Does gill boundary layer carbonic anhydrase contribute to carbon dioxide excretion: a comparison between dogfish (Squalus acanthias) and rainbow trout (Oncorhynchus mykiss).
In vivo experiments were conducted on spiny dogfish (Squalus acanthias) and rainbow trout (Oncorhynchus mykiss) in sea water to determine the potential role of externally oriented or gill boundary layer carbonic anhydrase in carbon dioxide excretion. This was accomplished by assessing pH changes in expired water using a stopped-flow apparatus. In dogfish, expired water was in acid-base disequilibrium as indicated by a pronounced acidification (delta pH=-0.11+/-0.01; N=22; mean +/- s.e.m.) during the period of stopped flow; inspired water, however, was in acid-base equilibrium (delta pH=-0.002+/-0.01; N=22). The acid-base disequilibrium in expired water was abolished (delta pH=-0.005+/-0.01; N=6) by the addition of bovine carbonic anhydrase (5 mg l-1) to the external medium. Addition of the carbonic anhydrase inhibitor acetazolamide (1 mmol l-1) to the water significantly reduced the magnitude of the pH disequilibrium (from -0.133+/-0.03 to -0.063+/-0.02; N=4). However, after correcting for the increased buffering capacity of the water caused by acetazolamide, the acid-base disequilibrium during stopped flow was unaffected by this treatment (control delta [H+]=99.8+/-22.8 micromol l-1; acetazolamide delta [H+]=81.3+/-21.5 micromol l-1). In rainbow trout, expired water displayed an acid-base disequilibrium (delta pH=0.09+/-0.01; N=6) that also was abolished by the application of external carbonic anhydrase (delta pH=0.02+/-0.01). The origin of the expired water acid-base disequilibrium was investigated further in dogfish. Intravascular injection of acetazolamide (40 mg kg-1) to inhibit internal carbonic anhydrase activity non-specifically and thus CO2 excretion significantly diminished the extent of the expired water disequilibrium pH after 30 min (from -0.123+/-0.01 to -0.065+/-0.01; N=6). Selective inhibition of extracellular carbonic anhydrase activity using a low intravascular dose (1.3 mg kg-1) of the inhibitor benzolamide caused a significant reduction in the acid-base disequilibrium after 5 min (from -0.11+/-0.01 to -0.07+/-0. 01; N=14). These results demonstrate that the expired water acid-base disequilibrium originates, at least in part, from excretory CO2 and that extracellular carbonic anhydrase in dogfish may have a significant role in carbon dioxide excretion. However, externally oriented carbonic anhydrase (if present in dogfish) plays no role in catalysing the hydration of the excretory CO2 in water flowing over the gills and thus is unlikely to facilitate CO2 excretion. (+info)
Synaptic activation of GABAA receptors induces neuronal uptake of Ca2+ in adult rat hippocampal slices.
Synaptically evoked transmembrane movements of Ca2+ in the adult CNS have almost exclusively been attributed to activation of glutamate receptor channels and the consequent triggering of voltage-gated calcium channels (VGCCs). Using microelectrodes for measuring free extracellular Ca2+ ([Ca2+]o) and extracellular space (ECS) volume, we show here for the first time that synaptic stimulation of gamma-aminobutyric acid-A (GABAA) receptors can result in a decrease in [Ca2+]o in adult rat hippocampal slices. High-frequency stimulation (100-200 Hz, 0.4-0.5 s) applied in stratum radiatum close (+info)
Modulation of chloride, potassium and bicarbonate transport by muscarinic receptors in a human adenocarcinoma cell line.
1. Short-circuit current (I(SC)) responses to carbachol (CCh) were investigated in Colony 1 epithelia, a subpopulation of the HCA-7 adenocarcinoma cell line. In Krebs-Henseleit (KH) buffer, CCh responses consisted of three I(SC) components: an unusual rapid decrease (the 10 s spike) followed by an upward spike at 30 s and a slower transient increase (the 2 min peak). This response was not potentiated by forskolin; rather, CCh inhibited cyclic AMP-stimulated I(SC). 2. In HCO3- free buffer, the decrease in forskolin-elevated I(SC) after CCh was reduced, although the interactions between CCh and forskolin remained at best additive rather than synergistic. When Cl- anions were replaced by gluconate, both Ca2+- and cyclic AMP-mediated electrogenic responses were significantly inhibited. 3. Basolateral Ba2+ (1-10 mM) and 293B (10 microM) selectively inhibited forskolin stimulation of I(SC), without altering the effects of CCh. Under Ba2+- or 293B-treated conditions, CCh responses were potentiated by pretreatment with forskolin. 4. Basolateral charybdotoxin (50 nM) significantly increased the size of the 10 s spike of CCh responses in both KH and HCO3- free medium, without affecting the 2 min peak. The enhanced 10 s spike was inhibited by prior addition of 5 mM apical Ba2+. Charybdotoxin did not affect forskolin responses. 5. In epithelial layers prestimulated with forskolin, the muscarinic antagonists atropine and 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP, both at 100 nM) abolished subsequent 10 microM CCh responses. Following addition of p-fluoro hexahydro-sila-difenidol (pF-HHSiD, 10 microM) or pirenzepine (1 microM), qualitative changes in the CCh response time-profile also indicated a rightward shift of the agonist concentration-response curve; however, 1 microM gallamine had no effect. These results suggest that a single M3-like receptor subtype mediates the secretory response to CCh. 6. It is concluded that CCh and forskolin activate discrete populations of basolateral K+ channels gated by either Ca2+ or cyclic AMP, but that the Cl- permeability of the apical membrane may limit their combined effects on electrogenic Cl- secretion. In addition, CCh activates a Ba2+-sensitive apical K+ conductance leading to electrogenic K+ transport. Both agents may also modulate HCO3- secretion through a mechanism at least partially dependent on carbonic anhydrase. (+info)
Effects of maternal acetazolamide treatment on body weights and incisor development of the fetal rat.
The incisor development of fetal rats on gestation day 19 was well correlated with their fetal weights. The number of odontoblasts in the mandibular incisors, an index of incisor development, increased more than that of the maxillary incisors with increase in fetal body weights. Maternal acetazolamide treatments were observed to suppress the mean fetal weight and to retard incisor development. A smaller incisor size, a thinner predentin layer, and fewer odontoblasts were characteristic of the acetazolamide group. There was also a good correlation between the fetal weights and the number of odontoblasts in the acetazolamide group. From these results, we postulated that the retarded incisor development of the fetal rats caused by the maternal acetazolamide treatment was related to their suppressed fetal weights. However, the regression coefficient of the fetal weights and the number of odontoblasts in the acetazolamide group was smaller than that of the vehicle control group. It may indicate that retarded incisor development in response to maternal acetazolamide treatment is to some extent independent of suppressed fetal weight. (+info)
Endogenous pH shifts facilitate spreading depression by effect on NMDA receptors.
Rapid extracellular alkalinizations accompany normal neuronal activity and have been implicated in the modulation of N-methyl-D-aspartate (NMDA) receptors. Particularly large alkaline transients also occur at the onset of spreading depression (SD). To test whether these endogenous pH shifts can modulate SD, the alkaline shift was amplified using benzolamide, a poorly permeant inhibitor of interstitial carbonic anhydrase. SD was evoked by microinjection of 1.2 M KCl into the CA1 stratum radiatum of rat hippocampal slices and recorded by a proximal double-barreled pH microelectrode and a distal potential electrode. In Ringer solution of pH 7.1 containing picrotoxin (but not at a bath pH of 7.4), addition of 10 microM benzolamide increased the SD alkaline shift from 0.20 +/- 0.07 to 0.38 +/- 0.17 unit pH (means +/- SE). This was correlated with a significant shortening of the latency and an increase in the conduction velocity by 26 +/- 16%. In the presence of the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV), benzolamide still amplified the alkaline transient, however, its effect on the SD latency and propagation velocity was abolished. The intrinsic modulation of SD by its alkaline transient may play an important role under focal ischemic conditions by removing the proton block of NMDA receptors where interstitial acidosis would otherwise limit NMDA receptor activity. (+info)
Inhibition of carbonic anhydrase accounts for the direct vascular effects of hydrochlorothiazide.
Hydrochlorothiazide has been shown to exert direct vasodilator effects by activation of calcium-activated potassium (KCa) channels in human and guinea pig isolated resistance arteries. Since hydrochlorothiazide binds to and inhibits the enzyme carbonic anhydrase and because KCa channel activation is pH sensitive, we investigated the role of intracellular and extracellular carbonic anhydrase in the vascular effects of thiazide diuretics. Small arteries were isolated from guinea pig mesentery and studied by use of a microvascular myograph technique. In some experiments, tone and intracellular pH (pHi) were measured simultaneously with 2', 7'-bis(2-carboxyethyl)-5(6)'-carboxyfluorescein (BCECF-AM). Bendroflumethiazide, a thiazide diuretic with minimal inhibitory effects on carbonic anhydrase, had little effect on noradrenaline-induced tone (16+/-8% relaxation) compared with hydrochlorothiazide (74+/-12% relaxation). In contrast to hydrochlorothiazide, the action of bendroflumethiazide was unaffected by 100 nmol/L charybdotoxin, a selective blocker of KCa channels. All inhibitors of carbonic anhydrase relaxed noradrenaline-induced tone in a concentration-dependent manner, and this effect was blocked by charybdotoxin. Hydrochlorothiazide and the inhibitors of carbonic anhydrase failed to relax tone induced by a depolarizing potassium solution. Acetazolamide and hydrochlorothiazide increased pHi by 0.27+/-0.07 and 0.21+/-0.04, respectively, whereas bendroflumethiazide had a much smaller effect: 0.06+/-0.03. The rise in pHi induced by any agent was not inhibited by charybdotoxin. The vasorelaxant effect of hydrochlorothiazide is shared by other inhibitors of carbonic anhydrase. Inhibitors of carbonic anhydrase, but not bendroflumethiazide, cause intracellular alkalinization, which is associated with KCa channel opening. These data suggest that the vasodilator effect of thiazide diuretics results primarily from inhibition of vascular smooth muscle cell carbonic anhydrase, which results in a rise in pHI, leading to KCa channel activation and vasorelaxation. (+info)
Topical therapies for glaucoma: what family physicians need to know.
Medication classes historically used in the management of glaucoma include beta blockers, miotics, sympathomimetics and carbonic anhydrase inhibitors. Because topically applied medications are more site specific, they are preferred in the treatment of glaucoma. Compared with oral medications, topical agents are associated with a decreased incidence of systemic side effects. With topical administration, conjunctival and localized skin allergic reactions are relatively common, whereas severe reactions, including death, are rare. Recently introduced topical agents for glaucoma therapy include dorzolamide and brinzolamide, the first topical carbonic anhydrase inhibitors; brimonidine and apraclonidine, more ocular-specific alpha agonists; and latanoprost, a prostaglandin analog, which is a new class of glaucoma medication. Latanoprost has the unique side effect of increasing iris pigmentation. Like their predecessors, the newer agents lower intraocular pressure by a statistically significant degree. Preservation of visual field, the more substantial patient-oriented end point, continues to be studied. (+info)
VCO2 and VE kinetics during moderate- and heavy-intensity exercise after acetazolamide administration.
The effect of carbonic anhydrase inhibition with acetazolamide (Acz) on CO2 output (VCO2) and ventilation (VE) kinetics was examined during moderate- and heavy-intensity exercise. Seven men [24 +/- 1 (SE) yr] performed cycling exercise during control (Con) and Acz (10 mg/kg body wt iv) sessions. Each subject performed step transitions (6 min) in work rate from 0 to 100 W [below ventilatory threshold (VET)]. VE and gas exchange were measured breath by breath. The time constant (tau) was determined for exercise VET by using a three-component model (fit from the start of exercise). VCO2 kinetics were slower in Acz (VET, MRT = 75 +/- 10 s) than Con (VET, MRT = 54 +/- 7 s). During VET kinetics were faster in Acz (MRT = 85 +/- 17 s) than Con (MRT = 106 +/- 16 s). Carbonic anhydrase inhibition slowed VCO2 kinetics during both moderate- and heavy-intensity exercise, demonstrating impaired CO2 elimination in the nonsteady state of exercise. The slowed VE kinetics in Acz during exercise +info)