A novel framework of circulatory equilibrium

A novel framework of circulatory equilibrium was developed by extending Guyton's original concept. In this framework, venous return (CO(V)) for a given stressed volume (V) was characterized by a flat surface as a function of right atrial pressure (P(RA)) and left atrial pressure (P(LA)) as follows: CO(V) = V/W - G(S)P(RA) - G(P)P(LA), where W, G(S), and G(P) denote linear parameters. In seven dogs under total heart bypass, CO(V), P(RA), P(LA), and V were varied to determine the three parameters in each animal with use of multivariate analysis. The coefficient of determination (r(2) = 0.92-0.99) indicated the flatness of the venous return surface. The averaged surface was CO(V) = V/0.129 - 19.61P(RA) - 3.49P(LA). To examine the invariability of the surface parameters among animals, we predicted the circulatory equilibrium in response to changes in stressed volume in another 12 dogs under normal and heart failure conditions. This was achieved by equating the standard surface with the individually measured cardiac output (CO) curve. In this way, we could predict CO [y = 0.90x + 5.6, r(2) = 0.95, standard error of the estimate (SEE) = 8.7 ml.min(-1).kg(-1)], P(RA) (y = 0.96x, r(2) = 0.98, SEE = 0.2 mmHg), and P(LA) (y = 0.89x + 0.5, r(2) = 0.98, SEE = 0.8 mmHg) reasonably well. We conclude that the venous return surface accurately represents the venous return properties of the systemic and pulmonary circulations. The characteristics of the venous return surface are invariable enough among animals, making it possible to predict circulatory equilibrium, even if those characteristics are unknown in individual animals.     

S(G), S(I), and EGP of exercise-trained middle-aged men estimated by a two-compartment labeled minimal model

To examine the effects of physical training on glucose effectiveness (S(G)), insulin sensitivity (S(I)), and endogenous glucose production (EGP) in middle-aged men, stable-labeled frequently sampled intravenous glucose tolerance tests (FSIGTT) were performed on 11 exercise-trained middle-aged men and 12 age-matched sedentary men. The time course of EGP during the FSIGTT was estimated by nonparametric stochastic deconvolution. Glucose uptake-specific indexes of glucose effectiveness (S(2*)(G) x 10(2): 0.81 +/- 0.08 vs. 0.60 +/- 0.05 dl. min(-1). kg(-1), P < 0.05) and insulin sensitivity [S(2*)(I) x 10(4): 24.59 +/- 2.98 vs. 11.89 +/- 2.36 dl. min(-1). (microU/ml)(-1). kg(-1), P < 0.01], which were analyzed using the two-compartment minimal model, were significantly greater in the trained group than in the sedentary group. Plasma clearance rate (PCR) of glucose was consistently greater in the trained men than in sedentary men throughout FSIGTT. Compared with sedentary controls, EGP of trained middle-aged men was higher before glucose load. The EGP of the two groups was similarly suppressed by approximately 70% within 10 min, followed by an additional suppression after insulin infusion. EGP returned to basal level at approximately 60 min in the trained men and at 100 min in the controls, followed by its overshoot, which was significantly greater in the trained men than in the controls. In addition, basal EGP was positively correlated with S(2*)(G) . The higher basal EGP and greater EGP overshoot in trained middle-aged men appear to compensate for the increased insulin-independent (S(2*)(G)) and -dependent (S(2*)(I)) glucose uptake to maintain glucose homeostasis.     

Effects of hypoxia on the venous circulation in rainbow trout (Oncorhynchus mykiss)

Hypoxia in fish is generally associated with bradycardia while cardiac output (Q) remains unaltered or slightly increased due to a compensatory increase in stroke volume (SV). Rainbow trout (Oncorhynchus mykiss) were subjected to severe (P(W)O2=7.3+/-0.2 kPa) or mild (P(W)O2=11.5+/-0.2 kPa) hypoxia. Central venous pressure (P(ven)), dorsal aortic pressure (P(da)), heart rate (f(H)) and Q, were recorded in vivo. Both levels of hypoxia triggered a significant increase in P(ven). Severe hypoxia was associated with bradycardia and unaltered Q, whereas mild hypoxia was associated with a small but significant increase in Q and no bradycardia. These findings indicate that an increase in P(ven) promotes an increase in SV during hypoxia. Since mild hypoxia increased P(ven), Q and SV without bradycardia or reduced systemic resistance (R(sys)), we hypothesize that an active increase in venous tone serving to mobilize blood to the central venous compartment in order to increase cardiac preload and consequently SV, is an important cardiovascular trait associated with hypoxia. Pharmacological pre-treatment with prazosin (1 mg kg(-1)) did not conclusively reveal the underlying mechanisms to the observed changes in P(ven). This study discusses the influence of venous pooling, reduced R(sys) and altered venous tone on changes in P(ven) observed during hypoxia.     

Carbon dioxide pressure-concentration relationship in arterial and mixed venous blood during exercise.

To calculate cardiac output by the indirect Fick principle, CO(2) concentrations (CCO(2)) of mixed venous (Cv(CO(2))) and arterial blood are commonly estimated from PCO(2), based on the assumption that the CO(2) pressure-concentration relationship (PCO(2)-CCO(2)) is influenced more by changes in Hb concentration and blood oxyhemoglobin saturation than by changes in pH. The purpose of the study was to measure and assess the relative importance of these variables, both in arterial and mixed venous blood, during rest and increasing levels of exercise to maximum (Max) in five healthy men. Although the mean mixed venous PCO(2) rose from 47 Torr at rest to 59 Torr at the lactic acidosis threshold (LAT) and further to 78 Torr at Max, the Cv(CO(2)) rose from 22.8 mM at rest to 25.5 mM at LAT but then fell to 23.9 mM at Max. Meanwhile, the mixed venous pH fell from 7.36 at rest to 7.30 at LAT and to 7.13 at Max. Thus, as work rate increases above the LAT, changes in pH, reflecting changes in buffer base, account for the major changes in the PCO(2)-CCO(2) relationship, causing Cv(CO(2)) to decrease, despite increasing mixed venous PCO(2). Furthermore, whereas the increase in the arteriovenous CCO(2) difference of 2.2 mM below LAT is mainly due to the increase in Cv(CO(2)), the further increase in the arteriovenous CCO(2) difference of 4.6 mM above LAT is due to a striking fall in arterial CCO(2) from 21.4 to 15.2 mM. We conclude that changes in buffer base and pH dominate the PCO(2)-CCO(2) relationship during exercise, with changes in Hb and blood oxyhemoglobin saturation exerting much less influence.     

Electron transfer from the Rieske iron-sulfur protein (ISP) to cytochrome f in vitro. Is a guided trajectory of the ISP necessary for competent docking?

The time course of electron transfer in vitro between soluble domains of the Rieske iron-sulfur protein (ISP) and cytochrome f subunits of the cytochrome b(6)f complex of oxygenic photosynthesis was measured by stopped-flow mixing. The domains were derived from Chlamydomonas reinhardtii and expressed in Escherichia coli. The expressed 142-residue soluble ISP apoprotein was reconstituted with the [2Fe-2S] cluster. The second-order rate constant, k(2)((ISP-f)) = 1.5 x 10(6) m(-1) s(-1), for ISP to cytochrome f electron transfer was <10(-2) of the rate constant at low ionic strength, k(2)((f-PC))(> 200 x 10(6) m(-1) s(-1)), for the reduction of plastocyanin by cytochrome f, and approximately 1/30 of k(2)((f-PC)) at the ionic strength estimated for the thylakoid interior. In contrast to k(2)((f-PC)), k(2)((ISP-f)) was independent of pH and ionic strength, implying no significant role of electrostatic interactions. Effective pK values of 6.2 and 8.3, respectively, of oxidized and reduced ISP were derived from the pH dependence of the amplitude of cytochrome f reduction. The first-order rate constant, k(1)((ISP-f)), predicted from k(2)((ISP-f)) is approximately 10 and approximately 150 times smaller than the millisecond and microsecond phases of cytochrome f reduction observed in vivo. It is proposed that in the absence of electrostatic guidance, a productive docking geometry for fast electron transfer is imposed by the guided trajectory of the ISP extrinsic domain. The requirement of a specific electrically neutral docking configuration for ISP electron transfer is consistent with structure data for the related cytochrome bc(1) complex.     

Sympathetic activity and the heterogenous blood pressure response to exercise training in hypertensives.

To test whether changes in sympathetic nervous system (SNS) activity or insulin sensitivity contribute to the heterogeneous blood pressure response to aerobic exercise training, we used compartmental analysis of [3H]norepinephrine kinetics to determine the extravascular norepinephrine release rate (NE2) as an index of systemic SNS activity and determined the insulin sensitivity index (S(I)) by an intravenous glucose tolerance test, before and after 6 mo of aerobic exercise training, in 30 (63 +/- 7 yr) hypertensive subjects. Maximal O2 consumption increased from 18.4 +/- 0.7 to 20.8 +/- 0.7 ml x kg(-1) x min(-1) (P = 0.02). The average mean arterial blood pressure (MABP) did not change (114 +/- 2 vs. 114 +/- 2 mmHg); however, there was a wide range of responses (-19 to +17 mmHg). The average NE2 did not change significantly (2.11 +/- 0.15 vs. 1.99 +/- 0.13 microg x min(-1) x m(-2)), but there was a significant positive linear relationship between the change in NE2 and the change in MABP (r = 0.38, P = 0.04). S(I) increased from 2.81 +/- 0.37 to 3.71 +/- 0.42 microU x 10(-4) x min(-1) x ml(-1) (P = 0.004). The relationship between the change in S(I) and the change in MABP was not statistically significant (r = -0.03, P = 0.89). When the changes in maximal O2 consumption, percent body fat, NE2, and S(I) were considered as predictors of the change in MABP, only NE2 was a significant independent predictor. Thus suppression of SNS activity may play a role in the reduction in MABP and account for a portion of the heterogeneity of the MABP response to aerobic exercise training in older hypertensive subjects.     

Interdependence of bronchial circulation and clearance of 99mTc-DTPA from the airway surface.

The extent to which the systemic vasculature is involved in soluble-particle uptake in the conducting airways has not been studied extensively. In anesthetized, ventilated sheep, 6-10 microl of technetium-99m-labeled diethylenetriamine pentaacetic acid (99mTc-DTPA) was delivered through a microspray nozzle to a fourth-generation airway. Perfusion of the cannulated bronchial artery was varied between control flow (0.6 ml x min(-1) x kg(-1)), high flow (1.8 ml x min(-1) x kg(-1)) or no flow (the infusion pump was stopped). Airway retention of the radioactive tracer was monitored using gamma camera imaging, and venous blood was sampled. During control perfusion, tracer retention at the site of deposition at 30 min averaged 20 +/- 6% (n = 7). With no flow, retention was significantly elevated to 32 +/- 8% (P = 0.03). In another group of sheep (n = 5) with a control retention of 13 +/- 4%, high flow resulted in an increase in tracer (25 +/- 4%; P = 0.04). Maximum blood uptake of tracer was calculated by estimating circulating blood volume and averaged 16% of total activity during control flow. Only during high-flow conditions was 99mTc-DTPA in the blood decreased (10%; P = 0.04). Most of the tracer was cleared by mucociliary clearance as visualized by imaging. This component was substantially decreased during no flow. The results demonstrate that both decreased and increased airway perfusion limit removal of soluble tracer applied to the conducting airways.     

Changes in microvascular fluid filtration capacity during 120 days of 6 degrees head-down tilt.

We used venous congestion strain gauge plethysmography (VCP) to measure the changes in fluid filtration capacity (K(f)), isovolumetric venous pressure (Pv(i)), and blood flow in six volunteers before, on the 118th day (D118) of head-down tilt (HDT), and 2 days after remobilization (Post). We hypothesized that 120 days of HDT cause significant micro- and macrovascular changes. We observed a significant increase in K(f) from 3.6 +/- 0.4 x 10(-3) to 5.7 +/- 0.9 x 10(-3) ml. min(-1). 100 ml(-1). mmHg(-1) (+51.4%; P < 0.003), which returned to pretilt values (4.0 + 0.4 x 10(-3) ml. min(-1). 100 ml(-1). mmHg(-1)) after remobilization. Similarly, Pv(i) increased from 13.4 +/- 2.1 mmHg to 28.9 +/- 2.8 mmHg (+105.8%; P < 0.001) at D118 and was not significantly different at Post (12.4 +/- 2.6 mmHg). Blood flow decreased significantly from 2.3 +/- 0.3 to 1.3 +/- 0.2 ml. min(-1). 100 ml tissue(-1) at D118 and was found elevated to 3.4 +/- 0.7 ml. min(-1). 100 ml tissue(-1) at Post. We believe that the increased K(f) is caused by a higher microvascular water permeability. Because this may result in edema formation, it could contribute to the alterations in fluid homeostasis after exposure to microgravity.     

Contribution of cooperativity and the Bohr effect to efficient oxygen transport by hemoglobins from five mammalian species

By using published experimental values of the standard oxygen (O2) equilibrium curve and the in vivo arterial and venous O2 pressure (PO2) of fetal and maternal blood in five mammalian species (human, cow, pig, sheep, and horse), we investigated the relationship between the efficiency of O2 delivery and the effectiveness of the Bohr shift, and discussed the significance of cooperativity for mammalian Hb. The O2 delivery of fetal blood was more efficient than that of maternal blood, and the effectiveness of the Bohr shift at both O2 loading and release sites of fetal blood was high. A linear relationship was observed between the efficiency of O2 delivery and the effectiveness of the Bohr shift at O2 loading sites of the five mammalian species. In both fetal and maternal blood, the theoretically obtained optimal P50 value for O2 delivery (optP50(OD)) was nearly equal to the optimal P50 value for the effectiveness of the Bohr shift at the O2 loading site (optP50(BS)(loading)). This phenomenon was favorable for fetal blood to uptake O2 from maternal blood with the aid of the Bohr shift and to deliver a large amount of O2 to the tissues. The optP50s for the effectiveness of the Bohr shift at given arterial PO2 (PaO2) and venous PO2 (PvO2) were derived as follows: optP50(BS)(loading) = PaO2((n+1)/(n-1))(1/n), and optP50(BS)(release) = PvO2((n+1)/(n-1))(1/n). The relationship between in vivo PO2s and n, PaO2/PvO2 = ((n+1)/(n-1))(2/n), was derived by letting optP50 for the efficiency of O2 delivery be equal to that for the effectiveness of the Bohr shift.     

Relationships between maximal muscle oxidative capacity and blood lactate removal after supramaximal exercise and fatigue indexes in humans.

The present study investigated whether blood lactate removal after supramaximal exercise and fatigue indexes measured during continuous and intermittent supramaximal exercises are related to the maximal muscle oxidative capacity in humans with different training status. Lactate recovery curves were obtained after a 1-min all-out exercise. A biexponential time function was then used to determine the velocity constant of the slow phase (gamma(2)), which denoted the blood lactate removal ability. Fatigue indexes were calculated during all-out (FI(AO)) and repeated 10-s cycling sprints (FI(Sprint)). Biopsies were taken from the vastus lateralis muscle, and maximal ADP-stimulated mitochondrial respiration (V(max)) was evaluated in an oxygraph cell on saponin-permeabilized muscle fibers with pyruvate + malate and glutamate + malate as substrates. Significant relationships were found between gamma(2) and pyruvate + malate V(max) (r = 0.60, P < 0.05), gamma(2) and glutamate + malate V(max) (r = 0.66, P < 0.01), and gamma(2) and citrate synthase activity (r = 0.76, P < 0.01). In addition, gamma(2), glutamate + malate V(max), and pyruvate + malate V(max) were related to FI(AO) (gamma(2) - FI(AO): r = 0.85; P < 0.01; glutamate + malate V(max) - FI(AO): r = 0.70, P < 0.01; and pyruvate + malate V(max) - FI(AO): r = 0.63, P < 0.01) and FI(Sprint) (gamma(2) - FI(Sprint): r = 0.74, P < 0.01; glutamate + malate V(max) - FI(Sprint): r = 0.64, P < 0.01; and pyruvate + malate V(max) - FI(Sprint): r = 0.46, P < 0.01). In conclusion, these results suggested that the maximal muscle oxidative capacity was related to blood lactate removal ability after a 1-min all-out test. Moreover, maximal muscle oxidative capacity and blood lactate removal ability were associated with the delay in the fatigue observed during continuous and intermittent supramaximal exercises in well-trained subjects.     

Stereoselectivity in the placental transfer and kinetic disposition of racemic bupivacaine administered to parturients with or without a vasoconstrictor

The aim of the present study was to investigate the stereoselectivity in the kinetic disposition and the transplacental distribution of bupivacaine in term parturients during labor. Maternal age ranged from 18-37 years and fetal gestational age from 37.6-41.5 weeks. Healthy parturients (n = 23) received epidural 0.5% racemic bupivacaine alone (group A) or combined with epinephrine (group B). Maternal venous blood was sampled at regular intervals until 8 h after drug administration and umbilical venous blood was obtained at delivery. Bupivacaine enantiomers were determined in plasma samples by HPLC using a Chiralcel(R) OD-R column and a UV detector. One- or two-compartment models were fitted to data and differences between the (+)-(R) and (-)-(S) enantiomers were compared with the paired Wilcoxon test (P< 0.05). The influence of epinephrine was evaluated using the unpaired Mann-Whitney test (P< 0.05). The disposition of bupivacaine in maternal plasma was stereoselective, with higher V(d/f) (140.60 vs. 132.81 L for group A and 197.86 vs. 169.46 L for group B) and C(l/f) (29.00 vs. 25.43 L/h for group A and 33.15 vs. 26.39 L/h for group B) and lower t(1/2)beta (3.24 vs. 3.30 h for group A and 4.36 vs. 4.45 h for group B) being observed for (+)-(R)-bupivacaine. The combined administration of epinephrine resulted in higher V(d/f) (197.86 vs. 140.60 L for (+)-(R) and 169.46 vs. 132.81 L for (-)-(S)) and t(1/2)beta values (4.36 vs. 3.24 h for (+)-(R) and 4.45 vs. 3.30 h for (-)-(S)). The transplacental distribution of bupivacaine was stereoselective only when bupivacaine was administered without epinephrine (group B), with a higher cord blood/maternal blood ratio being observed for (-)-(S)-bupivacaine (0.40 vs. 0.35). Chirality 16:65-71, 2004.     

Diurnal variations in ventilatory and cardiorespiratory responses to submaximal treadmill exercise in females

Diurnal variations in ventilatory and cardiorespiratory responses to submaximal treadmill exercise were analysed in 11 eumenorrhoeic women and in 10 women using monophasic oral contraceptives. Subjects performed submaximal treadmill exercise at three intensities averaging 7, 8, and 9 km x h(-1), each for 4 min at 0800, 1300 and 1700 hours, assigned randomly on 3 separate days. Rectal temperature was measured before (T(rec(b))) and after (T(rec(a))) exercise. Cardiac frequency (f(c)), ventilation (V(E)), oxygen uptake (VO(2)), carbon dioxide output (VCO(2)), and respiratory exchange ratio (R) were assessed in the last minute of each stage of the exercise. Both T(rec(b)) and T(rec(a)) increased from 0800 to 1700 hours (P < 0.001). For a given submaximal work rate, VO(2) and VCO(2) were higher in the afternoon compared to the morning. Similarly, R was increased at 1700 hours compared to 0800 hours during the recovery period following exercise (P < 0.05). However, V(E) did not vary significantly during the day at any of the running intensities. No significant interactions (group x time of day) were observed in any of the studied parameters. In contrast to ventilation, the VO(2) and VCO(2) of the females during submaximal exercise were both affected by the time of day, without any differences between eumenorrhoeic women and users of oral contraceptives.     

Dynamic cerebral autoregulation during exhaustive exercise in humans

We investigated whether dynamic cerebral autoregulation is affected by exhaustive exercise using transfer-function gain and phase shift between oscillations in mean arterial pressure (MAP) and middle cerebral artery (MCA) mean blood flow velocity (V(mean)). Seven subjects were instrumented with a brachial artery catheter for measurement of MAP and determination of arterial Pco(2) (Pa(CO(2))) while jugular venous oxygen saturation (Sv(O(2))) was determined to assess changes in whole brain blood flow. After a 10-min resting period, the subjects performed dynamic leg-cycle ergometry at 168 +/- 5 W (mean +/- SE) that was continued to exhaustion with a group average time of 26.8 +/- 5.8 min. Despite no significant change in MAP during exercise, MCA V(mean) decreased from 70.2 +/- 3.6 to 57.4 +/- 5.4 cm/s, Sv(O(2)) decreased from 68 +/- 1 to 58 +/- 2% at exhaustion, and both correlated to Pa(CO(2)) (5.5 +/- 0.2 to 3.9 +/- 0.2 kPa; r = 0.47; P = 0.04 and r = 0.74; P < 0.001, respectively). An effect on brain metabolism was indicated by a decrease in the cerebral metabolic ratio of O(2) to [glucose + one-half lactate] from 5.6 to 3.8 (P < 0.05). At the same time, the normalized low-frequency gain between MAP and MCA V(mean) was increased (P < 0.05), whereas the phase shift tended to decrease. These findings suggest that dynamic cerebral autoregulation was impaired by exhaustive exercise despite a hyperventilation-induced reduction in Pa(CO(2)).     

Gill (Na+,K+)-ATPase in diadromous, freshwater palaemonid shrimps: species-specific kinetic characteristics and alpha-subunit expression

To better comprehend physiological adaptation to dilute media and the molecular mechanisms underlying ammonia excretion in palaemonid shrimps, we characterized the (Na+,K+)-ATPase from Macrobrachium amazonicum gills, disclosing high- (K(0.5) = 4.2+/-0.2 micromol L(-1); V = 33.9+/-1.9 U mg(-1)) and low-affinity (K(0.5) = 0.144+/-0.010 mmol L(-1); V = 232.9+/-15.3 U mg(-1)) ATP hydrolyzing sites. Stimulation by Na+ (K(0.5) = 5.5+/-0.3 mmol L(-1); V = 275.1+/-15.1 U mg(-1)), Mg2+ (K(0.5) = 0.79+/-0.06 mmol L(-1); V = 261.9+/-18.3 U mg(-1)), K+ (K(M) = 0.88+/-0.04 mmol L(-1); V = 271.8+/-10.9 U mg(-1)) and NH4(+) (K(M) = 5.0+/-0.2 mmol L(-1); V = 385.9+/-15.8 U mg(-1)) obeys single saturation curves, activity being stimulated synergistically by NH4(+) and K+. There is a single K+ binding site, NH4(+) binding to a second, exclusive site, stimulating activity by 33%, modulating K+ affinity. (Na+,K+)-ATPase activity constitutes approximately 80% of total ATPase activity (K(Iouabain) = 147.5+/-8.9 micromol L(-1)); Na+-, K+-, Ca2+-, V- and F(o)F(1)-ATPases are also present. M. amazonicum microsomal fractions possess approximately 2-fold less (Na+,K+)-ATPase alpha-subunit than M. olfersi, consistent with a 2.6-fold lower specific activity. These differences in (Na+, K+)-ATPase stimulation by ATP and ions, and specific activities of other ATPases, suggest the presence of distinct biochemical adaptations to life in fresh water in these related species.     

Reconstituting the barrier properties of a water-tight epithelial membrane by design of leaflet-specific liposomes

To define aspects of lipid composition and bilayer asymmetry critical to barrier function, we examined the permeabilities of liposomes that model individual leaflets of the apical membrane of a barrier epithelium, Madin-Darby canine kidney type 1 cells. Using published lipid compositions we prepared exofacial liposomes containing phosphatidylcholine, sphingomyelin, glycosphingolipids, and cholesterol; and cytoplasmic liposomes containing phosphatidylethanolamine, phosphatidylserine, and cholesterol. The osmotic permeability of cytoplasmic liposomes to water (P(f)), solutes, and NH(3) was 18-90-fold higher than for the exofacial liposomes (P(f(ex)) = 2.4 +/- 0.4 x 10(-4) cm/s, P(f(cy)) = 4.4 +/- 0.3 x 10(-3) cm/s; P(glycerol(ex)) = 2.5 +/- 0.3 x 10(-8) cm/s, P(glycerol(cy)) = 2.2 +/- 0.02 x 10(-6) cm/s; P(NH3(ex)) = 0. 13 +/- 0.4 x 10(-4) cm/s, P(NH3(cy)) = 7.9 +/- 1.0 x 10(-3) cm/s). By contrast, the apparent proton permeability of exofacial liposomes was 4-fold higher than cytoplasmic liposomes (P(H+(ex)) = 1.1 +/- 0. 1 x 10(-2) cm/s, P(H+(cy)) = 2.7 +/- 0.6 x 10(-3) cm/s). By adding single leaflet permeabilities, we calculated a theoretical P(f) for a Madin-Darby canine kidney apical membrane of 4.6 x 10(-4) cm/s, which compares favorably with experimentally determined values. In exofacial liposomes lacking glycosphingolipids or sphingomyelin, permeabilities were 2-7-fold higher, indicating that both species play a role in barrier function. Removal of cholesterol resulted in 40-280-fold increases in permeability. We conclude: 1) that we have reconstituted the biophysical properties of a barrier membrane, 2) that the barrier resides in the exofacial leaflet, 3) that both sphingomyelin and glycosphingolipids play a role in reducing membrane permeability but that there is an absolute requirement for cholesterol to mediate this effect, 4) that these results further validate the hypothesis that each leaflet offers an independent resistance to permeation, and 5) that proton permeation was enhanced by sphingolipid/cholesterol interactions.     

Beta-adrenergic activation of solute coupled water uptake by toad skin epithelium results in near-isosmotic transport

Transepithelial potential (V(T)), conductance (G(T)), and water flow (J(V)) were measured simultaneously with good time resolution (min) in isolated toad (Bufo bufo) skin epithelium with Ringer on both sides. Inside application of 5 microM isoproterenol resulted in the fast increase in G(T) from 1.2+/-0.3 to 2.4+/-0.4 mS x cm(-2) and slower increases in equivalent short circuit current, I(SC)(Eqv) = -G(T) x V(T), from 12.7+/-3.2 to 33.1+/-6.8 microA cm(-2), and J(V) from 0.72+/-0.17 to 3.01+/-0.49 nL cm(-2) s(-1). Amiloride in the outside solution abolished I(SC)(Eqv) (-1.6+/-0.1 microA cm(-2)) while J(V) decreased to 0.50+/-0.15 nL cm(-2) x s(-1), which is significantly different from zero. Isoproterenol decreased the osmotic concentration of the transported fluid, C(osm) approximately 2 x I(SC)(Eqv)/J(V), from 351+/-72 to 227+/-28 mOsm (Ringer's solution: 252.8 mOsm). J(V) depicted a saturating function of [Na+]out in agreement with Na+ self-inhibition of ENaC. Ouabain on the inside decreased I(SC)(Eqv) from 60+/-10 to 6.1+/-1.7 microA cm(-2), and J(V) from 3.34+/-0.47 to 1.40+/-0.24 nL cm(-2) x s(-1). Short-circuited preparations exhibited a linear relationship between short-circuit current and J(V) with a [Na+] of the transported fluid of 130+/-24 mM ([Na+]Ringer's solution = 117.4 mM). Addition of bumetanide to the inside solution reduced J(V). Water was transported uphill and J(V) reversed at an excess outside osmotic concentration, deltaC(S,rev) = 28.9+/-3.9 mOsm, amiloride decreased deltaC(S,rev) to 7.5+/-1.5 mOsm. It is concluded that water uptake is accomplished by osmotic coupling in the lateral intercellular space (lis), and hypothesized that a small fraction of the Na+ flux pumped into lis is recirculated via basolateral NKCC transporters.     

Retinal venous oxygen saturation and cardiac output during controlled hemorrhage and resuscitation.

The objective was to test calibration of an eye oximeter (EOX) in a vitiligo swine eye and correlate retinal venous oxygen saturation (Srv(O(2))), mixed venous oxygen saturation (Sv(O(2))), and cardiac output (CO) during robust changes in blood volume. Ten anesthetized adult Sinclair swine with retinal vitiligo were placed on stepwise decreasing amounts of oxygen. At each oxygen level, femoral artery oxygen saturation (Sa(O(2))) and retinal artery oxygen saturation (Sra(O(2))) were obtained. After equilibration on 100% O(2), subjects were bled at 1.4 ml. kg(-1). min(-1) for 20 min. Subsequently, anticoagulated shed blood was reinfused at the same rate. During graded hypoxia, exsanguination, and reinfusion, Sra(O(2)) and Srv(O(2)) were measured by using the EOX, and CO and Sv(O(2)) were measured by using a pulmonary artery catheter. During graded hypoxia, Sra(O(2)) correlated with Sa(O(2)) (r = 0.92). Srv(O(2)) correlated with Sv(O(2)) (r = 0.89) during exsanguination and reinfusion. Sv(O(2)) and Srv(O(2)) correlated with CO during blood removal and resuscitation (r = 0.92). Use of vitiligo retinas improved the calibration of EOX measurements. In this robust hemorrhage model, Srv(O(2)) correlates with CO and Sv(O(2)) across the range of exsanguination and resuscitation.     

Consumption of an oral carbohydrate-protein gel improves cycling endurance and prevents postexercise muscle damage

Investigators have reported improved endurance performance and attenuated post-exercise muscle damage with carbohydrate-protein beverages (CHO+P) versus carbohydrate-only beverages (CHO). However, these benefits have been demonstrated only when CHO+P was administered in beverage-form, and exclusively in male subjects. Thus, the purposes of this study were to determine if an oral CHO+P gel improved endurance performance and post-exercise muscle damage compared to a CHO gel, and determine if responses were similar between genders. Thirteen cyclists (8 men, 5 women; VO(2)peak = 57.9 +/- 7.0 ml x kg(-1) x min(-1)) completed two timed cycle-trials to volitional exhaustion at 75% of VO(2)peak. At 15-minute intervals throughout these rides, subjects received CHO or CHO+P gels, which were matched for carbohydrate content (CHO = 0.15 g CHO x kg BW(-1); CHO+P = 0.15 g CHO + 0.038 g protein x kg BW(-1)). Trials were performed using a randomly counterbalanced, double-blind design. Subjects rode 13% longer (p < 0.05) when utilizing the CHO+P gel (116.6 +/- 28.5 minutes) versus the CHO gel (102.8 +/- 25.0 minutes). In addition, men (101.8 +/- 24.6; 114.8 +/- 26.2) and women (104.4 +/- 28.6; 119.6 +/- 34.9) responded similarly to the CHO and CHO+P trials, with no significant treatment-by-gender effect. Postexercise creatine kinease (CK) was not significantly different between treatments. However, CK increased significantly following exercise in the CHO trial (183 +/- 116; 267 +/- 214 U x L(-1)), but not the CHO+P trial (180 +/- 133; 222 +/- 141 U x L(-1)). Therefore, to prolong endurance performance and prevent increases in muscle damage, it is recommended that male and female cyclists consume CHO+P gels rather than CHO gels during and immediately following exercise.     

Endothelum-dependent and-independent relaxations in aortic rings obtained from hypertensive hooded (Aguti) rats.

Experimental hypertension studies are few in the hooded (Aguti) rat. The present study was designed to investigate the usefulness of this rat strain for experimental hypertension studies and to test the hypothesis that the hypertension may be associated with a diminution of endothelium dependent and independent relaxations. Hypertension was induced in inbred hooded rats (n=8 each) by administering 8% salt in the diet and /or 100 mg/kg/day Nomega-nitro-L-arginine-methyl-ester (L-NAME) in the drinking water for six and/or four weeks respectively. The rats were anaesthetized using a 25% urethane and 1% chloralose mixture given intraperitoneally at a dose of 5 mg/kg. Their blood pressure was measured invasively. Thereafter, relaxations of rat aortic preparations to acetylcholine, histamine and sodium nitroprusside (SNP) were assessed using standard organ bath conditions. Probabiliity level of 0.05 was taken as statistically significant. The mean arterial pressure (MAP;mm Hg) rose significantly in all test groups (Salt: 148.3 +/- 4.6; L-NAME: 181.7 +/-8.3; Salt+L-NAME:154.9 +/-8.7) compared with control (94.2 +/-6.8; [P < 0.05]. The MAP was significantly [P < 0.05] higher in the L-NAME group than in all the other groups. The heart rate fell significantly in the salt + L-NAME group compared to control [P <0.05].The IC50 of acetylcholine in aortic rings from L-NAME rats (7.9 x 10(-1) +/- 6.0 x 10(-3)) was significantly higher than in rings from control (9.4 x 10(-8) +/- 2.8 x 10(-8)), salt (7.8 x 10(-7) +/- 4.7 x 10(-7)) and salt + L-NAME (3.3 x 10 (-7) +/- 2.1 x 10(-7)) rats [P < 0.05]. The IC50 of histamine and SNP in the rings from the test groups of rats showed no significant difference from control. Also, endothelium dependent and independent relaxations were preserved in the various forms of hypertension studied except in chronic NOS inhibition where the former was attenuated in response to acetylcholine.     

Total weak acid concentration and effective dissociation constant of nonvolatile buffers in human plasma.

The strong ion approach provides a quantitative physicochemical method for describing the mechanism for an acid-base disturbance. The approach requires species-specific values for the total concentration of plasma nonvolatile buffers (A(tot)) and the effective dissociation constant for plasma nonvolatile buffers (K(a)), but these values have not been determined for human plasma. Accordingly, the purpose of this study was to calculate accurate A(tot) and K(a) values using data obtained from in vitro strong ion titration and CO(2) tonometry. The calculated values for A(tot) (24.1 mmol/l) and K(a) (1.05 x 10(-7)) were significantly (P < 0.05) different from the experimentally determined values for horse plasma and differed from the empirically assumed values for human plasma (A(tot) = 19.0 meq/l and K(a) = 3.0 x 10(-7)). The derivatives of pH with respect to the three independent variables [strong ion difference (SID), PCO(2), and A(tot)] of the strong ion approach were calculated as follows: dpH/dSID(+) = [1 + 10(pK(a)-pH)](2)/(2.303 x [SPCO(2)10(pH-pK'(1)[1 + 10(pK(a)-pH](2) + A(tot)10(pK(a)-PH]]; dpH/dPCO(2) = S10(-pK'(1)/[2.303[A(tot)10(pH)(10(pH + 10(pK(a))(-2) - SID(+)10(-pH)]], dpH/dA(tot) = -1/[2.303[SPCO(2)10(pH-pK'(1) + SID(+)10(pK(a)-pH)]], where S is solubility of CO(2) in plasma. The derivatives provide a useful method for calculating the effect of independent changes in SID(+), PCO(2), and A(tot) on plasma pH. The calculated values for A(tot) and K(a) should facilitate application of the strong ion approach to acid-base disturbances in humans.