Extramitochondrial domain rich in carbonic anhydrase activity improves myocardial energetics

Abstract

CO 2 is produced abundantly by cardiac mitochondria. Thus an efficient means for its venting is required to support metabolism. Carbonic anhydrase (CA) enzymes, expressed at various sites in ventricular myocytes, may affect mitochondrial CO 2 clearance by catalyzing CO 2 hydration (to H + and HCO 3 − ), thereby changing the gradient for CO 2 venting. Using fluorescent dyes to measure changes in pH arising from the intracellular hydration of extracellularly supplied CO 2 , overall CA activity in the cytoplasm of isolated ventricular myocytes was found to be modest (2.7-fold above spontaneous kinetics). Experiments on ventricular mitochondria demonstrated negligible intramitochondrial CA activity. CA activity was also investigated in intact hearts by 13 C magnetic resonance spectroscopy from the rate of H 13 CO 3 − production from 13 CO 2 released specifically from mitochondria by pyruvate dehydrogenase-mediated metabolism of hyperpolarized [1- 13 C]pyruvate. CA activity measured upon [1- 13 C]pyruvate infusion was fourfold higher than the cytoplasm-averaged value. A fluorescent CA ligand colocalized with a mitochondrial marker, indicating that mitochondria are near a CA-rich domain. Based on immunoreactivity, this domain comprises the nominally cytoplasmic CA isoform CAII and sarcoplasmic reticulum-associated CAXIV. Inhibition of extramitochondrial CA activity acidified the matrix (as determined by fluorescence measurements in permeabilized myocytes and isolated mitochondria), impaired cardiac energetics (indexed by the phosphocreatine-to-ATP ratio measured by 31 P magnetic resonance spectroscopy of perfused hearts), and reduced contractility (as measured from the pressure developed in perfused hearts). These data provide evidence for a functional domain of high CA activity around mitochondria to support CO 2 venting, particularly during elevated and fluctuating respiratory activity. Aberrant distribution of CA activity therefore may reduce the heart’s energetic efficiency.

Dietary fatty acids alter left ventricular myocardial gene expression in Wistar rats

“…[49] Specific to the myocardium, development of cellular or mitochondrial acidosis can obtund contractility through an array of mechanisms, including reduced calcium availability and responsiveness as well as impaired energetics. [50–52] In contrast, increased CAII and CAIV expression was measured in failing myocardium, and it was proposed that increased CA-mediated activation of the Na + /H + exchanger contributed to the hypertrophic process through sustained increases in cytosolic Ca ++ . [53] CAIII is distinct in that it has low CO 2 hydration activity compared to other isozymes and acts as a phosphatase,[54] possibly contributing to free radical scavenging activity.…”

Chemistry and biochemistry of13C hyperpolarized magnetic resonance using dynamic nuclear polarization

“…216 Several basic cellular processes have been investigated using HP [1- 13 C] pyruvate, including a recent study describing the role of carbonic anhydrase-rich domains in facilitating mitochondrial respiration in myocytes. 217 …”

Mammalian Target of Rapamycin (mTOR) Inhibition with Rapamycin Improves Cardiac Function in Type 2 Diabetic Mice

“…CA catalyzes CO 2 hydration and the reverse reaction between H ϩ and HCO 3 Ϫ ions, which helps in the maintenance of intracellular pH (66,67). CA II deficiency in humans is an autosomal recessive disease characterized by renal tubular acidosis, osteopetrosis, cerebral calcification, and growth retardation (68,69).…”