CO2 permeability and carbonic anhydrase activity of rat cardiomyocytes

AimTo determine the CO2 permeability (P-CO2) of plasma membranes of cardiomyocytes. These cells were chosen because heart possesses the highest rate of O-2 consumption/CO2 production in the body. MethodsCardiomyocytes 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 O-18-exchange technique deriving P-CO2. 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. ResultsLysed 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 P-CO2 of 0.10 cm s(-1) (SD 0.06, n = 15) at 37 degrees C. ConclusionIn comparison with the P-CO2 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 P-CO2 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 P-CO2 of 0.1 cm s(-1) from this level of cholesterol. We conclude that the low cholesterol establishes a P-CO2 high enough to render the membrane resistance to CO2 diffusion almost negligible, even under conditions of maximal O-2 consumption of the heart.