Date of Award

2-1987

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Medical Sciences

Supervisor

George J.F. Heigenhauser

Abstract

In intense exercise the maintenance of muscle contraction and metabolism is critically dependent on the regulation of intracellular homeostasis. The present studies have examined the physiological and biochemical effects of three factors which influence this regulation at rest and in exercise; these factors are the strong ions, the weak ions (weak acid and base electrolytes), and carbon dioxide (CO₂).

This thesis describes three series of experiments which were designed to demonstrate the importance of ion regulation in the maintenance of muscle performance and metabolism in intense exercise. The purposes of these studies were three-fold: (1) to quantify the intracellular composition of strong ions of fast- and slow-twitch skeletal muscles at rest and at the end of intense exercise; (2) to determine the relative contributions of strong ions, weak acids and bases, and CO₂ to the total intracellular ionic composition muscle at rest and in exercise; and (3) to demonstrate the between intracellular ion regulation, muscle metabolism and muscle performance during intense exercise.

The first series of experiments examined the ionic and metabolic composition rat hindlimb muscles (white gastrocnemius, WC; plantaris, PL; red gastrocnemius, RG; soleus, SL) sampled at rest and following 4.5 min of intense swimming exercise. Intracellular ionic status is dependent upon the PCO₂ strong ion difference ([SID]), and the total concentration of weak acids and bases ([ATOT]) of the intracellular fluids. PCO₂ was held constant at various levels and intracellular strong ion concentrations r were measured using instrumental neutron activation analysis. [ATOT] is a pooled term representing all of the intracellular weak electrolytes and cannot be directly measured. A method for determining [ATOT) indirectly from measurements of pH, [SID] and PCO₂ muscle homogenates was formulated and is described. Intracellular [SID) was found to be significantly higher in fast twitch WG (161 mEq/l) than in slow twitch SL (137 mEq/l); this was primarily due to a higher [K+] in resting WG. Muscle [ATOT] averaged 190 mEq/l at rest, and there was little difference between muscles. Intramuscular pH was determined using three methods: the distribution of the weak acid DMO, from pH measurements of muscle homogenates, and was calculated from the independent variables [SID], [ATOT}, and PCO₂. Corresponding to its higher [SID], the WG had a significantly higher intracellular pH at rest (6.94) than SL (6.72).

The second series of experiments examined changes in extra- and intra-cellular ion and metabolite concentrations, and quantified the ion and metabolite fluxes between muscle and blood at rest and during intense electrical stimulation using an isolated perfused rat hindlimb preparation. These studies confirmed that K⁺ and La⁻ leave the muscle cells during contraction and that Na⁺ and Cl⁻ enter, and that these ionic disturbances cause the increase in intracellular [H⁺j associated with the decrease in muscle performance. With stimulation, the major changes affecting intracellular ion status and metabolism were large increases in intracellular lactate concentration ([La⁻li) and [Na⁺]i, and a marked reduction in [K⁺]i. These changes were greatest in WG, a highly glycolytic muscle, and least in SL, a predominantly oxidative muscle; the rise in [La⁻]i accounted for 67% and 50% of the fall in [SID] in WG and SL, respectively. Correspondingly, the largest changes in ion status occurred in WG. The high initial [SID]i in resting we prevented excessive increases in [H⁺] and protein ionization state during muscle contraction.

The third series of experiments examined the effects of extracellular metabolic and respiratory alkalosis on muscle performance, metabolism and ion regulation, and provided an opportunity to test the hypothesis that changes in intracellular ionic status will affect the regulation of metabolism. With alkalosis, compared to controls, there were no differences with respect to performance and metabolism, however, the rate of La⁻ efflux from muscle was significantly increased and [La⁻]i was significantly reduced. Extracellular alkalosis was also associated with increased fluxes of Na⁺ from perfusate into muscles, resulting in large increases in [Na⁺] at rest and during stimulation; K⁺ efflux in alkalotic hindlimbs was significantly reduced, compared to controls, during stimulation. A theory is proposed whereby exercise-induced changes in intracellular [La⁻], [K⁺] and [Na⁺] exert direct effects on the ionized state of intracellular proteins and on metabolic regulation during exercise.

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