In studies 2 and 3, it was hypothesized that digoxin inhibition of NKA activity would reduce NKA glycolytic energy demands and subsequently reduce lactate production in contracting skeletal muscle. This was on the basis that NKA has a preference for ATP via glycolysis (Clausen 2003), and increased NKA activation is associated with increased lactate production (James et al, 1999; Bundgaard et al, 2002), whereas inhibition of NKA by ouabain is associated with reduced lactate production (James et al, 1999).
Plasma [Lac-]v and [Lac-]a-v were in fact lower with digoxin in study 2, and plasma Lac-
was also lower at several sub-maximal time points throughout exercise in study 2, but both of these responses not due to any apparent reduced muscle NKA activity. Whilst lactate production and peak workload are lower in CHF patients taking DIG compared to controls during exercise (Okita et al, 1998; Näveri et al, 1997), there is also numerous additional muscle morphology changes in CHF patients that preclude reliable comparisons with healthy muscle exposed to NKA inhibition.
Therefore these observations are either due to reduced Lac- production or release from muscle. Interestingly, Lac- flux into plasma at fatigue was greater with alkalosis in study 1, which reflects enhanced glycolytic ATP production, and is likely to be due in part to increased NKA activity. This glycolysis/NKA rationale in alkalosis is consistent with those previously observations made when lactate production increased with increased NKA activation (James et al, 1999; Bundgaard et al, 2002).
The effects of acutely inhibited NKA activity on glycolysis have not been comprehensively explored in healthy human skeletal muscle during exercise, thus require further investigation
7.2 CONCLUSIONS
The key conclusions for this thesis include; Study 1, Chapter 3
1. Oral sodium bicarbonate induced alkalosis attenuated muscle fatigue, evidenced by a ~25% improvement in submaximal finger flexion exercise performance by in healthy untrained humans.
2. Finger flexion exercise barely perturbed arterial plasma ions and acid-base status, but induced substantial arterio-venous changes.
3. Plasma [K+]a and [K+]v were systematically reduced with alkalosis, whereas the
[K+]a-v during exercise tended to be greater. Muscle K+ efflux at fatigue was
~49% greater in alkalosis, consistent with lower [K+] and a bigger gradient for K+ release. However the peak K+ uptake was elevated during recovery in alkalosis, which suggests increased muscle NKA activity.
4. Forearm blood flow, plasma volume, blood volume, muscle O2 content did not
change with alkalosis.
5. Alkalosis elevated arterial and venous [HCO3-], CO2 content and PCO2, and
lowered arterial and venous [H+] at rest, during exercise and recovery. Lower circulating [K+] and greater muscle K+ uptake, Na+ delivery and Cl- uptake with alkalosis, are all consistent with preservation of membrane excitability during exercise. This suggests that lesser exercise-induced membrane depolarisation may be an important mechanism underlying enhanced exercise performance with alkalosis.
6. During post-exercise recovery under control conditions, K+ re-uptake across previously exercising muscle demonstrates the important regulatory role that active tissue plays in recovery from K+ challenge in restoring K+ homeostasis.
Study 2, Chapter 4
1. Oral digoxin administration for 14 days in healthy humans achieved a clinically relevant serum digoxin concentration, but did not perturb plasma K+ homeostasis at rest, during exercise or recovery, nor did it contribute to fatigue during intermittent supramaximal finger flexion exercise.
2. Forearm blood flow increased substantially during exercise, but was not affected by DIG; nor were changes in plasma and blood volume, or forearm muscle O2
uptake and CO2 output.
3. The plasma [K+]a increased slightly and [K+]v increased dramatically with each
bout of exercise. The [K+]a-v decreased (more negative) rapidly from rest during
the first exercise bout and together with substantial K+ efflux from muscle, reflected net K+ loss from contracting muscle throughout exercise. However, no digoxin treatment effects were found for any K+ measures. A lack of DIG effect on K+ homeostasis might reflect inadequate digitalisation or adaptive compensatory NKA upregulation.
4. Arterial [HCO3-], PCO2, CaCO2 and the [Lac-]a-v were lower during exercise and
recovery in DIG; and venous [H+] was lower at fatigue.
5. Acid-base disturbances during exercise were decreased with digoxin, possibly associated with a decrease in glycolysis, although unlikely to be associated with a decrease in NKA activity. These changes had no impact on exercise performance.
Study 3, Chapter 5
1. Oral digoxin administration for 14 days in healthy humans did not perturb plasma K+ homeostasis or contribute to fatigue during submaximal cycling exercise of increasing intensity.
2. Blood flow was not measured across the inactive forearm, however it appears unlikely to have changed with DIG based on forearm blood flow results from
study 2, and that exercise
V
•O2 did not change with DIG. Plasma volume shiftsfrom rest and across the inactive forearm were unaffected by DIG.
3. Arterial [K+], venous [K+] and the arterio-venous [K+] difference across the inactive forearm were ~6.5, ~5 and ~1.25 mM at fatigue respectively, however [K+]a, [K+]v and [K+]a-v were not affected by DIG. The lack of DIG effect on plasma
K+ homeostasis during exercise involving large muscle mass was possibly due to adaptive compensatory NKA upregulation in healthy skeletal muscle tissue.
4. Plasma [Lac-]a tended to be lower during DIG at 67%
V
•O2peak, and plasma [Lac-
]v was lower during DIG at 33%
V
•O2peak. However,DIG effects on glycolysis were
small, and not associated with altered NKA. The [SID]a was lower in DIG at rest
and 33%
V
•O2peak, but did not affect [H+]a or exercise performance.5. Plasma [Cl-]a tended to be greater in DIG at rest to 67%
V
•O2peak, although the
associated mechanism is currently unknown.
6. Inactive muscle also plays a substantial role in the regulation of strong ions during sub maximal cycling exercise at increasing intensities
7. Adaptive compensatory NKA upregulation with DIG in healthy humans also demonstrates remarkable self-preservation in otherwise healthy tissue to maintain K+ homeostasis and subsequent muscle function.