CO 2 permeability and carbonic anhydrase activity of rat cardiomyocytes.

AIM: To determine the CO2 permeability (PCO 2 ) of plasma membranes of cardiomyocytes. These cells were chosen because heart possesses the highest rate of O2 consumption/CO2 production in the body.

METHODS: Cardiomyocytes were isolated from rat hearts using the Langendorff technique. Cardiomyocyte suspensions exhibited a vitality of 2-14% and were studied by the previously described mass spectrometric 18 O-exchange technique deriving PCO 2 . We showed by mass spectrometry and by carbonic anhydrase (CA) staining that non-vital cardiomyocytes are free of CA and thus do not contribute to the mass spectrometric signal, which is determined exclusively by the fully functional vital cardiomyocytes.

RESULTS: Lysed cardiomyocytes yielded an intracellular CA activity for vital cells of 5070; that is, the rate of CO2 hydration inside the cell is accelerated 5071-fold. Using this number, analyses of the mass spectrometric recordings from cardiomyocyte suspensions yield a PCO 2 of 0.10 cm s-1 (SD ± 0.06, n = 15) at 37 °C.

CONCLUSION: In comparison with the PCO 2 of other cells, this value is quite high and about identical to that of the human red cell membrane. As no major protein CO2 channels such as aquaporins 1 and 4 are present in rat cardiac sarcolemma, the high PCO 2 of this membrane is likely due to its low cholesterol content of about 0.2 (mol cholesterol)·(mol total membrane lipids)-1 . Previous work predicted a PCO 2 of ≥0.1 cm s-1 from this level of cholesterol. We conclude that the low cholesterol establishes a PCO 2 high enough to render the membrane resistance to CO2 diffusion almost negligible, even under conditions of maximal O2 consumption of the heart.