Carbonic anhydrase inhibition and calcium transients in soleus fibers

Carbonic anhydrase enzymes: Likely targets for inhalational anesthetics

Inhalational anesthetics such as isoflurane, desflurane and halothane are the mainstay medications for surgical procedures; upon inhalation, they produce anesthesia described as reversible unconsciousness with the features of amnesia, sleep, immobility and analgesia. To date, how they produce anesthesia is unknown. This study proposes that carbonic anhydrase enzymes are likely targets mediating the actions of inhalational anesthetics. Carbonic anhydrase enzymes, commonly expressed in living organisms, utilize carbon dioxide (CO₂) as a substrate and can generate H⁺ and HCO₃^- from CO₂ with a great efficiency. There are remarkable lines of evidence for their likely roles in mediating anesthetic actions. Firstly, carbonic anhydrase enzymes are extensively expressed in the brain and spinal cord, and their importance in the brain activity, especially for the GABA and NMDA receptor signaling pathways, has been demonstrated in numerous studies. According to these studies, they provide HCO₃^- for GABA-A receptor activities and also buffer HCO₃^- excess resulting from NMDA receptor activation. Activation of GABA-A and inhibition of NMDA receptors are associated with the induction of anesthesia by the intravenous general anesthetics propofol and ketamine, respectively. Secondly, the carbonic anhydrase inhibitors topiramate and zonisamide are effectively used in the treatment of epilepsy for decades; their chronic use results in the requirement of increased levels of amobarbital in order to produce anesthesia in the epileptic patients during WADA test. In addition, given that CO₂ is a substrate for these enzymes, their tertiary structure is likely has a hydrophobic pocket suitable for the anesthetic molecules to bind. Inhalational anesthetic molecules, which are lipophilic and inert in nature, have an ability to cross the membranes and inhibit carbonic anhydrases, which might not be accessible by topiramate and zonisamide. Unlike carbonic anhydrase inhibitors, they could bind to the hydrophobic pocket for CO₂ molecules and produce a profound effect called anesthesia. Finally, there is a great deal of similarities between the physiological actions of inhalational anesthetics and carbonic anhydrase inhibitors; moreover well-known side effects of inhalational anesthetics could be associated with the inhibition of carbonic anhydrases. Therefore, this article presents a hypothesis that the anesthetic actions of inhalational anesthetics could be due to their inhibitory effects on the carbonic anhydrases. Investigating this hypothesis might lead to the development of new safer anesthetics, and more importantly it might reveal an endogenous anesthetic pathway, in which the carbonic anhydrase system is a component along with the GABA-A and NMDA receptor systems.

Carbonic anhydrase III is not required in the mouse for normal growth, development, and life span

Carbonic anhydrase III is a cytosolic protein which is particularly abundant in skeletal muscle, adipocytes, and liver. The specific activity of this isozyme is quite low, suggesting that its physiological function is not that of hydrating carbon dioxide. To understand the cellular roles of carbonic anhydrase III, we inactivated the Car3 gene. Mice lacking carbonic anhydrase III were viable and fertile and had normal life spans. Carbonic anhydrase III has also been implicated in the response to oxidative stress. We found that mice lacking the protein had the same response to a hyperoxic challenge as did their wild-type siblings. No anatomic alterations were noted in the mice lacking carbonic anhydrase III. They had normal amounts and distribution of fat, despite the fact that carbonic anhydrase III constitutes about 30% of the soluble protein in adipocytes. We conclude that carbonic anhydrase III is dispensable for mice living under standard laboratory husbandry conditions.

Inhibition of muscle carbonic anhydrase slows the Ca(2+) transient in rat skeletal muscle fibers

A countertransport of H(+) is coupled to Ca(2+) transport across the sarcoplasmic reticulum (SR) membrane. We propose that SR carbonic anhydrase (CA) accelerates the CO(2)-HCO reaction so that H(+) ions, which are exchanged for Ca(2+) ions, are produced or buffered in the SR at sufficient rates. Inhibition of this SR-CA is expected to reduce the rate of H(+) fluxes, which then will retard the kinetics of Ca(2+) transport. Fura 2 signals and isometric force were simultaneously recorded in fiber bundles of the soleus (SOL) and extensor digitorum longus (EDL) from rats in the absence and presence of the lipophilic CA inhibitors L-645151, chlorzolamide (CLZ), and ethoxzolamide (ETZ), as well as the hydrophilic inhibitor acetazolamide (ACTZ). Fura 2 and force signals were analyzed for time to peak (TTP), 50% decay time (t(50)), and their amplitudes. L-645151, CLZ, and ETZ significantly increased TTP of fura 2 by 10-25 ms in SOL and by 5-7 ms in EDL and TTP of force by 6-30 ms in both muscles. L-645151 and ETZ significantly prolonged t(50) of fura 2 and force by 20-55 and 40-160 ms, respectively, in SOL and EDL. L-645151, CLZ, and ETZ also increased peak force of single twitches and amplitudes of fura fluorescence ratio (R(340/380)) at an excitation wavelength of 340 to 380 nm. All effects of CA inhibitors on fura 2 and force signals could be reversed. ACTZ did not affect TTP, t(50), and amplitudes of fura 2 signals or force. L-645151, CLZ, and ETZ had no effects on myosin-, Ca(2+)-, and Na(+)-K(+)-ATPase activities, nor did they affect the amplitude and half-width of action potentials. We conclude that inhibition of SR-CA by impairing H(+) countertransport is responsible for deceleration of intracellular Ca(2+) transients and contraction times.

Carbon dioxide transport and carbonic anhydrase in blood and muscle

CO(2) produced within skeletal muscle has to leave the body finally via ventilation by the lung. To get there, CO(2) diffuses from the intracellular space into the convective transport medium blood with the two compartments, plasma and erythrocytes. Within the body, CO(2) is transported in three different forms: physically dissolved, as HCO(3)(-), or as carbamate. The relative contribution of these three forms to overall transport is changing along this elimination pathway. Thus the kinetics of the interchange have to be considered. Carbonic anhydrase accelerates the hydration/dehydration reaction between CO(2), HCO(3)(-), and H(+). In skeletal muscle, various isozymes of carbonic anhydrase are localized within erythrocytes but are also bound to the capillary wall, thus accessible to plasma; bound to the sarcolemma, thus producing catalytic activity within the interstitial space; and associated with the sarcoplasmic reticulum. In some fiber types, carbonic anhydrase is also present in the sarcoplasm. In exercising skeletal muscle, lactic acid contributes huge amounts of H(+) and by these affects the relative contribution of the three forms of CO(2). With a theoretical model, the complex interdependence of reactions and transport processes involved in CO(2) exchange was analyzed.

Metabolic and contractile influence of carbonic anhydrase III in skeletal muscle is age dependent

Carbonic anhydrase (CA) III is very abundant in type I skeletal muscle, but its function is still debated. Our aims were to examine CA III expression during growth and determine whether the effects of CA inhibition previously observed in adult muscles could be seen in younger rats in which CA III levels are lower. CA III content and activity were measured in soleus muscles from 10- to 100-day-old rats, and the influence of CA inhibitor on fatigue and hexosemonophosphate content was quantified in vitro. CA III activity and content increased fivefold between 10 and 100 days of age. Data analysis revealed that the influence of CA inhibitor on fatigue was to some extent positively and linearly related to the level of CA III activity. Hexosemonophosphate accumulation with CA inhibition also became more significant with age. In conclusion, CA III level in soleus muscle does not stabilize before 3 mo after birth; data also confirm that the effects of CA inhibitors are due to inhibition of the CA III isoform.

Inhibition and kinetic properties of membrane-bound carbonic anhydrases in rabbit skeletal muscles

It was the aim of this study to investigate whether the carbonic anhydrases associated with the sarcoplasmic reticulum (SR) and sarcolemmal membranes differ in their kinetic and inhibitory properties. To this end, sarcolemmal and SR membrane vesicle fractions were prepared from rabbit white and red skeletal muscles, the white muscle sarcolemmal fraction (WSL), the red muscle sarcolemmal fraction (RSL), the white muscle SR fraction (WSR), and the red muscle SR fraction (RSR). WSL displayed a specific carbonic anhydrase activity of 22.1 U . ml/mg and RSL of 7.5 U . ml/mg, whereas the SR fractions showed a much lower activity of 0.5 U . ml/mg for WSR and of 2.4 U . ml/mg for RSR. In both SR fractions phase separation experiments with Triton X-114 demonstrated that the carbonic anhydrase activity is due to a membrane-bound enzyme and not due to a cytosolic isozyme. The kinetic properties of carbonic anhydrase from the four distinct membane fractions were evaluated by determination of the Michaelis constant, Km, and of the catalytic centre activity kcat. Km appears to be somewhat lower for SR than for SL. Inhibition constants of SR and SL carbonic anhydrases were determined applying six carbonic anhydrase inhibitors: chlorzolamide, ethoxzolamide, methazolamide, benzolamide, and acetazolamide, and also cyanate. The inhibition constants of the SR fractions were significantly different from those of the corresponding sarcolemmal fractions, indicating that the carbonic anhydrase measured in the SR fractions does not originate from contaminating sarcolemmal membrane vesicles, but appears to represent a distinct carbonic anhydrase associated with the SR membrane.

Localization of carbonic anhydrase IV in rat and human heart muscle

We investigated carbonic anhydrase IV (CA IV) in rat and human heart with immunohistochemical methods by both light and electron microscopy. In cryosections that were incubated with anti-CA IV/FITC, the capillaries showed a strong reaction for CA IV. In paraffin and semithin sections treated with anti-CA IV/ABC (avidin-biotin-peroxidase complex) blood vessels, capillaries, and sarcolemma (SL) were positively stained. By staining ultrathin sections with anti-CA IV/immunogold, CA IV could also be demonstrated at the latter two locations, including the specialized sarcolemmal structures intercalated discs, and T-tubules. In addition, by this method CA IV was seen to be associated with the sarcoplasmic reticulum (SR). The absence of immunostaining in SR and/or SL with some techniques probably indicates a problem of accessibility of the antigenic sites. In line with the immunohistochemical results, CA IV mRNA expression was visualized in both endothelial and muscle cells by in situ hybridization histochemistry.

Acetazolamide reduces peripheral afferent transmission in humans

Carbonic anhydrase has been localized in skeletal muscle and nerve, thus, inhibition with acetazolamide (ACZ) may alter nerve and/or muscle function in healthy humans. ACZ (3 oral doses 14, 8, and 2 h prior to testing) reduced isometric force (37%) and peak to peak electromyographic (EMG) amplitude (1.38 mV to 0.83 mV), while increasing EMG latency associated with a unilateral Achilles tendon-tap. Reflex recovery profiles, following a contralateral conditioning tap, were similar in both placebo and ACZ experiments. ACZ led to significant changes in Hmax/Mmax ratio (52.19/14.42 to 45.73/15.65) and H-reflex latency (34.18 +/- 2.54 ms to 35.24 +/- 2.74 ms). Motor nerve conduction velocity and maximal voluntary isometric torque (knee extensors) were unaltered by ACZ. These data suggest that inhibition of the tendon-tap reflex and associated isometric force, following ACZ, is related to impairment of synaptic integrity between la fibers of the muscle spindle and the alpha motor neuron and not impairment of the muscle spindle or force-generating capacity.

Adult fast myosin pattern and Ca2+-induced slow myosin pattern in primary skeletal muscle culture

A primary muscle cell culture derived from newborn rabbit muscle and growing on microcarriers in suspension was established. When cultured for several weeks, the myotubes in this model develop the completely adult pattern of fast myosin light and heavy chains. When Ca2+ ionophore is added to the culture medium on day 11, raising intracellular [Ca2+] about 10-fold, the myotubes develop to exhibit properties of an adult slow muscle by day 30, expressing slow myosin light as well as heavy chains, elevated citrate synthase, and reduced lactate dehydrogenase. The remarkable plasticity of these myotubes becomes apparent, when 8 days after withdrawal of the ionophore a marked slow-to-fast transition, as judged from the expression of isomyosins and metabolic enzymes, occurs.

Membrane-associated carbonic anhydrase IV in skeletal muscle: subcellular localization

Carbonic anhydrase IV (CA IV) was examined by light microscopy and electron microscopy in rat soleus muscle. Semithin sections of aldehyde-fixed Epon-embedded muscle were stained with rabbit anti-rat lung CA IV and the avidin-biotin-peroxidase complex. With this technique, capillaries and sarcolemma showed positive CA IV staining. For electron microscopy, rat soleus specimens were aldehyde-fixed, with or without subsequent osmication, and embedded in Epon. Ultrathin sections were immunostained with anti-rat lung CA IV/immunogold. Omitting osmium allowed ample antigen-antibody reactions but could not prevent the release of glycosylphosphatidylinositol-anchored CA IV from the membranes, which led to apparent background staining. Postosmication significantly reduced tissue antigenicity but kept the antigen bound to the membranes and thus allowed a very precise localization of CA IV. By electron microscopy, membrane-bound CA IV is found to be associated with capillary endothelium, sarcolemma, and sarcoplasmic reticulum (SR). Conceivably, the presence of SR staining in ultrathin sections and its absence in semithin sections reflect a problem of accessibility of the antigenic sites.

Effects of carbonic anhydrase inhibitors on oxygen consumption and lactate accumulation in skeletal muscle

In isolated rat soleus and extensor digitorum longus (EDL) muscles, the effects of carbonic anhydrase inhibitors were studied on oxygen consumption as well as lactate release and accumulation after incubation in inhibitors lasting long enough to produce marked changes in contractile parameters and in the concentrations of energy-rich phosphates. The inhibitors used were chlorzolamide (10(-3) M) and NaCNO (10(-2) M). Compared with control muscles, muscles treated with either of the two inhibitors showed a decrease in force, and an increase in time-to-peak as well as in relaxation time. Lactate content and release in soleus and in EDL were increased by factors of 2-3 with both inhibitors. With both inhibitors, oxygen consumption in the red soleus increased by approximately 27%, whereas in EDL, no significant change could be observed. The increase in aerobic metabolic rate in the red soleus only might indicate that the isozyme CA III, which is present only in this type of muscle, is in some way involved in keeping the oxygen consumption low. The increase in anaerobic metabolic rate occurring in both muscles can possibly be explained by increases in Pi and ADP.

Contractile function of papillary muscles with carbonic anhydrase inhibitors

Contractile parameters of directly stimulated rabbit papillary muscles were studied during incubation in baths containing the carbonic anhydrase inhibitors chlorzolamide (1*10(-3) M) or ethoxzolamide (1*10(-4) M). Both inhibitors caused an at least partly reversible decrease in isometric force as it has been observed in skeletal muscle, and--in contrast to the results in skeletal muscles--a decrease in time-to-peak and half-relaxation time. It is postulated that inhibition of the membrane-bound carbonic anhydrase of heart muscle might induce an intracellular acidosis and that this acidosis causes the observed effects on contractile parameters.

Ca(2+)-dependent heat production under basal and near-basal conditions in the mouse soleus muscle

1. The rate of energy expended for the clearance of sarcoplasmic Ca2+ by sarcoreticular Ca2+ uptake process(es), plus the concomitant metabolic reactions, was evaluated from measurements of resting heat production by mouse soleus muscle before and after indirect inhibition of Ca2+ uptake by sarcoplasmic reticulum (SR). 2. Direct inhibition of the Ca2+, Mg(2+)-ATPase of SR membrane in intact muscle preparations exposed to the specific inhibitor 2,5-di(tert-butyl-1,4-benzohydroquinone (tBuBHQ) slowly increased the rate of heat production (E). Indirect inhibition of SR Ca2+ uptake was obtained by reducing sarcoplasmic Ca2+ concentration (Ca2+i) as a consequence of reducing Ca2+ release from the SR using dantrolene sodium. This promptly decreased E by 12%. Exposure of the preparations to an Mg(2+)-enriched environment (high Mg2+) or to the chemical phosphatase 2,3-butanedione monoxime (BDM), two other procedures aimed at decreasing SR Ca2+ release, also acutely decreased E, by 20 and 24%, respectively. 3. Subthreshold-for-contracture depolarization of the sarcolemma achieved by increasing extracellular K+ concentration to 11.8 mM induced a biphasic increase of E: an initial peak to 290% of basal E, followed by a plateau phase at 140% of basal E during which resting muscle tension was increased by less than 3%. Most, if not all, of the plateau-phase metabolic response was quickly suppressed by dantrolene or high Mg2+ or BDM. Another means of increasing SR Ca2+ cycling was to partially remove the calmodulin-dependent control of SR Ca2+ release using the calmodulin inhibitor W-7. The progressive increase in E with 30 microM-W-7 was largely reduced by dantrolene or high Mg2+ or BDM. 4. In the presence of either dantrolene or BDM to prevent the effect of W-7 on SR Ca2+ release, exposure of the muscle to W-7 acutely suppressed about 3% of E. This and the above results confirm that the plasmalemmal, calmodulin-dependent Ca(2+)-ATPase, although a qualitatively essential part of the Ca2+i homeostatic system of the cell, can only be responsible for a very minor part of the energy expenditure devoted to the homeostasis of Ca2+i. Active Ca2+ uptake by SR which, at least in the submicromolar range of Ca2+i, is expected to be responsible for most of this Ca(2+)-dependent energy expenditure, might dissipate up to 25-40% of total metabolic energy in the intact mouse soleus under basal and near-basal conditions.