Influence of feeding schedules on the chronobiology of renin activity,urinary electrolytes and blood pressure in dogs

The contribution of the renin–angiotensin–aldosterone system (RAAS) to the development of congestive heart failure (CHF) and hypertension (HT) has long been recognized. Medications that are commonly used in the course of CHF and HT are most often given with morning food for the sake of convenience and therapeutic compliance. However, biological rhythms and their responsiveness to environmental clues such as food intake may noticeably impact the effectiveness of drugs used in the management of cardiovascular disorders. Only sparse information about the effect of feeding schedules on the biology of the RAAS and blood pressure (BP) is presently available. Two studies were designed to explore the chronobiology of renin activity (RA), BP, renal sodium (U_Na,fe) and potassium (U_K,fe) handling in relation to meal timing in dogs. In a first experiment (Study a), blood and urinary samples for measurement of RA, U_Na,fe and U_K,fe were drawn from 18 healthy beagle dogs fed a normal-sodium diet at either 07:00, 13:00 or 19:00?h. In a second experiment (Study b), BP was recorded continuously from six healthy, telemetered beagle dogs fed a similar diet at 07:00, or 19:00?h. Data were collected throughout 24-h time periods, and analyzed by means of nonlinear mixed-effects models. Differences between the geometric means of early versus late time after feeding observations were further compared using parametric statistics. In agreement with our previous investigations, the results indicate that RA, U_Na,fe, U_K,fe, systolic, and diastolic BP oscillate with a circadian periodicity in dogs fed a regular diet at 07:00?h. A cosine model with a fixed 24-h period was found to fit the variations of RA, U_K,fe and BP well, whereas cyclic changes in U_Na,fe were best characterized by means of a combined cosine and surge model, reflecting a postprandial sodium excretion followed by a monotonous decay. Our data show that feeding time has a marked influence on the chronobiology of the renin cascade, urinary electrolytes, and BP. Introducing a 6- or 12-h delay in the dogs’ feeding schedule caused a shift of similar magnitude (05:06 and 12:32?h for Studies a and b, respectively) in the rhythm of these biomarkers. In all study groups, RA and BP exhibited a marked fall just after food intake. The drop in RA is consistent with sodium and water-induced body fluid expansion, while the reduction of BP could be related to the decreased activity of renin and the secretion of vasodilatory gut peptides. An approximately 1.5-fold (1.2–1.6-fold) change between the average early and late time after feeding observations was found for RA (p?<?0.0001), U_Na,fe (p?<?0.01) and U_K,fe (p?<?0.05). Postprandial variations in BP, albeit small (ca. 10?mmHg), were statistically significant (p?<?0.01) and supported by the model-based analysis.

In conclusion, the timing of food intake appears to be pivotal to the circadian organization of the renin cascade and BP. This synchronizing effect could be mediated by feeding-related signals, such as dietary sodium, capable of entraining circadian oscillators downstream of the master, light–dark-adjusted pacemaker in the suprachiasmatic nucleus.     

Impaired Pressure Natriuresis in Obese Youths

Objective: To compare the response and recovery of blood pressure (BP) and sodium excretion (U_NaV) in response to a behavioral stressor in overweight/obese and lean adolescents. Research Methods and Procedures: Twenty‐five lean (12% to 20% body fat) and 59 overweight/obese (>25% body fat) normotensive adolescents were provided all meals for 3 days (average sodium intake, 4000 ± 200 mg/d), before performing the stressor on the third day. There was a 2‐hour pre‐stress rest, followed by a 1‐hour stress (involving a video game task), and a 2‐hour recovery. Percentage of body fat was obtained from DXA. U_NaV was measured hourly, whereas systolic BP and diastolic BP measurements were obtained at 15‐minute intervals, and averaged for each 1‐hour period. Results: There was no significant difference between the lean and overweight/obese group for the response of systolic BP and diastolic BP (group by time interaction, p = 0.60 and p = 0.64, respectively). However, the lean group had a significantly greater increase in U_NaV in response to the stressor compared with the overweight/obese group (p = 0.02). U_NaV remained elevated compared with baseline in both groups at the 1‐hour (p ≤ 0.0001) and 2‐hour (p ≤ 0.0001) post‐time points. Furthermore, there was a tendency for a larger number of sodium retainers in the overweight/obese group compared with the lean group (39.0% vs. 20.0%; χ² = 2.85, df = 1, p = 0.09). Discussion: This study provided evidence that sodium regulation was impaired during a behavioral stress in overweight/obese individuals compared with lean individuals.     

Relationships among sodium current,permeability, and Na activities in control and glucocorticoid-stimulated rabbit descending colon

Summary Effects of a potent synthetic glucocorticoid, methylprednisolone (MP), on transepithelial Na transport were examined in rabbit descending colon. Current-voltage (I–V) relations of the amiloride-sensitive apical Na entry pathway were measured in colonic tissues of control and MP-treated (40 mg im for 2 days) animals. Tissues were bathed mucosally by solutions of various Na activities, (Na)_m, ranging from 6.2 to 75.6mm, and serosally by a high K solution. TheseI–V relations conformed to the constant field flux equation permitting determination of the permeability of the apical membrane to Na,P _Na ^m , and the intracellular Na activity, (Na)_c. The following empirical relations were observed for both control and MP-treated tissues: (i) Na transport increases hyperbolically with increasing (Na)_m obeying simple Michaelis-Mentin kinetics; (ii)P _Na ^m decreased hyperbolically with increasing (Na)_m, but was unrelated to individual variations in (Na)_c; (iii) (Na)_c increased hyperbolically with (Na)_m; (iv) both spontaneous and steroid-stimulated variations in Na entry rate could be attributed entirely to parallel variations inP _Na ^m at each mucosal Na activity. Comparison of these empirical, kinetic relations between control and MP-treated tissues revealed: (i) maximal Na current andP _Na ^m were greater in MP tissues, but the (Na)_m's at which current andP _Na ^m were half-maximal were markedly reduced; (ii) (Na)_c was significantly increased in MP tissues at each (Na)_m while the (Na)_m at half-maximal (Na)_c was unchanged. These results provide direct evidence that glucocorticoids cause marked stimulation of Na absorption across rabbit colon primarily by increasing the Na permeability of the apical membrane. While the mechanism for the increased permeability remains to be determined, the altered relation betweenP _Na ^m and (Na)_m suggests possible differences in the conformation or environment of the Na channel in MP-treated tissues.     

Circadian Variability of Cystatin C,Creatinine, and Glomerular Filtration Rate (GFR) in Healthy Men during Normal Sleep and after an Acute Shift of Sleep

The estimation of the glomerular filtration rate (GFR) is essential for the evaluation of patients with kidney disease and for the treatment of patients with medications that are eliminated by the kidneys. Plasma cystatin C has been shown in several studies to be superior to plasma creatinine for the estimation of GFR. However, there is limited information on the circadian variation of cystatin C and estimated GFR using cystatin C (eGFR_CystC) or “The Modification of Diet in Renal Disease Study” (MDRD) (eGFR_MDRD) equations. We studied the circadian variation of cystatin C and creatinine during night‐ and day‐sleep conditions in seven healthy volunteers. Serum samples were collected every hour (48 samples per individual) to evaluate the effect of different sampling times on the test results. The median intra‐individual coefficients of variations for the studied markers were 4.2% for creatinine, 4.7% for eGFR_MDRD, 5.5% for cystatin C, and 7.7% for eGFR_CystC. Neither cystatin C nor creatinine differed significantly between the night‐ and day‐sleep conditions. Cystatin C differed significantly with time of day (p=.0003), but this was not the case for creatinine (p=.11). The circadian variation of cystatin C was minor. Small but significant increases in creatinine values and a decrease of eGFR_MDRD were observed after food intake. Thus, cystatin C and creatinine sampling does not have to be restricted to specific times of the day.     

Addressing the theoretical and clinical advantages of combination therapy with inhibitors of the renin–angiotensin–aldosterone system: Antihypertensive effects and benefits beyond BP control

AimsThis article reviews the importance of the renin–angiotensin–aldosterone system (RAAS) in the cardiometabolic continuum; presents the pros and cons of dual RAAS blockade with angiotensin-converting enzyme inhibitors (ACEIs) and angiotensin receptor blockers (ARBs); and examines the theoretical and practical benefits supporting the use of direct renin inhibitors (DRIs) in combination with ACEIs or ARBs.Main methodsThe author reviewed the literature for key publications related to the biochemical physiology of the RAAS and the pharmacodynamic effects of ACEIs, ARBs, and DRIs, with a particular focus on dual RAAS blockade with these drug classes.Key findingsAlthough ACEI/ARB combination therapy produces modest improvement in BP, it has not resulted in the major improvements predicted given the importance of the RAAS across the cardiorenal disease continuum. This may reflect the fact that RAAS blockade with ACEIs and/or ARBs leads to exacerbated renin release through loss of negative-feedback inhibition, as well as ACE/aldosterone escape through RAAS and non-RAAS-mediated mechanisms. Plasma renin activity (PRA) is an independent predictor of morbidity and mortality, even for patients receiving ACEIs and ARBs. When used alone or in combination with ACEIs and ARBs, the DRI aliskiren effectively reduces PRA. Reductions in BP are greater with these combinations, relative to the individual components alone.SignificanceIt is possible that aliskiren plus either an ACEI or ARB may provide greater RAAS blockade than monotherapy with ACEIs or ARBs, and lead to additive improvement in BP and clinically important outcomes.     

CARDIOVASCULAR RHYTHMS AND CARDIAC BAROREFLEX SENSITIVITY IN AT1A RECEPTOR GAIN-OF-FUNCTION MUTANT MICE

A mutant mouse expressing a gain-of-function of the AT_1A angiotensin II receptor was engineered to study the consequences of a constitutive activation of this receptor on blood pressure (BP). Cardiovascular rhythms and spontaneous cardiac baroreflex sensitivity (BRS) were evaluated using telemetric BP recordings of five transgenic (AT_1AMUT) and five wild (AT_1AWT) mice. The circadian rhythms were described with the Chronos-Fit program. The gain of the transfer function between systolic BP (SBP) and pulse intervals used to estimate the spontaneous BRS (ms/mmHg) was calculated in the low frequency (0.15–0.60?Hz) band. Transgenic AT_1AMUT exhibited higher BP and heart rate (HR) levels compared to controls (SBP AT_1AMUT 134.6?±?5.9?mmHg vs. AT_1AWT 110.5?±?5.9; p?<?0.05; HR AT_1AMUT 531.0?±?14.9 vs. AT_1AWT 454.8?±?5.4 beats/min; p?=?0.001). Spontaneous BRS was diminished in transgenic mice (AT_1AMUT 1.23?±?0.17?ms/mmHg vs. AT_1AWT 1.91?±?0.18?ms/mmHg; p?<?0.05). Motor activity did not differ between groups. These variables exhibited circadian changes, and the differences between the strains were maintained throughout the cycle. The highest values for BP, HR, and locomotor activity were observed at night. Spontaneous BRS varied in the opposite direction, with the lowest gain estimated when BP and HR were elevated (i.e., at night, when the animals were active). It is likely the BP elevation of the mutant mice results from the amplification of the effects of AngII at different sites. Future studies are necessary to explore whether AT_1A receptor activation at the central nervous system level effectively contributed to the observed differences. (Author correspondence: jean-luc.elghozi@nck.aphp.fr)     

Higher plant phosphoenolpyruvate carboxylase kinase is regulated at the level of translatable mRNA in response to light or a circadian rhythm

Phosphoenolpyruvate carboxylase is regulated by reversible phosphorylation in response to light in C₃ and C₄ plants and to a circadian oscillator in CAM plants. Increases in phosphoenolpyruvate carboxylase kinase activity require protein synthesis. This requirement has been analysed by quantifying translatable mRNA for this protein kinase using in vitro translation of isolated RNA followed by direct assay of kinase activity. In leaves of the CAM plant Bryophyllum (Kalanchoë) fedtschenkoi, in normal diurnal conditions, kinase mRNA was 20-fold more abundant at night than in the day. In constant environmental conditions (continuous darkness, CO₂-free air, 15°C) kinase mRNA exhibited circadian oscillations. The circadian disappearance of kinase mRNA and kinase activity was delayed by lowering the temperature to 4°C and accelerated by raising the temperature to 30°C. The appearance of kinase mRNA and activity was blocked by cordycepin and puromycin. In maize and barley, kinase mRNA increased in response to light. For all three plants, the phosphoenolpyruvate carboxylase kinase activity generated during in vitro translation was Ca²⁺-independent. These results demonstrate that phosphoenolpyruvate carboxylase kinase activity is regulated at the level of translatable mRNA in C₃, C₄ and CAM plants.     

Effect of Na i on activity and voltage dependence of the Na/K pump in adult rat cardiac myocytes

We have measured the voltage dependence of the Na/K pump in isolated adult rat cardiac myocytes using the whole-cell patch-clamp technique. In the presence of 1–2 mM Ba and 0.1 mm Cd and nominally Ca-free, Na/K pump current (I _p) was measured as the change in current due to 1 mM ouabain. Voltage dependence of I _pwas measured between –140 and +40 or +60 mV using square voltage-pulse and voltage-ramp protocols, respectively. With 150 mM extracellular Na (Na _o ) and 5.4 mM extracellular K (K _o ), we found that the Na/K pump shows a strong positive voltage dependence between –140 and 0 mV and is voltage independent at positive potentials. Removing Na _o reduced the voltage dependence at negative potentials with no effect at positive potentials. When K _o was reduced, a negative slope appeared in the current-voltage (I-V) curve at positive potentials. We have investigated whether Na _i (intracellular Na) might also affect the voltage dependence of I _pby varying Na in the patch pipette (Na_pip) between 20 and 85 mM. We found, as expected, that I _pincreased markedly as Na_pip was raised, saturating at about 70 mM Na_pip under these conditions. In contast, while I _psaturated near +20 mV and declined to about 40% of maximum at –120 mV, there was no effect of Na_pip under these conditions. In contrast, while I _psaturated near +20 mV and declined to about 40% of maximum at –120 mV, there was no effect of Na_pip on the voltage dependence of I _p. This suggests that neither Na _i binding to the Na/K pump nor the conformational changes dependent on Na _i binding are voltage dependent. These results are consistent with extracellular ion binding within the field of the membrane but do not rule out the possibility that other steps, such as Na translocation, are also voltage dependent.We wish to thank Ms. Melinda Price, Ms. Meei Liu and Mr. Randall Anderson for their technical assistance. This work was supported in part by National Institutes of Health grant HL44660.     

EXPRESSION PROFILING REVEALS A POSITIVE REGULATION BY MPER2 ON CIRCADIAN RHYTHM OF CYTOTOXICITY RECEPTORS: LY49C AND NKG2D

The mammalian circadian gene, mPer2, an indispensable component of the mammalian circadian clock, not only modulates endogenous circadian rhythms but also plays a crucial role in regulating innate immune function. Previously, we showed that mPer2 plays a crucial role in regulating cytotoxic response. To investigate the molecular mechanism for mPer2-controlled cytotoxic response, in the present study we conducted mRNA expression for 11 genes participating in cytotoxicity regulation in wild-type (WT) and mPer2 knockout (mPer2 ^{?;?;?/ ?;?;?}) mice bone marrow, that is, Dap-10, Ly49C, Ly49I, Rac1, Mapk1, Map2k1, Nkg2d, Shp-1, Pak1, Pik3ca, and Vav1. The mRNA levels of Ly49C (p?<?0.001), Ly49I (p?=?0.039), and Nkg2d (p?=?0.038) were significantly downregulated in mPer2 ^{?;?;?/ ?;?;?} mice. Time-dependence of expression profiling was then conducted for four core clock genes (Per1, Bmal1, Clock, Rev-erbα), and six out of these 11 cytotoxic regulation genes (Ly49C, Ly49I, Mapk1, Nkg2d, Shp-1, Pik3ca) in WT and mPer2 ^{?;?;?/ ?;?;?} entrained in light/dark (LD) or dark/dark (DD) cycles. Consistently, circadian oscillations were observed for Per1, Rev-erbα, Ly49C, and Nkg2d in WT mice under LD and DD cycles. However, these rhythmic expressions were either disrupted or dampened in mPer2 ^{?;?;?/ ?;?;?} mice. Comparison of gene expression between WT and mPer2 ^{?;?;?/ ?;?;?} mice showed that mPer2 knockout had systematically downregulated the mRNA expression of two cytotoxicity regulators, Ly49C and Nkg2d. FACS analysis further confirmed that the circadian expression of these genes was not due to the daily difference in cell numbers of NK, NKT, or T cells in bone marrow. Taken together, our results reveal that mPer2 is a critical clock component in modulating circadian rhythms in bone marrow. Furthermore, it implies that Ly49C and Nkg2d are two clock-controlled genes that may play an important role in mediating mPer2-controlled cytotoxic response. (Author correspondence: zsunusa@yahoo.com)     

Aldosterone control of the density of sodium channels in the toad urinary bladder

Summary Near-instantaneous current-voltage relationships and shot-noise analysis of amiloride-induced current fluctuations were used to estimate apical membrane permeability to Na (P _Na), intraepithelial Na activity (Na _c ), single-channel Na currents (i) and the number of open (conducting) apical Na channels (N₀), in the urinary bladder of the toad (Bufo marinus). To facilitate voltageclamping of the apical membrane, the serosal plasma membranes were depolarized by substitution of a high KCl (85mm) sucrose (50mm) medium for the conventional Na-Ringer's solution on the serosal side.Aldosterone (5×10^–7 m, serosal side only) elicited proportionate increases in the Na-specific current (I _Na and inP _Na, with no significant change in the dependence ofP _Na on mucosal Na (Na _o ).P _Na and the control ofP _Na by aldosterone were substrate-dependent: In substrate-depleted bladders, pretreatment with aldosterone markedly augmented the response to pyruvate (7.5×10^–3 m) which evoked coordinate and equivalent increases inI _Na andP _Na.The aldosterone-dependent increase inP _Na was a result of an equivalent increase in the area density of conducting apical Na channels. The computed single-channel current did not change. We propose that, following aldosterone-induced protein synthesis, there is a reversible metabolically-dependent recruitment of preexisting Na channels from a reservoir of electrically undetectable channels. The results do not exclude the possibility of a complementary induction of Na-channel synthesis.     

Modulation of apical Na permeability of the toad urinary bladder by intracellular Na,Ca, and H

Summary The Na conductance of the apical membrane of the toad urinary bladder was measured at different concentrations of Na both in the external medium and in the cell. Bladders were bathed in high K-sucrose medium to reduce basal-lateral resistance and voltage, and the transepithelial currents measured under voltage-clamp conditions. Amiloride was used as a specific blocker of the apical Na channel. At constant external Na, the internal Na concentration was increased by blocking the basallateral Na pump with ouabain. With high Na activity in the mucosal medium (86mm), increases in intracellular Na activity from 10 to over 40mm increased the amiloride-sensitive slope conductance at zero voltage while apical Na permeability, estimated from current-voltage plots using the constant field equation, decreased by less than 20%. Lowering the serosal Ca concentration from 1 to 0.1mm had no effect on the change inP _Na with increasing Na_c, but increasing serosal Ca to 5mm enhanced the reduction inP _Na with increasing Na _c , presumably by increasing Ca influx into the cell.P _Na was also reduced by serosal vanadate (0.5mm), a putative blocker of ATP-dependent Ca extrusion from the cell, and by acute exposure to CO₂, which presumably acidifies the cytoplasm. Current-voltage relationships of the amiloridesensitive transport pathway were also measured in the absence of a Na gradient across the apical membrane. These plots show that outward current passes through the channels somewhat less easily than does inward current. The shape of theI-V relationships was not significantly altered by changes in cellular Na, Ca or H, indicating that the effects of these ions onP _Na are voltage independent.     

A Chronobiological Approach to Circulating Levels of Renin,Angiotensin-Converting Enzyme,Aldosterone, ACTH,and Cortisol in Addison's Disease

This study deals with a chronobiological approach to the circadian rhythm of the renin-angiotensin-aldosterone system (RAAS) and the ACTH-cor-tisol axis (ACA) in patients with Addison's disease (PAD). The aim is to explore the mechanism(s) for which the circadian rhythmicity of the RAAS and ACA takes place. The study has shown that both the RAAS and ACA are devoid of a circadian rhythm in PAD. The lack of rhythmicity for renin and ACTH provides indirect evidence that their rhythmic secretion is in some way related to the circadian oscillation of aldosterone and cortisol. This implies a new concept: a positive feedback may be included among the mechanisms which chronoregulate the RAAS and ACA.     

A MODEL OF HIGH AFFINITY CHOLINE TRANSPORT IN RAT CORTICAL SYNAPTOSOMES

Abstract— Initial velocity of choline uptake by cortical synaptosomes from the Long-Evans rat has been measured as a function of both choline and sodium concentration. These data were then fitted to the rate equation for each of several possible models which characterize the participation of sodium in the transport process, and the models giving best fit were identified. Although one cannot unequivocally distinguish between a model including a high affinity carrier component plus diffusion and one including both high affinity and low affinity carriers, the conclusions concerning the high affinity component are the same in both cases. The major conclusions from the model are as follows: (1) The carrier may first combine with either choline or sodium; if the first reaction is with sodium, then there is an obligatory reaction with a second sodium before choline can interact with the carrier. (2) Translocation may occur as either CS or CNa₂S (C= carrier; S= choline; CS= carrier-substrate complex). (3) The apparent maximal velocity (Va) is dependent on the sodium concentration. (4) K₁, the choline concentration giving Va/2. is also dependent on the sodium concentration. K₁ increases with [Na] from 0 to 38.41 mm ; above 38.41 mm -[Na]. K₁ declines with [Na]. (5) There is a sigmoidal relationship between velocity of uptake and [Na]; however, uptake is not zero at [Na] = 0. (6) Jm. uptake at a given [choline] and infinite [Na]. is hyperbolically related to the choline concentration, but changes slowly over the range of 0.5–5.0 ± 10^-6m . (7) K_Na, the sodium concentration giving a velocity equal to Jm/2, is related to the choline concentration by a quadratic equation, and was found to be greater than physiological [Na] at choline concentrations of 0.5, 0.6, or 1.0 ± 10^-6m . but less than physiological [Na] at choline concentrations of 2.0 or 5.0 ± 10^-6m . The best fit model for the high affinity uptake of choline is very similar to what has been found in previous studies for the high affinity uptake of glutamic acid and GABA, thus raising the question of whether or not all high affinity synaptosomal mechanisms may be variations of a common model.     

A Chronobiological Approach to Circulating Levels of Renin, Angiotensin-Converting Enzyme, Aldosterone, ACTH, and Cortisol in Addison's Disease

This study deals with a chronobiological approach to the circadian rhythm of the renin-angiotensin-aldosterone system (RAAS) and the ACTH-cor-tisol axis (ACA) in patients with Addison's disease (PAD). The aim is to explore the mechanism(s) for which the circadian rhythmicity of the RAAS and ACA takes place. The study has shown that both the RAAS and ACA are devoid of a circadian rhythm in PAD. The lack of rhythmicity for renin and ACTH provides indirect evidence that their rhythmic secretion is in some way related to the circadian oscillation of aldosterone and cortisol. This implies a new concept: a positive feedback may be included among the mechanisms which chronoregulate the RAAS and ACA.     

Ionic exchanges in isolated and open-circuited toad skin

Summary Net influxes of Na and Cl and effluxes of K and H (J _Na,J _Cl,J _K andJ _H) and volume flowJ _v across isolated open-circuited toad skins were measured using rotating chambers and a small volume of external solution, the ion fluxes being determined by chemical analysis of the external solution, in the range of 0.2 to 5.0mm external Na concentration. In this concentration range, with skin potential varying with (Na) _e ,J _Na is a linear function of the Na electrochemical potential difference across the skin, as expected on irreversible thermodynamic grounds. TheL _Na coefficient calculated asJ _Na/ _Na is equal to 5.5×10^–12 mole² joule^–1 cm^–2 min^–1, which is similar to values obtained for the same species in the short-circuited state and in higher ranges of (Na) _e . A positive correlation is observed betweenJ _Na andJ _K whenJ _Na varied with (Na) _e and also whenJ _Na varies in randomly selected skins. Antidiuretic hormone stimulatesJ _Na,J _K andJ _v in the range of 0.2 to 5.0mm (Na) _e and lowers the Na concentration in the equivalent solution absorbed by the skin (calculated asJ _Na/J _v ). Substitution of external Cl by SO₄ has no effect onJ _Na,J _K andJ _H and also in the skin potential in the range of (Na) _e studied. Substitution of external Na by K abolishesJ _Cl and reverses the skin polarity, the external solution now being positive to the internal one. Na removal from the external solution also reducesJ _K almost to zero.J _H is significantly reduced in this condition; however, a basal secretion still persists with (Na) _e equal to zero. The results of these experiments can be tentatively interpreted in terms of electrical coupling between ion fluxes, since only the procedures that result in alterations of skin potential are followed by changes in the rates of ion transport. The existence of other coupling mechanisms cannot be ruled out.     

Ion transport and regulation ofTetrahymena pyriformis in isotonic and hypotonic media

Summary The ion and volume regulatory mechanisms ofTetrahymena pyriformis were studied in normal or hypotonic nutrient media and in isotonic inorganic media with different Na/K ratios, in conjunction with the effects of a general metabolic inhibitor (low temperature) and a specific inhibitor (iodoacetate). For K two mechanisms of active influx were found: The first is sensitive to IAc and seems to be the basic mechanism for the maintenance of the K_i/K_o gradient. The second is sensitive to cooling and related to the function of the contractile vacuole; it is also responsible for the high intracellular levels of K. The passive K efflux seems to be a basic factor for volume regulation, together with the contractile vacuole. It increases in hypotonic media and this seems to be related to structural changes of the membranes occurring in hypotonic media. For Na two mechanisms of active transport were also found: One for active Na efflux with highK _m, which is associated with the contractile vacuole and another, for active Na influx with lowK _m, which is inhibited by high levels of intracellular K.The electrochemical potentials of Na and K and the membrane potential (Cl Nernst potential) were also studied in isotonic inorganic media. The membrane is negatively polarized, except if Na_o<5 mM when it becomes positive. In normal conditions, Na is transported outwards actively and leaks passively, while K is transported inwards actively and leaks 56 times more rapidly than Na ions.A model for the overall transport and regulation of ions inTetrahymena is proposed.Abbreviations IAc iodoacetate - PCV packed cell volume - Na _i,K _i,Cl intracellular concentrations ofNa ⁺,K ⁺,Cl ^–, respectively - Na _o,K _o, Cl_o extracellular concentrations of Na⁺, K⁺, Cl^–, respectively - DR distribution ratio - HyN hypotonic nutrient medium - IsN isotonic nutrient medium - HyS IsS hypotonic, and isotonic salt medium, respectively     

Effects of 2-deoxy-d-glucose,amiloride, vasopressin,and ouabain on active conductance andE Na in the toad bladder

Summary The effects of various agents on active sodium transport were studied in the toad bladder in terms of the equivalent circuit comprising an active conductanceK ^a, an electromotive forceE _Na, and a parallel passive conductanceK ^p. For agents which affectK ^a, but notE _Na orK ^p, the inverse slope of the plot of total conductance against short-circuit currentI ₀ evaluatesE _Na, and the intercept representsK ^p. Studies employing 5×10^–7 m amiloride to depressK ^a indicate a changingE _Na, invalidating the use of the slope technique with this agent. An alternative suitable technique employs 10^–5 m amiloride, which reducesI ₀ reversibly to near zero without effect onK ^p. Despite curvilinearity of the -I₀ plot under these conditions,K ^p may therefore be estimated fairly precisely from the residual conductance. It then becomes possible to follow the dynamic behavior ofK ^a andE _Na (in the absence of 10^–5 m amiloride) by frequent measurements of andI ₀, utilizing the relationshipsK ^a=K-K ^p, andK _Na=I _O/(K-K ^p). 2-deoxy-d-glucose (7.5×10^–3 m) depressedK ^a without affectingE _Na. Amiloride (5×10^–7 m) depressedK ^a and enhancedE _Na. Vasopressin (100 mU/ml) enhancedK ^a markedly and depressedE _Na slightly. Ouabain (10^–4 m) depressed bothK ^a andE _Na. All of the above effects were noted promptly;K ^p was unaffected. The electromotive force of Na transportE _Na appears not to be a pure energetic parameter, but to reflect kinetic factors as well, in accordance with thermodynamic considerations.     

A High‐Energy NASICON‐Type Cathode Material for Na‐Ion Batteries

Over the last decade, Na‐ion batteries have been extensively studied as low‐cost alternatives to Li‐ion batteries for large‐scale grid storage applications; however, the development of high‐energy positive electrodes remains a major challenge. Materials with a polyanionic framework, such as Na superionic conductor (NASICON)‐structured cathodes with formula Na_xM₂(PO₄)₃, have attracted considerable attention because of their stable 3D crystal structure and high operating potential. Herein, a novel NASICON‐type compound, Na₄MnCr(PO₄)₃, is reported as a promising cathode material for Na‐ion batteries that deliver a high specific capacity of 130 mAh g^?1 during discharge utilizing high‐voltage Mn^2+/3+ (3.5 V), Mn^3+/4+ (4.0 V), and Cr^3+/4+ (4.35 V) transition metal redox. In addition, Na₄MnCr(PO₄)₃ exhibits a high rate capability (97 mAh g^?1 at 5 C) and excellent all‐temperature performance. In situ X‐ray diffraction and synchrotron X‐ray diffraction analyses reveal reversible structural evolution for both charge and discharge.     

Disubstituted diaryl diselenides inhibit δ-ALA-D and Na+, K+-ATPase activities in rat brain homogenates in vitro

Toxicological and pharmacological studies demonstrated that the introduction of functional groups into the aromatic ring of diphenyl diselenide alter its effect. The aim of this study was to evaluate the in vitro effect of m-trifluoromethyl-diphenyl diselenide (m-CF₃–C₆H₄Se)₂, p-chloro-diphenyl diselenide (p-Cl–C₆H₄Se)₂ and p-methoxyl-diphenyl diselenide (p-CH₃O–C₆H₄Se)₂ on δ-aminolevulinate dehydratase (δ-ALA-D) and Na⁺, K⁺-ATPase activities in rat brain homogenates. Diselenides inhibited δ-ALA-D activity (IC₅₀ 4–6 μM [concentration inhibiting 50%]), and dithiothreitol (DTT) restored the enzyme activity. ZnCl₂ (100 μM) did not restore δ-ALA-D inhibition caused by (p-Cl–C₆H₄Se)₂ and (m-CF₃–C₆H₄Se)₂. Na⁺, K⁺-ATPase activity was more sensitive to (p-Cl–C₆H₄Se)₂ and (m-CF₃–C₆H₄Se)₂ (IC₅₀ 6 μM) than (p-CH₃O–C₆H₄Se)₂ and (PhSe)₂ (IC₅₀ 45 and 31 μM, respectively). DTT restored the activity of Na⁺, K⁺-ATPase inhibited by diselenides. The effect of diselenides on Na⁺/K⁺-ATPase is dependent on their substitutions in the aromatic ring. The mechanism through which diselenides inhibit δ-ALA-D and Na⁺, K⁺-ATPase activities involves the oxidation of thiol groups.     

Salt stress affects in vitro growth and in situ symbioses of ectomycorrhizal fungi

Ten isolates of six species of ectomycorrhizal fungi were grown in vitro at nine concentrations of three sodium salts (NaCl, Na₂SO₄, Na₃C₆H₅O₇) for 4 weeks. Colony diamater, biomass and protein content of fungi were evaluated. Isolates of Pisolithus tinctorius and Suillus luteus were more tolerant of NaCl and Na₂SO₄ than of Na₃C₆H₅O₇. Fungi in the genera Cenococcum, Laccaria, and Thelephora were highly intolerant of Na₃C₆H₅O₇ and Na₂SO₄ in vitro. Biomass and protein content of fungi generally declined with increasing substrate salinity in solution culture. In situ ectomycorrhizal colonization by Laccara laccata and P. tinctorius and the dry weight of Pinus taeda seedlings were significantly reduced by 80 mM NaCl after 14 weeks. Only select ectomycorrhizal fungi appear capable of growth and symbiosis in saline soils.