Year

1993

Degree Name

Doctor of Philosophy

Department

University of Wollongong, Dept. of Biological Science; Dept. of Biomedical Science

Abstract

Despite the great importance of lipids in many biological functions, little is known about the effects of different types of fats on muscle composition and physical performance. In this study, the levels of essential polyunsaturated fatty acids in the diets of male rats were manipulated in order to determine whether the composition of skeletal muscle membranes would be altered and if so, the function of isolated skeletal muscles and whole animal performance.

Three isocaloric diets (providing 10% fat), were developed. An essential fatty acid deficient (EFAD) diet contained 1 % n - 6 and 0.2% n - 3 fatty acids; a polyunsaturated fatty acid diet (PUFA) enriched diet contained 3 5 % n - 6 and 21 % n - 3 fatty acids and a Control diet contained 5 1 % n - 6 and 9 % n - 3 fatty acids.

Weanling litter-mates were assigned at random to dietary groups and maintained on their test diets for nine weeks, followed by zero, two or six weeks recovery on normal rat pellets. Groups of rats were tested at each of these stages and results were analysed by 2-way ANOVA , with diet and litter as factors.

Membrane fatty acid analysis revealed that the total proportions of saturated and unsaturated fatty acids remained constant between the three dietary groups. However, the proportions of unsaturated fats in the membranes reflected the proportions of unsaturated fats in the diets. In the EFAD group, essential fatty acid deficiency was verified using the triene-tetraene ratio (> 0.5 in both soleus and EDL ) . Effects on membrane composition in soleus muscles ("slow-twitch") and extensor digitorum longus (EDL) muscles ("fast-twitch") were similar, but the two muscles varied in their rates of recovery. After nine weeks, there was a significant increase, in both muscles, in n - 9 fatty acids in the EFAD group, compared with the Control and PUFA groups. There was also a significant increase in n - 3 fatty acids in the PUFA group, compared with the Control and EFAD groups. After two weeks recovery, the difference in the level of n - 9 fatty acids in the EFAD group was no longer was apparent in soleus, but persisted in EDL. After six weeks recovery, it was no longer apparent in EDL. However, the increased proportion of n - 3 fatty acids in the PUFA group was present in both soleus and EDL muscles, even after six weeks recovery. Thus, the fatty acid composition of muscle membranes recovered from the effects of the EFAD diet (although at different rates), but did not return to control levels following the PUFA diet.

Effects of changes in membrane fatty acid composition on skeletal muscle function were determined in isolated soleus and EDL muscles from groups of five to seven rats. Muscles were isolated and stimulated electrically after nine weeks on the test diets, followed by zero, two or six weeks recovery. Parameters measured included tensions generated, response times (latent time, contraction time and half-relaxation time), fatigue time and endurance. Overall, the EFAD diet resulted in significantly lower tensions and reduced response times compared with the Control and PUFA diets. Results for muscles from the Control and PUFA groups were very similar, as was the composition of the two diets. Although muscles from the EFAD rats had significantly reduced tensions and response times after nine weeks on the test diets, they recovered from these effects, although again, soleus recovered within two weeks and EDL within six weeks.

Effects of changes in dietary fatty acids on whole animal physical performance were investigated in groups of nine to 11 rats. A series of performance-related tests was conducted including treadmill endurance, grip strength, basal oxygen consumption (VO2 basal) and peak oxygen consumption (VO 2 peak)- Each rat was tested twice; after nine weeks on the test diets and after six weeks recovery. Changes in dietary fatty acids had no effect on grip strength, VO2 basal or V O2 peak, but had a highly significant effect on endurance. In the rats on the PUFA diet, endurance was 44 % less than for the the Control rats after nine weeks, and there was no sign of recovery six weeks later.

Effects of changes in dietary fatty acids on Na+,K+-ATPase (in the sarcolemma) and Ca2+-ATPase (in the sarcoplasmic reticulum membrane) were examined in groups of nine rats since these two membrane-bound enzyme systems are very important for muscle contraction and relaxation. Concentration of Na+,K+-ATPase was estimated using a vanadate-facilitated [3H]ouabain binding assay and activity of Na+,K+-ATPase was measured by the production of phosphate by muscle homogenate during the hydrolysis of ATP in the presence and absence of ouabain. Activity of Ca2+-ATPase was measured by the hydrolysis of ATP in the presence of low and high concentrations of calcium. Changes in dietary fatty acids had no significant effect on activity of either enzyme. Although the concentration of Na+,K+-ATPase was also unaffected, there was a significant effect of diet on affinity of Na+,K+-ATPase for ouabain; affinity appeared to be reduced in muscles from rats on the PUFA diet.

Summary

Changes in the composition of dietary fatty acids had the following effects in rats:

1. The composition of skeletal muscle membrane phospholipid fatty acid composition was altered. The pattern of unsaturated fatty acids reflected the unsaturated fatty acid composition of the diets. Muscle membrane composition of EFAD rats recovered to Control levels, but EDL muscles took longer to recover than soleus muscles. Muscle membrane composition of PUFA rats did not recover to Control levels, since they retained a high level of n -3 fatty acids after the recovery period.

2. The function of isolated muscles from rats on the EFAD diet was altered. Although all changes in isolated muscle function recovered to control levels, EDL muscles took longer to recover than soleus muscles.

3. Endurance was markedly reduced in whole rats which had been on the PUFA diet and there was no sign of recovery after six weeks.

4. Activity of Na+,K+-ATPase and Ca2+-ATPase were unaffected, as was the concentration of Na+,K+-ATPase, but affinity for ouabain in muscles from rats on the PUFA group appeared to be decreased.

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