A study of temporal relation between intracellular pH and contractile performance in toad ventricular strips during hypercapnic acidosis

Experiments were performed on toad (Bufo marinus) ventricular strips to determine the temporal relation between the decrease in intracellular pH (pH_i) and the changes in mechanical performance accompanying increases in P_CO2. Changes in pH_i were measured with a dual wavelength spectrophotometer using the pH sensitive dye neutral red. Regardless of whether the superfusate pH (pH₀) or [HCO₃^?] was held constant, changes in P_CO2 were accompanied by a monotonic, biphasic change in pH_i. A plot of 1n ΔpH_i v. t consistently yielded a curvilinear line which was well fit by two straight lines. The developed tension also exhibited a biphasic response to ΔP_CO2: an initial decrease followed by a rebound which returned τ to control values (98.4 ± 1.6%) after 30 min. The onset of the second, slow phase of acidification correlated well (r = 0.83) with the onset of the mechanical rebound. The time of this onset was affected by [HCO₃^?]. Further experiments showed that with P_CO2 held constant, a decrease in [HCO₃^?] was accompanied by a slow decrease in pH_i. The data from these experiments suggest that: (1) changes in P_CO2 affect the pH of at least two intracellular compartments and these effects are temporally dissociated; (2) the response of the faster compartment correlates with the initial decrease of τ; the response of the slower with the rebound of τ; (3) when P_CO2 is held constant, decreases in pH₀ are accompanied by a slow decrease in pH_i suggesting a possible H⁺ or [HCO₃^?] leak.     

CO2 back-diffusion in the rete aids O2 secretion in the swimbladder of the eel

In order to estimate the relative importance of back-diffusion of O2 and CO2 for their partial pressure enhancement in the swimbladder, we have determined O2 and CO2 partial pressure and content and pH in microsamples collected non-obstructively from the afferent and efferent rete vessels in the European eel. 1. The PO2 increased significantly along the arterial vessels of the rete (from 33 to 303 Torr), with no change in O2 content, suggesting O2 not to be exchanged in the rete counter-current system. 2. A corresponding increase of PCO2 (from 4 to 35 Torr) was accompanied by a significant rise in CO2 content (from 8 to 15 mmol.L-1), suggesting significant CO2 back-diffusion in the rete. 3. Changes in PO2 during passage of blood through the swimbladder epithelium were variable and small, and the PO2 in rete venous blood was similar to that in rete arterial blood, explaining the lack of O2 back-diffusion. 4. Using blood CO2 dissociation data, about 70% of the rise in arterial CO2 content was estimated to derive from diffusion of CO2, the remaining 30% from diffusion of HCO3-, from venous to arterial rete capillaries, or from H+ transport in the opposite direction. The data indicate that CO2 back-diffusion in the rete does not only raise the rete arterial PCO2; it also reduces the O2 capacity (Root effect) and thus enhances the arterial PO2.     

Different frequencies of active interruptions to sitting have distinct effects on 22 h glycemic control in type 2 diabetes

Background & aimsWhether the frequency of interruptions to sitting time involving simple resistance activities (SRAs), compared to uninterrupted sitting, differentially affected 22 h glycemic control in adults with medication-controlled type 2 diabetes (T2D).Methods & resultsTwenty-four participants (13 men; mean ± SD age 62 ± 8 years) completed three 8 h laboratory conditions: SIT: uninterrupted sitting; SRA3: sitting interrupted with 3 min of SRAs every 30 min; and, SRA6: sitting interrupted with 6 min of SRAs every 60 min. Flash glucose monitors assessed glycemic control over a 22 h period. No differences were observed between conditions for overall 22 h glycemic control as measured by AUC_total, mean glucose and time in hyperglycemia. During the 3.5 h post-lunch period, mean glucose was significantly lower during SRA6 (10.1 mmol·L⁻¹, 95%CI 9.2, 11.0) compared to SIT (11.1 mmol·L⁻¹, 95%CI 10.2, 12.0; P = 0.006). Post-lunch iAUC_net was significantly lower during SRA6 (6.2 mmol·h·L⁻¹, 95%CI 3.3, 9.1) compared to SIT (9.9 mmol·h·L⁻¹, 95%CI 7.0, 12.9; P = 0.003). During the post-lunch period, compared to SIT (2.2 h, 95%CI 1.7, 2.6), time in hyperglycemia was significantly lower during SRA6 (1.5 h, 95%CI 1.0, 1.9, P = 0.001). Nocturnal mean glucose was significantly lower following the SRA3 condition (7.6 mmol·L⁻¹, 95%CI 7.1, 8.1) compared to SIT (8.1 mmol·L⁻¹, 95%CI 7.6, 8.7, P = 0.024).ConclusionsWith standardized total activity time, less-frequent active interruptions to sitting may acutely improve glycemic control; while more-frequent interruptions may be beneficial for nocturnal glucose in those with medication-controlled T2D.     

Solute back-diffusion raises the gas concentrating efficiency in counter-current flow

We have extended the counter-current model of the rete mirabile of the fish swimbladder to include the effects of inert gas secretion into the swimbladder as well as solute back-diffusion in the rete capillaries. (1) Gas secretion attenuates the inert gas concentrating efficiency of the rete, i.e. its ability to produce high inert gas partial pressures in blood at the swimbladder pole. The maximum attainable gas secretion rate depends on the salting-out effect, i.e. on the ratio of solubility in venous and arterial blood of the rete. (2) Solute back-diffusion leads to a significant increase in the concentrating efficiency, and this is due to the salting-out effect produced by the solute when it diffuses back into the arterial capillary blood. This enhancement is particularly prominent when gas and solute permeabilities of the rete vessels are high. (3) Estimates for physiologic parameters in the European eel suggest that lactate back-diffusion may contribute significantly to the gas concentrating efficiency of the rete mirabile.     

Acidified seawater impacts sea urchin larvae pH regulatory systems relevant for calcification

Calcifying echinoid larvae respond to changes in seawater carbonate chemistry with reduced growth and developmental delay. To date, no information exists on how ocean acidification acts on pH homeostasis in echinoderm larvae. Understanding acid–base regulatory capacities is important because intracellular formation and maintenance of the calcium carbonate skeleton is dependent on pH homeostasis. Using H⁺-selective microelectrodes and the pH-sensitive fluorescent dye BCECF, we conducted in vivo measurements of extracellular and intracellular pH (pH_e and pH_i) in echinoderm larvae. We exposed pluteus larvae to a range of seawater CO₂ conditions and demonstrated that the extracellular compartment surrounding the calcifying primary mesenchyme cells (PMCs) conforms to the surrounding seawater with respect to pH during exposure to elevated seawater pCO₂. Using FITC dextran conjugates, we demonstrate that sea urchin larvae have a leaky integument. PMCs and spicules are therefore directly exposed to strong changes in pH_e whenever seawater pH changes. However, measurements of pH_i demonstrated that PMCs are able to fully compensate an induced intracellular acidosis. This was highly dependent on Na⁺ and HCO₃⁻, suggesting a bicarbonate buffer mechanism involving secondary active Na⁺-dependent membrane transport proteins. We suggest that, under ocean acidification, maintained pH_i enables calcification to proceed despite decreased pH_e. However, this probably causes enhanced costs. Increased costs for calcification or cellular homeostasis can be one of the main factors leading to modifications in energy partitioning, which then impacts growth and, ultimately, results in increased mortality of echinoid larvae during the pelagic life stage.     

Central chemoreceptor stimulus in the terrestrial,pulmonate snail,Helix aspersa

We studied the effect of hypercapnic and fixed acid central chemoreceptor stimulation on the pneumostone in the pulmonate snail, Helix aspersa. We found that focal stimulation of the central chemoreceptor area of the pulmonate snail brain with hypercapnic solutions more effectively increased the pneumostonal area than did fixed acid stimulation at the same extracellular pH. Disrupting intracellular pH regulation by inhibiting Cl⁻ transport, either pharmacologically (DIDS) or by ion substitution (Cl⁻-free perfusate), enhanced pneumostomal responses to CO₂. While maintaining a constant perfusate pH, addition of NH₄Cl to the perfusate resulted in pneumostomal closure; whereas removal of NH₄Cl from the bath resulted in pneumostomal opening. In conclusion, the ventilatory response to CO₂ in H. aspersa does not require Cl⁻ transport or conductance. Furthermore, changing pH_i alone is an adequate stimulus for the central chemoreceptors in the snail.     

Water and lactate movement in the swimbladder of the eel, Anguilla anguilla

Hemoglobin (Hb) and lactate (La) concentrations were measured in small (150 microliters) blood samples collected with micropipettes from the inflow and outflow vessels of the rete mirabile of the eel swimbladder. Hemoglobin was used as a marker of the intravascular space. 1. Hemoglobin concentrations suggest that there was no significant water movement between the arterial and venous capillaries in the rete, but a significant 6% water efflux from the vascular space into the swimbladder epithelium. No significant differences in osmolality were observed between the sites of measurement. 2. Of the lactate present in the blood entering the venous capillaries of the rete, 30% derived from release by the swimbladder epithelium; 38% of the lactate entering the venous capillaries diffused back in the rete tissue into its arterial capillaries. 3. Theoretical models suggest that any water movement in the swimbladder, leading to blood flow mismatch in the rete counter-current system, reduces its efficiency to concentrate inert gases, whereas lactate back-diffusion enhances this efficiency.     

Duodenal acidity “sensing” but not epithelial HCO3? supply is critically dependent on carbonic anhydrase II expression

Carbonic anhydrase (CA) is strongly expressed in the duodenum and has been implicated in a variety of physiological functions including enterocyte HCO₃⁻ supply for secretion and the “sensing” of luminal acid and CO₂. Here, we report the physiological role of the intracellular CAII isoform involvement in acid-, PGE_2, and forskolin-induced murine duodenal bicarbonate secretion (DBS) in vivo. CAII-deficient and WT littermates were studied in vivo during isoflurane anesthesia. An approximate 10-mm segment of the proximal duodenum with intact blood supply was perfused under different experimental conditions and DBS was titrated by pH immediately. Two-photon confocal microscopy using the pH-sensitive dye SNARF-1F was used to assess duodenocyte pH_i in vivo. After correction of systemic acidosis by infusion of isotonic Na₂CO₃, basal DBS was not significantly different in CAII-deficient mice and WT littermates. The duodenal bicarbonate secretory response to acid was almost abolished in CAII-deficient mice, but normal to forskolin- or 16,16-dimethyl PGE₂ stimulation. The complete inhibition of tissue CAs by luminal methazolamide and i.v. acetazolamide completely blocked the response to acid, but did not significantly alter the response to forskolin. While duodenocytes acidified upon luminal perfusion with acid, no significant pH_i change occurred in CAII-deficient duodenum in vivo. The results suggest that CA II is important for duodenocyte acidification by low luminal pH and for eliciting the acid-mediated HCO₃⁻ secretory response, but is not important in the generation of the secreted HCO₃⁻ ions.     

Persistent increased lung response to methacholine after normobaric hyperoxia in rabbits

This study was performed to determine the occurrence and time course of airway hyperreactivity following exposure to normobaric hyperoxia. Twenty-six rabbits were studied. Twelve served as control (group 1), and 14 were exposed to normobaric hyperoxia (Fi_O2 ≥ 95%) for 48 h: 9 rabbits (group 2) were studied after 1 day of recovery in room air and 5 (group 3) after 7 days. The rabbits were anesthetized, curarized and artificially ventilated. Respiratory resistance (Rrs) and elastance (Ers) and their respective changes induced by cumulative doses of aerosolized methacholine were assessed by the multiple linear regression analysis of airway pressure, tidal flow and volume. Weight-specific Rrs and Ers were significantly higher in group 2 (respectively, 87.7 ± 6.5 cmH₂O·L⁻¹·sec·kg and 1100.2 ± 87.1 cmH₂O·L⁻¹·kg, mean ± SEM) than in group 1 (respectively, 65.2 ± 3.2 cmH₂O·L⁻¹·sec·kg and 904.4 ± 49.7 cmH₂O·L⁻¹·kg (P < 0.05)), but were not different from group 3 (79.4 ± 7.9 cmH₂O·L⁻¹·sec·kg and 952.3 ± 125.0 cmH₂O·L⁻¹·kg, respectively). The dose of methacholine required to increase Rrs by 50% (PD_Rrs50) was significantly lower in both treated groups: 0.37 ± 0.11 mg in group 2 and 0.51 ± 0.19 mg in group 3 vs 2.07 ± 0.51 mg in group 1 (P < 0.05)). PD_Ers50 was significantly lower in group 2 (0.45 ± 0.15 mg) and 3 (0.75 ± 0.43 mg) compared with controls (1.11 ± 0.26 mg (P < 0.05)). These results show that hyperoxia induces an increase in Rrs and Ers, and airway hyperreactivity in the rabbit. The latter is prolonged beyond the immediate post-exposure period.     

Intracellular pH regulation in heart

Intracellular pH (pH_i) is an important modulator of cardiac excitation and contraction, and a potent trigger of electrical arrhythmia. This review outlines the intracellular and membrane mechanisms that control pH_i in the cardiac myocyte. We consider the kinetic regulation of sarcolemmal H⁺, OH⁻ and HCO₃⁻ transporters by pH, and by receptor-coupled intracellular signalling systems. We also consider how activity of these pH_i effector proteins is coordinated spatially in the myocardium by intracellular mobile buffer shuttles, gap junctional channels and carbonic anhydrase enzymes. Finally, we review the impact of pH_i regulatory proteins on intracellular Ca²⁺ signalling, and their participation in clinical disorders such as myocardial ischaemia, maladaptive hypertrophy and heart failure. Such multiple effects emphasise the fundamental role that pH_i regulation plays in the heart.     

Venous blood gas and metabolite response to low-intensity muscle contractions with external limb compression

The effect of low-intensity resistance exercise with external limb compression (100 [EC100] and 160 [EC160] mm Hg) on limb blood flow and venous blood gas-metabolite response was investigated and compared with that of high-intensity resistance exercise (no external compression). Unilateral elbow flexion muscle contractions were performed at 20% (75 repetitions, 4 sets, 30-second rest intervals) and 70% of 1-repetition maximum (1-RM; 3 sets, each set was until failure, 3-minute rest intervals). Precontraction brachial arterial blood flow (Doppler ultrasound) was reduced with EC100 or EC160 (56% and 39% of baseline value, respectively) compared with no external compression (control). At 20% 1-RM, brachial arterial blood flow increased after contractions performed with EC160 (190%), but not with the others. Decreases in venous oxygen partial pressure (P_vO₂) and venous oxygen saturation (S_vO₂) were greater during EC100 and EC160 than control (mean [SE]: P_vO₂, 28 [3] vs 26 [2] vs 33 [2] mm Hg; S_vO₂, 41% [5%] vs 34% [4%] vs 52% [5%], respectively). Changes in venous pH (pH_v), venous carbon dioxide partial pressure (P_vCO₂), and venous lactate concentration ([L⁻]_v) were greater with EC160 than EC100 and/or control (pH_v, 7.19 [0.01] vs 7.25 [0.01] vs 7.27 [0.02]; P_vCO₂, 72 [3] vs 64 [2] vs 60 [3] mm Hg; [L⁻]_v, 5.4 [0.6] vs 3.7 [0.4] vs 3.0 [0.4] mmol/L, respectively). Seventy percent 1-RM contractions resulted in greater changes in pH_v (7.14 [0.02]), P_vCO₂ (91 [5] mm Hg), and [L⁻]_v (7.0 [0.5] mmol/L) than EC100 and EC160, but P_vO₂ (30 [4] mm Hg) and S_vO₂ (40% [3%]) were similar. In conclusion, changes in pH_v, P_vCO₂, and [L⁻]_v, but not in P_vO₂ and S_vO₂, are sensitive to changes in relative, “internal” intensity of low-intensity muscle contractions caused by reduced blood flow (EC160) or high-intensity muscle contractions. Given the magnitude of the changes in pH_v, P_vCO₂, and [L⁻]_v, it appears plausible that they may be involved in stimulating the observed increase in muscle activation via group III and IV afferents.     

Glucose utilization by edematous rat lungs

Edema was produced in the isolated perfused rat lung by raising left atrial pressure. Eleven control lungs consumed 18±3.9 µmoles glucose/lung · hr⁻¹ and released 17.1±4.2 µmoles lactate/lung · hr⁻¹. During pulmonary edema in 13 isolated perfused lungs, glucose consumption was 35.5±8.8 µmoles/lung · hr⁻¹ (P<.05) and lactate production was 37±5.9 µmoles/lung · hr⁻¹ (P<.05). Separation of radiolabeled glucose and lactate indicated that all lactate was derived from glucose in control and edematous lungs. We found no important difference in¹⁴CO₂ production from 1-¹⁴C, 6-¹⁴C, or¹⁴C(U)-glucose. Tissue slices of lungs made edematous in vivo had differences in glucose consumption and lactate production which were similar to those observed in the isolated lungs. Oxygen consumption by 1 mm thick lung slices was 224±9.7 µl O₂/mg DNA · hr⁻¹ in control and 218±18 µl O₂/mg DNA · hr⁻¹ in edematous lungs. When dinitrophenol was added to the medium, the QO₂ was greater in the control than in the edematous lung slices (391±22 µl O₂/mg DNA · hr⁻¹ control vs. 334±33 µl O₂/mg DNA · hr⁻¹ edema,P<.05). We concluded that pulmonary edema in the isolated rat lung is accompanied by: 1) greater glucose consumption; 2) greater lactate production; 3) no important difference in¹⁴CO₂ production from pentose pathway or tricarboxylic cycle activity; and 4) lower response of edematous tissue slices to dinitrophenol stimulation.     

The effect of pH on 86Rubidium efflux from pancreatic islet cells

The influence of pH on the K⁺ permeability of pancreatic islet cells was investigated by measuring ⁸⁶Rb⁺ fluxes in isolated rat islets perifused or incubated in the absence of glucose. Acidification of the medium (to pH 6.5), by decreasing the concentration of HCO^?₃ or by increasing pCO₂ at constant HCO^?₃ reversibly reduced the rate of ⁸⁶Rb⁺ efflux from perifused islets. Alkalinization of the medium (to pH 7.8), by decreasing the pCO₂, reversibly increased ⁸⁶Rb⁺ efflux. Similar changes were recorded upon alteration of the pH in a bicarbonate-free, Hepesbuffered medium with or without calcium. Alteration of the CO₂ level at constant external pH — in order to modify internal pH — produced only a small and transient increase in efflux rate when CO₂ was lowered, and a decrease in efflux rate when CO₂ was raised. NH₄Cl reversibly augmented ⁸⁶Rb⁺ efflux in the presence and in the absence of HCO^?₃. At low pH (6.5), ⁸⁶Rb⁺ uptake by islet cells was reduced after 10 min (25%) and 30 min (11%), but not after 60 min, of incubation; it was not significantly affected by high pH (7.8). Calcium uptake and insulin release were reduced at low pH and increased at high pH.These results show that the K⁺ permeability of islet cells is affected by changes in extracellular but not in intracellular pH. They suggest that the endogenous production of protons that accompanies glucose stimulation of islet cells is not the mediator of the decrease in K⁺ permeability induced by the sugar.     

Characteristics of normal rabbit pleural fluid: Physiologic and biochemical implications

Each normal rabbit pleural space contains approximately 0.2 ml of collectable, clear liquid. This fluid has a total cell count of 1503±414 cells/mm³, the majority of which are monocytic cells. Pleural fluid total protein concentration (1.4±0.1 gm/100 ml) is approximately 20%, and lactic dehydrogenase concentration (129±6 Wrobleski U/ml) is approximately 40% of that found in blood. Pleural fluid and blood glucose were equivalent, and venous serum chloride concentration was slightly higher than the simultaneously measured pleural fluid value. Normal rabbit pleural fluid is alkaline; the value of 7.66±0.02 probably is slightly higher than the actual pH value owing to the escape of CO₂. A 8–9 mEq/liter HCO ₃ ⁻ gradient was found between pleural fluid and venous blood. The HCO ₃ ⁻ gradient cannot be explained by the Gibbs-Donnan equilibrium, suggesting that a process of active transport, possibly chloride pumping, is involved.     

Distal acidification defect induced by phosphate deprivation

The effect of phosphate deprivation on urinary acidification was investigated in rats fed a phosphate-deficient diet and in control rats fed the same diet supplemented with phosphate. Phosphate-deprived animals developed hypophosphatemia, hypercalcemia, and hypophosphaturia, but failed to develop hyperchloremic metabolic acidosis following 30 or 60 days of phosphate deprivation. Baseline urine pH was significantly higher in phosphate-deprived rats than in controls, but baseline urine HCO₃ excretion was not significantly different between the two groups. The pattern of HCO₃ reabsorption in phosphate-deprived rats was identical to that of controls at both low and high plasma HCO₃ levels. During chronic NH₄Cl administration, both 30- and 60-day phosphate-deprived rats had a significantly higher minimal urine pH and lower titratable acid and net acid excretion than seen in controls. NH₄ excretion was significantly lower than controls in the 60-day phosphate-deprived rats only. During Na²SO₄ administration the minimal urine pH was significantly lower in controls than in phosphate-deprived rats, but there was overlap of urine pH values. At comparable levels of urine pH, NH₄ excretion was significantly lower in phosphate-deprived rats than in controls. Phosphate-deprived rats were able to raise urine-blood CO₂ pressure to the same levels as controls during both HCO₃ loading and Tris buffer administration. Phosphate-deprived rats had greater extrarenal buffering capacity than controls as evidenced by a lower decline in blood pH and HCO₃ during HCl infucion in phosphate-deprived rats. These data demonstrate that phosphate deprivation is associated with distal acidification defect, impaired NH₃ excretion, and increased extrarenal buffering capacity. The increased availability of buffer in phosphate deprivation may play an important role in acid-base homeostasis in this condition.     

Gas conductance and metabolism of shorebird eggs: variation within and between species

Fresh egg mass (M₀; g), water vapor conductance of the egg shell (G_H2O; mg·[Torr·d]⁻¹), and neonate mass (M_n; g) were measured in the ruff (Philomachus pugnax), common redshank (Tringa totanus), northern lapwing (Vanellus vanellus), black-tailed godwit (Limosa limosa), and Eurasian curlew (Numenius arquata). In addition, the development of embryonic O₂ consumption (Ṁ_O2; ml·d⁻¹) and CO₂ production (Ṁ_CO2; ml·d⁻¹) were measured in these species, except the ruff. In northern lapwing and black-tailed godwit eggs the coefficients of variation for G_H2O were 3.8 and 2.3 times higher, respectively, than those for M₀. In these two species only about 10% of the variation for G_H2O was attributable to M₀, and about 77% to differences between clutches, suggesting a strong maternal component. In the northern lapwing, embryonic Ṁ_O2 plateaued prior to internal pipping, but not in the common redshank and black-tailed godwit. The latter result is in contrast to embryonic patterns previously described for other precocial species. In shorebirds the occurrence of an embryonic Ṁ_O2 plateau is not related to the neonatal level of cold-induced thermogenesis.     

The effect of 4,4′-diisothiocyanato-stilbene-2,2′-disulfonate on CO2 permeability of the red blood cell membrane

It has long been assumed that the red cell membrane is highly permeable to gases because the molecules of gases are small, uncharged, and soluble in lipids, such as those of a bilayer. The disappearance of ¹²C¹⁸O¹⁶O from a red cell suspension as the ¹⁸O exchanges between labeled CO₂ + HCO₃⁻ and unlabeled HOH provides a measure of the carbonic anhydrase (CA) activity (acceleration, or A) inside the cell and of the membrane self-exchange permeability to HCO₃⁻ (P_m,HCO⁻3). To test this technique, we added sufficient 4,4′-diisothiocyanato-stilbene-2,2′-disulfonate (DIDS) to inhibit all the HCO₃⁻/Cl⁻ transport protein (Band III or capnophorin) in a red cell suspension. We found that DIDS reduced P_m,HCO⁻3 as expected, but also appeared to reduce intracellular A, although separate experiments showed it has no effect on CA activity in homogenous solution. A decrease in P_m,CO2 would explain this finding. With a more advanced computational model, which solves for CA activity and membrane permeabilities to both CO₂ and HCO₃⁻, we found that DIDS inhibited both P_m,HCO⁻3 and P_m,CO2, whereas intracellular CA activity remained unchanged. The mechanism by which DIDS reduces CO₂ permeability may not be through an action on the lipid bilayer itself, but rather on a membrane transport protein, implying that this is a normal route for at least part of red cell CO₂ exchange.     

The role of sodium in the uptake of ursodeoxycholic acid in isolated hamster hepatocytes

The uptake of ursodeoxycholic acid (UDCA) was studied in isolated hamster hepatocytes. The uptake was rapid and linear up to 60 seconds for each concentration studied. When the uptake rate was plotted against UDCA concentration, the curve was nonlinear, indicating both saturable and nonsaturable uptake mechanisms. The nonsaturable process had a diffusion constant of 0.01 nmol·s⁻¹·g of cell·μmol/L⁻¹. The saturable component was characterized by a maximum rate of uptake (V_max) of 5.68 nmol·s⁻¹·g of cell⁻¹ and a Michaelis constant (K_m) of 224 μmol/L. In the presence of monensin, ouabain, and amiloride, the uptake of UDCA was significantly decreased by 35% to 55%, whereas the sodium-independent uptake of UDCA was not affected by either monensin or amiloride, thereby confirming sodium dependence of UDCA uptake. The sodium-dependent uptake of UDCA was characterized by a V_max and a K_m of 1.57 nmol·s⁻¹·g of cell⁻¹ and 46 μmol/L, respectively. The rate of uptake of UDCA was maximal at extracellular sodium concentrations ≥20 mmol/L. Furthermore, the uptake of UDCA was competitively inhibited by both taurocholic acid and cholic acid with an inhibitory constant (K₁) of 60 μmol/L and 48 μmol/L, respectively. Finally, 1 mmol/L of 4,4′-diisothiocyano-2,2′-disulfonic stilbene (DIDS) inhibited solely the sodium-dependent uptake of cholic acid and UDCA. These findings confirm that the hepatocellular uptake of UDCA involves, at least in part, a sodiumdependent, ouabain, amiloride, and DIDS-sensitive transporter.     

Microemboli reduce phase III slopes of CO2 and invert phase III slopes of infused SF6

We investigated the effect of increasing doses of intravenously infused glass microspheres (mean diameter 125 μm) on gas exchange in anesthetized, heparinized, mechanically ventilated goats (Vt= 16–18ml/kg). Breath-by-breath CO₂ expirograms were collected using a computerized system (Study A) during the infusion of a total of 15 g of microspheres. We found a 50% decrease in extravascular lung water by indicator dilution with a corresponding doubling of alveolar dead space (Vdalv). Airways deadspace (Vdaw) decreased by 13 ml (10%) and mean normalized phase III slope for CO₂ decreased from 0.23 to −0.08 L⁻ becoming negative in 3 of 5 animals. In a second study (Study B), simultaneous breath-by-breath CO₂ and infused SF₆ expirograms were collected using a infrared CO₂ analyzer and a mass spectrometer. Under baseline conditionsVdaw for CO₂ was smaller than for SF₆ and the ratio of the phase III slope for SF₆ to the phase III slope for CO₂ was 1.39. Following embolization there were no differences inVdaw between the two gases, however, the phase III slope for CO₂ became either slightly negative or extremely flat, while the phase III slope for SF₆ became negative in 73% of the breaths (−0.17 L⁻¹, P < 0.05). Negative phase III slopes have been predicted by a single path model when blood flow is confined to the most mouthward generations of the acinus (Schwardtet al., Ann. Biomed. Engin, 19: 679–697, 1991). The agreement between the numerical model and the experimental data is consistent with a serial distribution of blood flow within the acinus.