Plasma [Na+], [Ca2+], and volume shifts and thermoregulation during exercise in man

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Abstract
Whether the equilibrium level of rectal temperature (Tre) with various work loads could be attributed to concomitant changes in plasma Na+ and Ca2+ concentrations, or their ratio was determined. Rectal and mean skin temperatures, T-1824 space and various plasma electrolyte and protein concentrations were measured in 6 trained male runners (19-23 yr) during a 30 min resting (sitting) control period, 60 min of sitting at 6% of .ovrhdot.VO2 max [maximal oxygen uptake], and ergometer exercise at 23%, 43% and 62% of .ovrhdot.VO2 max, followed by 30 min recovery. Ta [ambient temperature] was 24.7-26.0.degree. C and relative humidity 49-53%. The decrease in plasma volume (PV) was 170 ml (-5.9%) after 60 min rest, -3.4% after work load 23%, -4.5% after work load 43% and 428 ml (-14.4%) after work load 62%. Equilibrium levels of Tre were significantly correlated, 0.78 with [Na+], 0.56 with [Cl-], 0.58 with [Osm] and -0.46 with %.DELTA.PV. Tre correlated only -0.06 with ionized [Ca2+], -0.01 with PV and 0.08 with [Na+]/[Ca2+]. The increase in total [Ca] from 5.1 to 5.2-5.5 meq/l (2-8%) during exercise was not significantly related to Tre. With these graded exercise levels, the loss of PV accounts for the progressive increases in plasma electrolyte concentrations that result in their positive correlations with equilibrium levels of Tre.