PLASMA [H+] REGULATION AND WHOLE-BLOOD [CO2] IN EXERCISING PONIES

The major objective was to determine in ponies whether factors in addition to changes in blood PCO2 contribute to changes in plasma [H+] during submaximal exercise. Measurements were made to establish in vivo plasma [H+] at rest and during submaximal exercise, and CO2 titration of blood was completed for both in vitro and acute in vivo conditions. In 19 ponies arterial plasma [H+] was decreased from rest 4.5 neq/l (P<0.05) during the 7th min of treadmill running at 6 mph, 5% grade (P<0.5). A 5.6-Torr exercise hypocapnia accounted for ~2.9 neq/l of this reduced [H+]. The non-PCO2 component of this alkalosis was ~1.6 neq/l, and it was due presumably to a 1.7-meq/l increase from rest in the plasma strong ion difference (SID). Despite the arterial hypocapnia, mixed nous PCO2 was 2.7 Torr above rest during steady-state exercise. Nevertheless, mixed venous plasma [H+] was 1.2 neq/l above rest during exercise, which was presumably due to the increase in SID. Also studied was the effect of submaximal exercise on whole blood CO2 content (CCO2). In vitro, at a given PCO2 there was minimal difference in CCO2 between rest and exercise blood, but plasma [HCO3̄] was greater for exercise blood than for rest blood. In vivo, during steady-state exercise, arterial plasma [HCO3̄] was unchanged or slightly elevated from rest, but Ca(CO2) was 4 vol% below rest. Plasma [HCO3̄] and whole blood CCO2 did not change by the same amount during exercise because when the high plasma and the relatively lower erythrocyte CO2 are combined, the dilution effect of the low erythrocyte CO2 is directly related to the hemoglobin concentration. We conclude that during submaximal exercise in ponies arterial hypocapnia and an increase in plasma SID are the primary causes of a reduction from rest in plasma [H+].