainbow trout (Oncorhynchus mykiss Walbaum) white muscle studied in vitro.

]). The acid-base disturbance was corrected by 8-12 h. There was little lactate accumulation in the blood, with levels rarely greater than 1-2 mmol/liter, and at all times the blood metabolic acid load (ΔH⁺m) was in excess of the blood lactate load (ΔLa⁻). Blood [glucose] increased by 50%. Whole-body extracellular fluid volume (ECFV) fell by 17% owing to a shift of fluid into the intercellular fluid volume (ICFV), causing a general hemoconcentration. Exercise also caused an acidosis in the whole-body intracellular compartment, with intracellular pH dropping from a rest value of 7.58 to a low of 7.24. The whole-body intracellular acidosis was corrected ~4 h sooner than the extracellular disturbance and became alkalotic at 8 h, returning to normal at 12 h. Associated with this acid-base disturbance was an increased efflux of acidic equivalents (H⁺) to the environmental water, coincident with a large increase in the titratable-acidity flux. Ammonia excretion increased only slightly. Analysis of the distribution of metabolic acid between the ECF, ICF, and environmental water revealed that until 4 h postexercise, the bulk of the acid load remained in the intracellular compartment. Approximately 20% passed through the extracellular fluid and was transiently stored in the environmental water at 4-12 h. This flux of H⁺ to the water was associated with an intracellular alkalosis and thus appeared to hasten correction of intracellular acid-base status, perhaps as a means of aiding metabolic recovery.

Adrenergic Analysis of Extracellular and Intracellular Lactate and H⁺ Dynamics after Strenuous Exercise in the Starry Flounder Platichthys stellatus

The role of key Cori cycle organs, white muscle and liver, and a lactate oxidizing organ, heart, in the clearance of blood-borne lactate after strenuous exercise in winter flounder was examined. During recovery from exercise, the fish were injected with uniformly labeled 14C-lactate via a caudal vein catheter Overall, 29%6, 12.0%, and 0.9% of 14C was recovered as CO₂, glucose, and glycogen, respectively, suggesting that blood lactate was primarily cleared via aerobic oxidation and glucose production as opposed to glycogenesis. Although white muscle has a low oxidative potential, by virtue of its sheer bulk it is estimated to make the major contribution to lactate oxidation, perhaps to fuel glycogen resynthesis. The bulk of the lactate produced during strenuous exercise is retained within the white-muscle mass, where it is utilized for in situ glycogenesis. This scenario is consistent with observations for other ectotherms, which indicates that the primary fate of muscle lactate is glycogenic, not oxidative.