Chronic NaHCO3 supplementation taken during 4 wks of RSE training induced the
opposite effect to acute NaHCO3 with a decrease in β1 and also an increase in β3 protein
abundance (treatment main effects). A consistent finding was the non-significant effect of chronic NaHCO3 and training on the protein abundance of any NKA α isoform.
The lack of increase in NKA α1 - α3 and β2 protein abundance following 4 wks RSE
training was unexpected. Exercise training in humans results in an up-regulation of NKA content, as seen after 7 wks of high-intensity sprint interval training (McKenna et al., 1993; Harmer et al., 2000), and also endurance training (Green et al., 2004). Provided that the stimulus is sufficient, up-regulation of the NKA can be seen within the first week of training (Green et al., 1993). High intensity training increased α2 protein abundance in
(Thomassen et al., 2010), while sprint training in runners increased the α1 protein
abundance (Iaia et al., 2008). The lack of increased α abundance here may be due to the RSE training having an insufficient total load to stimulate increases in NKA isoform protein, via insufficient sprint duration, total exercise duration, or total work. Interestingly, only the β3 isoform was found to have an elevated protein abundance
following training here.
The effects of NaHCO3-induced hypokalaemia should be contrasted with previously
demonstrated training induced changes in NKA protein abundance. Whilst acute NaHCO3 ingestion and associated metabolic alkalosis reduces arterial [K+] at rest
(Sostaric et al., 2006), chronic alkalosis can lead to hypokalaemia through increased renal excretion of K+ (Clausen, 2010). Studies utilising animal models have found that 2 wks of dietary-induced hypokalaemia resulted in a large decrease in NKA α2 abundance
(Azuma et al., 1991; Hsu & Guidotti, 1991). Ten days K+ deprivation resulted in large decreases in α2 and β2 abundances in rat red and white gastrocnemius and DL, while β1
was reduced in red gastrocnemius (Thompson & McDonough, 1996), a muscle of similar fibre composition to the human vastus lateralis sampled in this study (Hundal et al., 1993). Whilst not inducing hypokalaemia at rest, the NaHCO3 supplementation protocol
utilised in the current study resulted in a post-exercise lowering of antecubital venous plasma [K+] to ~0.6 mmol.L-1 below rest, compared to ~0.3 mmol.L-1 below rest for PRE-CaCO3 (Varley, 2013). If this small difference in post-exercise [K+] was repeated
during training it is possible that this may have induced signalling changes in muscle and contributed to the decrease seen in NKA β1 protein abundance following RSE training
activity levels (Geering, 2001), the reduced β1 protein abundance seen post-training may
then adversely affect K+ regulation.
Performance measurements taken during the POST-CaCO3 trial demonstrated that
following 4 weeks training with HCO3 supplementation there was no improvement of
mean power output during RSE testing (Varley, 2013). This finding contrasts previous research in this laboratory demonstrating improved RSE performance following similar training conducted without NaHCO3 supplementation (Serpiello et al., 2011). Therefore,
whilst acute NaHCO3 supplementation may improve acute exercise performance, it is
possible that chronic supplementation may prevent or reduce training adaptations in NKA in skeletal muscle and therefore limit any performance gains. Further research is recommended to examine the effects of NaHCO3 supplementation during training that
comprises exercise of moderate-to-high intensities but of longer durations that are known to enhance NKA content.
4.4.4
Limitations
There were a number of limitations of this study, including the low number of subjects, the typical variability evident in the NKA isoform western blot results, and the lack of additional post-exercise biopsy sampling times to detect any delayed responses in NKA adaptations. Hence, care should be taken when interpreting these protein expression results. On the other hand, the repeated trials give strength to the study design, as the participants acted as their own controls. To reduce the inherent variability in western blotting the protein loading and antibody concentrations were optimised prior to analysis. Nevertheless, variability of results is typical of western blotting and an increased number of participants would have been beneficial. However, he training commitment and NaHCO3 supplementation were very demanding and combined with the invasive
measures made recruitment more difficult than anticipated. It is also possible that the order of the pre-training trials had a time/learning effect on the results, and while the inclusion of familiarisations sessions was intended to minimise this, the randomisation of these trials should also been considered for future studies.
Measurement of muscle [3H]oubain binding site content would also have been beneficial to demonstrate whether training increased total NKA content. Whilst the aim of this study was to investigate individual isoform responses, which was done via western blotting, the lack of changes seen in the individual α isoforms does however suggest that total NKA content would also be unlikely to have increased with RSE.
4.4.5
Conclusions
Acute RSE did not significantly increase the protein abundance of any NKA isoform post-exercise, which is consistent with previous research with short-term continuous exercise. Acute NaHCO3 supplementation resulted in an increased skeletal muscle NKA
β1 isoform protein, which contrasts chronic NaHCO3 supplementation during RSE
training, which significantly decreased muscle β1, as well as increased β3 protein
abundance. It appears likely that the RSE protocol utilised was insufficient to induce training-related increases in skeletal muscle NKA isoform proteins and any upregulatory effect may in fact have been blocked by chronic NaHCO3 supplementation.