In vivo regulation of plasma [H+] in ponies during acute changes in PCO2.

The research study explores how the change in plasma hydrogen ion concentration ([H+]) due to changes in carbon dioxide partial pressure (PCO2) is less significant under acute in vivo conditions than in vitro conditions. This experiment involving ponies elaborates on the relationship between the concentration of hydrogen ions, carbon dioxide partial pressure, bicarbonate concentration, and pH levels both in vitro and in vivo.

Research Methodology

The researchers manipulated the inspired CO2 and O2 levels, causing hyperventilation in the ponies, to purposely increase and decrease arterial PCO2 levels ranging from 5 to 8 Torr from normal standards. The experiment was conducted on 12 ponies where arterial and mixed venous blood was sampled during eucapnia and 5-60 minutes post-change in arterial PCO2 levels.

For in vitro data, five different levels of PCO2 were achieved by blood equilibration in a tonometer.

Results and Observations

• The in vitro relationship slope between H+ and PCO2 for oxygenated and partially oxygenated blood was found to be 0.73 +/- 0.01 and 0.69 +/- 0.01 neq.1-1.Torr-1 respectively. These slopes were notably higher than the in vivo slopes of 0.61 +/- 0.03 and 0.57 +/- 0.03 for arterial and mixed venous blood.
• The change in bicarbonate concentration with respect to change in pH (Slykes) for in vitro oxygenated and partially deoxygenated blood was documented as 15.4 +/- 1.1 and 17.0 +/- 1.1 respectively. On comparing, these were lower than in vivo values of 23.3 +/- 2.7 and 25.2 +/- 4.7 Slykes for arterial and mixed venous blood.
• In vitro experiments, the plasma strong ion difference (SID) saw a rise of 4.5 +/- 0.2 meq/l when PCO2 levels spiked from 25 to 55 Torr. This change was accounted for by a 3.5 meq/l fall in the chloride ion concentration and a 1.3+/- 0.1 meq/l uptick in sodium ion concentration.

Conclusion

Overall, the results support the hypothesis that under acute in vivo conditions, the variation in plasma [H+] due to a change in PCO2 is lesser than that observed under in vitro conditions.