Carbonic anhydrase inhibitors modify intracellular pH transients and contractions of rat middle cerebral arteries during CO₂/HCO₃^– fluctuations

Abstract

The CO/HCO buffer minimizes pH changes in response to acid-base loads, HCO provides substrate for Na,HCO-cotransporters and Cl/HCO-exchangers, and H and HCO modify vasomotor responses during acid-base disturbances. We show here that rat middle cerebral arteries express cytosolic, mitochondrial, extracellular, and secreted carbonic anhydrase isoforms that catalyze equilibration of the CO/HCO buffer. Switching from CO/HCO-free to CO/HCO-containing extracellular solution results in initial intracellular acidification due to hydration of CO followed by gradual alkalinization due to cellular HCO uptake. Carbonic anhydrase inhibition decelerates the initial acidification and attenuates the associated transient vasoconstriction without affecting intracellular pH or artery tone at steady-state. Na,HCO-cotransport and Na/H-exchange activity after NH-prepulse-induced intracellular acidification are unaffected by carbonic anhydrase inhibition. Extracellular surface pH transients induced by transmembrane NH flux are evident under CO/HCO-free conditions but absent when the buffer capacity and apparent H mobility increase in the presence of CO/HCO even after the inhibition of carbonic anhydrases. We conclude that (a) intracellular carbonic anhydrase activity accentuates pH transients and vasoconstriction in response to acute elevations of pCO, (b) CO/HCO minimizes extracellular surface pH transients without requiring carbonic anhydrase activity, and

PTPRG is an ischemia risk locus essential for HCO3–-dependent regulation of endothelial function and tissue perfusion

“…The current study adds to our growing appreciation that acid-base equivalents, and HCO 3 – in particular, fulfill multifaceted functions ( Boedtkjer et al, 2016a ). In cerebral resistance arteries, HCO 3 – (a) contributes to buffering of acute acid loads ( Rasmussen and Boedtkjer, 2018 ), (b) serves as substrate particularly for the Na + ,HCO 3 – -cotransporter NBCn1 that protects against intracellular acidification ( Thomsen et al, 2014 ), and (c) is sensed by RPTPγ to regulate cerebral perfusion ( Figure 6 ). In migrating vascular smooth muscle cells from conduit arteries, the high spatial mobility of the CO 2 /HCO 3 – buffer system also contributes to dissipating local pH gradients in diffusion-restricted spaces of filopodia ( Boedtkjer et al, 2016b ).…”

A New Kid on the Block? Carbonic Anhydrases as Possible New Targets in Alzheimer’s Disease

“…Immunocytochemical experiments demonstrated that astrocytes and neurons express the mitochondrial CA VA suggesting that this isozyme has a cell-specific, physiological role in the nervous system [28]. There is instead a lack of studies on CA VB in the brain, although it was recently detected in rat middle cerebral arteries [29]. CA VII is endowed with a good CO 2 hydrase activity and CA VIII is instead acatalytic—the two isoforms show comparable expression in the cortex, hippocampus and thalamus [30,31].…”

“…With regard to brain vessel wall effects, a recent study by Rasmussen and colleagues on isolated rat middle cerebral arteries showed that switching from CO 2 /HCO 3 − free to CO 2 /HCO 3 − containing bath solution at a fixed pH of 7.4 caused a biphasic pH response with initial acidification followed by a gradual recovery of pH and vasoconstriction [29]. In the presence of AAZ the rate of intracellular acidification and vasoconstriction were reduced, whereas applying the cell-impermeable CAI AMB (4-(aminomethyl)benzenesulfonamide) did not affect the rate of intracellular acidification.…”

“…On the other hand, reduced CA activity was found in post-mortem AD brain tissue [11,14,15] and AAZ acute administration was found to induce amnesic effect in rodents [42,45] and humans [46]. It is important to note though that recent clinical trials using this drug even in very high doses did not reveal any cognitive impairments [29,30,31]. Therefore, due to these contradictions between preclinical and clinical findings and acute versus chronic administrations, if the best therapeutic strategy would be to use activators or inhibitors and during what phase of disease progression is still not clear.…”

Acidosis inhibits rhythmic contractions of human thoracic ducts

“…Using out‐of‐equilibrium conditions – based on rapid mixing and laminar propulsion of solutions with differing H + /HCO 3 – /CO 2 composition – separate signaling consequences of acid‐base equivalents have been identified in resistance arteries (Boedtkjer et al. ; Rasmussen and Boedtkjer ). The response of resistance arteries to extracellular acid‐base disturbances is complex (Boedtkjer ) and relies not only on sensing of extracellular H + but also on separate signaling effects of intracellular H + (Boedtkjer et al.…”