Transgenic Over-Expression of Carbonic Anhydrase III in Cardiac Muscle Demonstrates a Mechanism to Resist Acidosis

Abstract

Carbonic anhydrase III (CAIII) is an abundant protein in skeletal muscle, liver and adipose cells. A cytosolic enzyme that catalyzes conversions between CO2 and HCO3- in regulating intracellular pH, its physiological function in muscle is unclear. Mice lacking CAIII showed lower than wild type intracellular pH in skeletal muscle cells during fatigue treatment. To further understand the role of CAIII in muscle functions and stress adaptation, we developed transgenic mice overexpressing CAIII in the heart under the control of a cloned myosin heavy chain promoter for phenotype comparisons with wild type mouse hearts that are CAIII negative. Three months old transgenic mice showed normal cardiac phenotypes under non-stress conditions. Cardiac function was examined using ex vivo working heart preparations under normal and low pH conditions to investigate CAIII function in pH regulation of cardiac muscle. With equilibration of 5% CO2 generating pH 7.4 in normal Kreb's perfusion buffer, 10% CO2 was used to lower pH to 7.0. Functional data showed that transgenic and wild type hearts had similar pumping functions under normal pH. Perfused with low pH buffer, heart functions of both groups were decreased. In comparison with wild type controls at low pH, CAIII transgenic mouse hearts showed higher left ventricular pressure development and systolic and diastolic velocities under both baseline conditions and increased afterload stress, indicating a better tolerance to acidosis. The results suggest that CAIII may function in compensating for intracellular pH under acidotic conditions, a tractive novel approach to develop new treatment of chronic congestive heart failure.