Measurement of pulmonary venous and arterial pH oscillations in dogs using catheter tip pH electrodes

The disequilibrium pH: a tool for the localization of carbonic anhydrase

The disequilibrium pH is defined as any discrepancy between the measured pH and the pH which would exist if CO2-HCO3-H+ reactions were at equilibrium. Measurement of the disequilibrium pH can be used to assess the status of CO2-HCO3--H+ reactions and, in combination with carbonic anhydrase (CA) or CA inhibitor treatments, may also be used to localize CA. Renal physiologists have used disequilibrium experiments to determine that HCO3- reabsorption in the kidney tubule occurs via proton secretion, and that CA activity is available to ultrafiltrate CO2-HCO3-H+ reactions in the proximal convoluted tubule, but not the distal tubule. Disequilibrium experiments were also used in investigating the availability of CA to CO2-HCO3--H+ reactions in water at the fish gill; the opposing results obtained in two studies have not yet been resolved. Respiratory physiologists have used the disequilibrium technique in vivo and with saline-perfused preparations to assess the availability of CA to plasma CO2-HCO3--H+ reactions following gas exchange. Saline-perfused preparations enable direct localization of CA activity, while in vivo measurements encompass the numerous factors affecting CO2-HCO3--H+ equilibration in a multi-phase solution. Given the many organs in which membrane-bound CA activity has now been identified, the usefulness of the disequilibrium pH technique has increased beyond its original applications in renal and pulmonary physiology.

Acid-base disequilibrium in the arterial blood of rainbow trout

An extracorporeal blood circulation and a stopflow technique were used to examine the acid-base status of arterial blood in the rainbow trout, Oncorhynchus mykiss. Arterial blood was routed from the coeliac artery through an external circuit in which pH (pHa), partial pressure of oxygen (PaO2) and partial pressure of carbon dioxide (PaCO2) were monitored continuously. The stopflow condition was imposed by turning off the pump which drove the external loop. A radioisotopic CO2 excretion assay was performed on blood samples collected periodically to evaluate plasma carbonic anhydrase (CA) activity and hence red blood cell (rbc) lysis. An acid-base disequilibrium was found in the post-branchial blood; pHa increased by 0.04-0.06 units, and PaCO2 by 0.03-0.10 Torr, during the stopflow period. The disequilibrium appeared to arise primarily from the slow (uncatalyzed) rate of plasma H2CO3 dehydration. This was confirmed by the intra-arterial injection of bovine CA (22 mg kg-1) prior to the stopflow; the disequilibrium was abolished. When the CA inhibitor acetazolamide (30 mg kg-1) was injected, a negative pH disequilibrium of 0.04 units, accompanied by a rise in PaCO2 of 0.57 Torr, was observed during the stopflow. These results can be explained by the acetazolamide-induced inhibition of rbc CA, which leads to continuing rbc CO2 "excretion" in the post-branchial blood.

Inhibition of carbonic anhydrase in dog plasma

Using a pH stat method, we measured the activity of carbonic anhydrase (CA) from dog erythrocytes in the presence of various amounts of dog plasma. A plasma factor appeared to be able to inhibit about 86% of the total CA activity, corresponding to the relative activity of CA II. Naiodoacetate was shown to inhibit the total CA activity up to about 13%, corresponding to the relative activity of CA I. C1- inhibited the total CA activity up to about 20%, presumably mainly through its strong influence on Ca I. It is concluded that with a degree of haemolysis of up to 3%, no appreciable plasma CA activity will occur.