Skeletal muscle myofibrillar and sarcoplasmic protein synthesis rates are affected differently by altitude-induced hypoxia in native lowlanders

[EN] As a consequence to hypobaric hypoxic exposure skeletal muscle atrophy is often reported. The underlying mechanism has been suggested to involve a decrease in protein synthesis in order to conserve O(2). With the aim to challenge this hypothesis, we applied a primed, constant infusion of 1-(13)C-leucine in nine healthy male subjects at sea level and subsequently at high-altitude (4559 m) after 7-9 days of acclimatization. Physical activity levels and food and energy intake were controlled prior to the two experimental conditions with the aim to standardize these confounding factors. Blood samples and expired breath samples were collected hourly during the 4 hour trial and vastus lateralis muscle biopsies obtained at 1 and 4 hours after tracer priming in the overnight fasted state. Myofibrillar protein synthesis rate was doubled; 0.041+/-0.018 at sea-level to 0.080+/-0.018%hr(-1) (p<0.05) when acclimatized to high altitude. The sarcoplasmic protein synthesis rate was in contrast unaffected by altitude exposure; 0.052+/-0.019 at sea-level to 0.059+/-0.010%hr(-1) (p>0.05). Trends to increments in whole body protein kinetics were seen: Degradation rate elevated from 2.51+/-0.21 at sea level to 2.73+/-0.13 micromolkg(-1)min(-1) (p = 0.05) at high altitude and synthesis rate similar; 2.24+/-0.20 at sea level and 2.43+/-0.13 micromolkg(-1)min(-1) (p>0.05) at altitude. We conclude that whole body amino acid flux is increased due to an elevated protein turnover rate. Resting skeletal muscle myocontractile protein synthesis rate was concomitantly elevated by high-altitude induced hypoxia, whereas the sarcoplasmic protein synthesis rate was unaffected by hypoxia. These changed responses may lead to divergent adaptation over the course of prolonged exposure.

Similar carbohydrate but enhanced lactate utilization during exercise after 9 wk of acclimatization to 5,620 m

[EN] We hypothesized that reliance on lactate as a means of energy distribution is higher after a prolonged period of acclimatization (9 wk) than it is at sea level due to a higher lactate Ra and disposal from active skeletal muscle. To evaluate this hypothesis, six Danish lowlanders (25 +/- 2 yr) were studied at rest and during 20 min of bicycle exercise at 146 W at sea level (SL) and after 9 wk of acclimatization to 5,260 m (Alt). Whole body glucose Ra was similar at SL and Alt at rest and during exercise. Lactate Ra was also similar for the two conditions at rest; however, during exercise, lactate Ra was substantially lower at SL (65 micro mol. min(-1). kg body wt(-1)) than it was at Alt (150 micro mol. min(-1). kg body wt(-1)) at the same exercise intensity. During exercise, net lactate release was approximately 6-fold at Alt compared with SL, and related to this, tracer-calculated leg lactate uptake and release were both 3- or 4-fold higher at Alt compared with SL. The contribution of the two legs to glucose disposal was similar at SL and Alt; however, the contribution of the two legs to lactate Ra was significantly lower at rest and during exercise at SL (27 and 81%) than it was at Alt (45 and 123%). In conclusion, at rest and during exercise at the same absolute workload, CHO and blood glucose utilization were similar at SL and at Alt. Leg net lactate release was severalfold higher, and the contribution of leg lactate release to whole body lactate Ra was higher at Alt compared with SL. During exercise, the relative contribution of lactate oxidation to whole body CHO oxidation was substantially higher at Alt compared with SL as a result of increased uptake and subsequent oxidation of lactate by the active skeletal muscles.

Regional differences in modelled net production and shallow remineralization in the North Atlantic subtropical gyre

[EN] We used 5-yr concomitant data of tracer distribution from the BATS (Bermuda Time-series Study) and ESTOC (European Station for Time-Series in the Ocean, Canary Islands) sites to build a 1-D tracer model conservation including horizontal advection, and then compute net production and shallow remineralization rates for both sites. Our main goal was to verify if differences in these rates are consistent with the lower export rates of particulate organic carbon observed at ESTOC. Net production rates computed below the mixed layer to 110m from April to December for oxygen, dissolved inorganic carbon and nitrate at BATS (1.34±0.79 molO2 m?2, ?1.73±0.52 molCm?2 and ?125±36 mmolNm?2) were slightly higher for oxygen and carbon compared to ESTOC (1.03±0.62 molO2 m?2, ?1.42±0.30 molCm?2 and ?213±56 mmolNm?2), although the differences were not statistically significant. Shallow remineralization rates between 110 and 250m computed at ESTOC (?3.9±1.0 molO2 m?2, 1.53±0.43 molCm?2 and 38±155 mmolNm?2) were statistically higher for oxygen compared to BATS (?1.81±0.37 molO2 m?2, 1.52± 0.30 molCm?2 and 147±43 mmolNm?2). The lateral advective flux divergence of tracers, which was more significant at ESTOC, was responsible for the differences in estimated oxygen remineralization rates between both stations. According to these results, the differences in net production and shallow remineralization cannot fully explain the differences in the flux of sinking organic matter observed between both stations, suggesting an additional consumption of nonsinking organic matter at ESTOC.