Ventilatory physiology of the numbat (<Emphasis Type="Italic">Myrmecobius fasciatus</Emphasis>)

This study examines the ventilatory physiology of the numbat (Myrmecobius fasciatus), a small to medium-sized (550 g) termitivorous marsupial. Ventilatory parameters at thermoneutrality reflect the slightly low (83% of predicted) basal metabolic rate of the numbat, with ventilation frequency (ƒ_R; 30.6±3.65 breaths min^–1), tidal volume [V_T; 6.0±0.66 ml at body temperature and pressure, saturated (BTPS)] and consequently minute volume (V_I; 117.7±15.22 ml min^–1; BTPS) all being 80–87% of that expected for a marsupial of similar body mass. Oxygen extraction was 27.7±1.37% in the thermoneutral zone. As is typical of marsupials, numbats accommodated increased oxygen consumption rates at ambient temperatures (T _a) below the thermoneutral zone by increasing minute volume (up to 411.2±43.98 ml min^–1; BTPS at T _a=10 °C) rather than oxygen extraction. Minute volume at 10 °C increased more by changes in ventilation frequency (up to 45.5±4.85 breaths min^–1) than tidal volume (9.4±1.03 ml, BTPS), as is also typical for a small-medium sized marsupial.Abbreviations BMR basal metabolic rate - BTPS body temperature and pressure, saturated - EO ₂ oxygen extraction - ƒ _R ventilation frequency - STPD standard temperature and pressure, dry - T _a ambient temperature - T _b body temperature - TNZ thermoneutral zone - V _I minute volume - V _T tidal volume - O ₂ oxygen consumption rate Communicated by I.D. Hume     

Regulation of transpiration in coffee hedgerows: covariation of environmental variables and apparent responses of stomata to wind and humidity

Stomatal regulation of transpiration was studied in hedgerow coffee (Coffea arabica L.) at different stages of canopy development encompassing a range of leaf area indices (L) from 0·7 to 6·7. Stomatal (g_s) and crown (g_c) conductance attained maximum values early during the day and then declined as both leaf-to-bulk air water vapour mole fraction difference (V_a) and photosynthetically active photon flux density (I) continued to increase. Covariation of environmental variables during the day, particularly V, I, and wind speed (u), obscured stomatal responses to individual variables. This also caused diurnal hysteresis in the relationship between g_c and individual variables. Normalization of g_s and g_c by I removed the hysteresis and revealed a strong stomatal response to humidity. At the crown scale, transpiration (E) increased linearly with net radiation (R_n) and seemed to increase with increasing wind speed. Increasing wind speed imposed higher leaf interior to leaf surface water vapour mole fraction differences (V_s) at given levels of V_a. However, strong relationships between declining g_c and E and increasing wind speed were obtained when g_c and E were normalized by I and R_n, respectively, without invoking additional potential interactions involving temperature or CO₂ concentration at the leaf surface. Apparent stomatal responses to wind were thus at least partially a reflection of the stomatal response to humidity.     

Maximal oxygen uptake, anaerobic threshold and running economy in women and men with similar performances level in marathons

Sex differences in running economy (gross oxygen cost of running, C_R), maximal oxygen uptake (VO_2max), anaerobic threshold (Th_an), percentage utilization of aerobic power (% VO_2max), and Th_an during running were investigated. There were six men and six women aged 20–30 years with a performance time of 2 h 40 min over the marathon distance. The VO_2max, Th_an, and CR were measured during controlled running on a treadmill at 1° and 3° gradient. From each subject's recorded time of running in the marathon, the average speed (v _M) was calculated and maintained during the treadmill running for 11 min. The VO_{2 max} was inversely related to body mass (m _b), there were no sex differences, and the mean values of the reduced exponent were 0.65 for women and 0.81 for men. These results indicate that for running the unit ml·kg^–0.75·min^–1 is convenient when comparing individuals with different m _b. The VO_2max was about 10% (23 ml·kg^–0.75·min^–1) higher in the men than in the women. The women had on the average 10–12 ml·kg^–0.75·min^–1 lower VO₂ than the men when running at comparable velocities. Disregarding sex, the mean value of C_R was 0.211 (SEM 0.005) ml·kg^–1·m^–1 (resting included), and was independent of treadmill speed. No sex differences in Th_an expressed as % VO_2max or percentage maximal heart rate were found, but Than expressed as VO₂ in ml·kg^–0.75·min^–1 was significantly higher in the men compared to the women. The percentage utilization of f _emax and concentration of blood lactate at v _M was higher for the female runners. The women ran 2 days more each week than the men over the first 4 months during the half year preceding the marathon race. It was concluded that the higher VO_2max and Th_an in the men was compensated for by more running, superior C_R, and a higher exercise intensity during the race in the performance-matched female marathon runners.     

Cluster analysis of respiratory time series

We have investigated the respiratory control system with the hypothesis that, although many variables such as minute ventilation (V _I), tidal volume (V _T),breathing period (T _T),inspiratory duration (T _I),and exspiratory duration (T _E),may be observed, the controller functions more simply by manipulating only 2 or 3 of these. Thus, if tidal volume is the only independent variable, T _Ibeing determined by the off-switch threshold, these variables should have very similar time courses. Anesthetized dogs were subjected to CO₂ breathing and carotid sinus perfusion to stimulate both chemoreceptors. The time series of the variables V _I, V _T, T_T, T_Eand T _Ias well as P _{A CO 2}were determined on a breath by breath basis. Derived characteristics of these time series were compared using Cluster Analysis and the latent dimensionality of respiratory control determined by Factor Analysis. The characteristics of the time series clustered into 4 groups: magnitude (of the response), speed, variability and relative change. One respiratory factor accounted for 86% of the variance for the variability characteristics, 2 factors for magnitude (91%) and relative change (85%) and 3 factors for speed (89%). The respiratory variables were analysed for each of the 4 groups of characteristics with the following results: V _Tand T _I clustered together only for the magnitude and relative change characteristics where as T _Tand T _Eclustered closely for all four characteristics. One latent factor was associated with the [T _T-T_E]group and the other usually with P _{A CO 2}.Supported by USPHS 5t01 01919-05, NIH HL 12564 and GM 07033     

The role of different anions in ionic liquids on Pseudomonas cepacia lipase catalyzed transesterification and hydrolysis

Lipase Pseudomonas cepacia (PS) catalyzed transesterification of ethyl 3-phenylpropanoate with eleven alcohols was investigated in three ionic liquids [ILs], [Bmim]BF₄, [Bmim]PF₆, and [Bmim]Tf₂N, consisting of an identical cation and different anions. The yields were higher in hydrophobic ILs [Bmim]Tf₂N (55–96%) and [Bmim]PF₆ (22–95%), than in hydrophilic [Bmim]BF₄ (0–19%). The incubation of lipase PS in hydrophobic ILs for a period of 20–300 days at room temperature resulted in an increased yield of 62–98% in [Bmim]Tf₂N and 45–98% in [Bmim]PF₆, respectively. The lipase PS-hydrophobic IL mixture was recycled five times without any decrease in the yield of the products. In another set of experiments, the hydrolytic activity of the enzyme was determined after incubation in each of the three ILs and in hexane for 20 days at room temperature. It was found to be 1.8- and 1.6-fold higher in [Bmim]Tf₂N and [Bmim]PF₆, respectively, remained unchanged in [Bmim]BF₄ and was 1.6 times lower in hexane as compared to the non-incubated enzyme.     

Chronic toxicity of methylparathion to the rotifer Brachionus calyciflorus fed on Nannochloris oculata and Chlorella pyrenoidosa

The effect of sublethal levels of methylparathion (0, 1, 3, 5, 7 mg l^–1) on the freshwater rotifer, Brachionus calyciflorus, during their entire life cycle was studied. Rotifers were fed on two species of unicellular algae: Nannochloris oculata and Chlorella pyrenoidosa; both algal concentrations were 5 × 10⁵ cell ml^–1.The parameters used to determine the toxicity of this compound were survival, fecundity, net reproductive rate (R)_o, generation time (T), intrinsic rate of natural increase (r), reproductive value (V _x/V_o) and life expectancy at hatching (eo). All the demographic parameters studied were affected by methyl-parathion exposure on rotifers fed on both species of algae, but the toxic effect was larger when animals were fed on Chlorella pyrenoidosa; in this case, animals showed a decreased in fertility and also a delayed first reproduction. Sublethal methylparathion levels produced a reduction in most of the parameters selected, especially after exposure to 7 mg l^–1, where the animals died before reproducing.     

Modifications of current properties by expression of a foreign potassium channel gene in Xenopus embryonic cells

The pH-sensitivity of transepithelial K⁺ transport was studied in vitro in isolated vestibular dark cell epithelium from the gerbil ampulla. The cytosolic pH (pH _iwas measured microfluorometrically with the pH-sensitive dye 2,7-bicarboxyethyl-5(6)-carboxyfluorescein (BCECF) and the equivalent short-circuit current (I _sc), which is a measure for transepithelial K⁺ secretion, was calculated from measurements of the transepithelial voltage (V _t)and the transepithelial resistance (R _t) in a micro-Ussing chamber. All experiments were conducted in virtually HCO ₃ ^– -free solutions. Under control conditions, pH _iwas 7.01±0.04 (n=18), V _twas 9.1±0.5 mV, R _t16.7±0.09 cm², and I _sc was 587±30 A/cm² (n=49). Addition of 20 mm propionate^– caused a biphasic effect involving an initial acidification of pH _i, increase in V _tand I _sc and decrease in R _tand a subsequent alkalinization of pH _i, decrease of V _tand increase of R _t. Removal of propionate^– caused a transient effect involving an alkalinization of pH _i, a decrease of V _tand I _sc and an increase in R _t. pH _iin the presence of propionate^– exceeded pH _iunder control conditions. Effects of propionate ^– on V _t, R _tand I _sc were significantly larger when propionate^– was applied to the basolateral side rather than to the apical side of the epithelium. The pH _i-sensitivityof I _sc between pH 6.8 and 7.5 was –1089 A/(cm² · pH-unit) suggesting that K⁺ secretion ceases at about pH _i7.6. Acidification of the extracellular pH (pH _o)caused an increase of V _tand I _sc and a decrease of R _tmost likely due to acidification of pH _i. Effects were significantly larger when the extracellular acidification was applied to the basolateral side rather than to the apical side of the epithelium. The pH _osensitivity of I _sc between pH 7.4 and 6.4 was –155 A/(cm² · pH unit). These results demonstrate that transepithelial K⁺ transport is sensitive to pH _iand pH _oand that vestibular dark cells contain propionate^– uptake mechanism. Further, the data suggest that cytosolic acidification activates and that cytosolic alkalinization inactivates the slowly activating K⁺ channel (I _sK)in the apical membrane. Whether the effect of pH _ion the I _sK channel is a direct or indirect effect remains to be determined.The authors wish to thank Drs. Daniel C. Marcus, Zhjiun Shen and Hiroshi Sunose for helpful discussions. This work was supported by grants NIH-R29-DC01098 and NIH-R01-DC00212.     

Dynamic versus isometric electromechanical delay in non-fatigued and fatigued muscle: A combined electromyographic,mechanomyographic, and force approach

This study used a combined electromyographic, mechanomyographic, and force approach to identify electromechanical delay (EMD) from the onsets of the electromyographic to force signals (EMD_E-F), onsets of the electromyographic to mechanomyogrpahic signals (EMD_E-M), and onsets of mechanomyographic to force signals (EMD_M-F). The purposes of the current study were to examine: (1) the differences in EMD_E-F, EMD_E-M, and EMD_M-F from the vastus lateralis during maximal, voluntary dynamic (1 repetition maximum [1-RM]) and isometric (maximal voluntary isometric contraction [MVIC]) muscle actions; and (2) the effects of fatigue on EMD_E-F, EMD_M-F, and EMD_E-M. Ten men performed pretest and posttest 1-RM and MVIC leg extension muscle actions. The fatiguing workbout consisted of 70% 1-RM dynamic constant external resistance leg extension muscle actions to failure. The results indicated that there were no significant differences between 1-RM and MVIC EMD_E-F, EMD_E-M, or EMD_M-F. There were, however, significant fatigue-induced increases in EMD_E-F (94% and 63%), EMD_E-M (107%), and EMD_M-F (63%) for both the 1-RM and MVIC measurements. Therefore, these findings demonstrated the effects of fatigue on EMD measures and supported comparisons among studies which examined dynamic or isometric EMD measures from the vastus lateralis using a combined electromyographic, mechanomyographic, and force approach.     

Using combined measurements of gas exchange and chlorophyll fluorescence to estimate parameters of a biochemical C3 photosynthesis model: a critical appraisal and a new integrated approach applied to leaves in a wheat (Triticum aestivum) canopy

We appraised the literature and described an approach to estimate the parameters of the Farquhar, von Caemmerer and Berry model using measured CO₂ assimilation rate (A) and photosystem II (PSII) electron transport efficiency (Φ₂). The approach uses curve fitting to data of A and Φ₂ at various levels of incident irradiance (I_inc), intercellular CO₂ (C_i) and O₂. Estimated parameters include day respiration (R_d), conversion efficiency of I_inc into linear electron transport of PSII under limiting light [κ_2(LL)], electron transport capacity (J_max), curvature factor (θ) for the non‐rectangular hyperbolic response of electron flux to I_inc, ribulose 1·5‐bisphosphate carboxylase/oxygenase (Rubisco) CO₂/O₂ specificity (S_c/o), Rubisco carboxylation capacity (V_cmax), rate of triose phosphate utilization (T_p) and mesophyll conductance (g_m). The method is used to analyse combined gas exchange and chlorophyll fluorescence measurements on leaves of various ages and positions in wheat plants grown at two nitrogen levels. Estimated S_c/o (25 °C) was 3.13 mbar µbar^?1; R_d was lower than respiration in the dark; J_max was lower and θ was higher at 2% than at 21% O₂; κ_2(LL), V_cmax, J_max and T_p correlated to leaf nitrogen content; and g_m decreased with increasing C_i and with decreasing I_inc. Based on the parameter estimates, we surmised that there was some alternative electron transport.     

Effects of anions on cellular volume and transepithelial Na+ transport across toad urinary bladder

Summary The effects of complete substitution of gluconate for mucosal and/or serosal medium Cl^– on transepithelial Na⁺ transport have been studied using toad urinary bladder. With mucosal gluconate, transepithelial potential difference (V _T) decreased rapidly, transepithelial resistance (R _T) increased, and calculated short-circuit current (I _sc) decreased. CalculatedE _Na was unaffected, indicating that the inhibition of Na⁺ transport was a consequence of a decreased apical membrane Na⁺ conductance. This conclusion was supported by the finding that a higher amiloride concentration was required to inhibit the residual transport. With serosal gluconateV _T decreased,R _T increased andI _sc fell to a new steady-state value following an initial and variable transient increase in transport. Epithelial cells were shrunken markedly as judged histologically. CalculatedE _Na fell substantially (from 130 to 68 mV on average). Ba²⁺ (3mm) reduced calculatedE _Na in Cl^– Ringer's but not in gluconate Ringer's. With replacement of serosal Cl^– by acetate, transepithelial transport was stimulated, the decrease in cellular volume was prevented andE _Na did not fall. Replacement of serosal isosmotic Cl^– medium by a hypo-osmotic gluconate medium (one-half normal) also prevented cell shrinkage and did not result in inhibition of Na⁺ transport. Thus the inhibition of Na⁺ transport can be correlated with changes in cell volume rather than with the change in Cl^– per se. Nystatin virtually abolished the resistance of the apical plasma membrane as judged by measurement of tissue capacitance. With K⁺ gluconate mucosa, Na⁺ gluconate serosa, calculated basolateral membrane resistance was much greater, estimated basolateral emf was much lower, and the Na⁺/K⁺ basolateral permeability ratio was much higher than with acetate media. It is concluded the decrease in cellular volume associated with substitution of serosal gluconate for Cl^– results in a loss of highly specific Ba²⁺-sensitive K⁺ conductance channels from the basolateral plasma membrane. It is possible that the number of Na⁺ pump sites in this membrane is also decreased.     

Growth and energy budget of F2'all-fish' growth hormone gene transgenic common carp

The growth and energy budget for F₂‘all‐fish’ growth hormone gene transgenic common carp Cyprinus carpio of two body sizes were investigated at 29·2° C for 21 days. Specific growth rate, feed intake, feed efficiency, digestibility coefficients of dry matter and protein, gross energy intake (I_E), and the proportion of I_E utilized for heat production (H_E) were significantly higher in the transgenics than in the controls. The proportion of I_E directed to waste products [faecal energy (F_E) and excretory energy loss (Z_E+U_E) where Z_E is through the gills and U_E through the kidney], and the proportion of metabolizable energy (M_E) for recovered energy (R_E) were significantly lower in the transgenics than in the controls. The average energy budget equation of transgenic fish was as follows: 100 I_E= 19·3 F_E+ 6·0 (Z_E+U_E) + 45·2 H_E+ 29·5 R_E or 100 M_E= 60·5 H_E+ 39·5 R_E. The average energy budget equation of the controls was: 100 I_E= 25·2 F_E+ 7·4 (Z_E+U_E) + 35·5 H_E+ 31·9 R_E or 100 M_E= 52·7 H_E+ 47·3 R_E. These findings indicate that the high growth rate of ‘all‐fish’ transgenic common carp relative to their non‐transgenic counterparts was due to their increased feed intake, reduced lose of waste productions and improved feed efficiency. The benefit of the increased energy intake by transgenic fish, however, was diminished by their increased metabolism.     

Ventilatory responses to exercise and carbon dioxide in elderly and younger humans

The present investigation examined the relationship between CO₂ sensitivity [at rest (S _R) and during exercise (S _E)] and the ventilatory response to exercise in ten elderly (61–79 years) and ten younger (17–26 years) subjects. The gradient of the relationship between minute ventilation and CO₂ production ( _E/ CO₂) of the elderly subjects was greater than that of the younger subjects [mean (SEM); 32.8 (1.6) vs 27.3 (0.4); P<0.01]. At rest, S _R was lower for the elderly than for the younger group [10.77 (1.72) vs 16.95 (2.13) 1 · min^–1 · kPa^–1; 1.44 (0.23) vs 2.26 (0.28) 1 · min^–1 · mmHg^–1; P<0.05], but S _E was not significantly different between the two groups [17.85 (2.49) vs 19.17 (1.62) l · min^–1 · kPa^–1; 2.38 (0.33) vs 2.56 (0.21) 1 · min^–1 · mmHg^–1]. There were significant correlations between both S _R and S _E, and _E/ CO₂ (P<0.05; P<0.001) for the younger group, bot none for the elderly. The absence of a correlation for the elderly supports the suggestion that _E/ CO₂ is not an appropriate index of the ventilatory response to exercise for elderly humans.     

The effect of trait type and strength of selection on heritability and evolvability in an island bird population

The heritability (h²) of fitness traits is often low. Although this has been attributed to directional selection having eroded genetic variation in direct proportion to the strength of selection, heritability does not necessarily reflect a trait's additive genetic variance and evolutionary potential (“evolvability”). Recent studies suggest that the low h² of fitness traits in wild populations is caused not by a paucity of additive genetic variance (V_A) but by greater environmental or nonadditive genetic variance (V_R). We examined the relationship between h² and variance‐standardized selection intensities (i or β_σ), and between evolvability (I_A:V_A divided by squared phenotypic trait mean) and mean‐standardized selection gradients (β_μ). Using 24 years of data from an island population of Savannah sparrows, we show that, across diverse traits, h² declines with the strength of selection, whereas I_A and I_R (V_R divided by squared trait mean) are independent of the strength of selection. Within trait types (morphological, reproductive, life‐history), h², I_A, and I_R are all independent of the strength of selection. This indicates that certain traits have low heritability because of increased residual variance due to the age at which they are expressed or the multiple factors influencing their expression, rather than their association with fitness.     

Gill (Na+,K+)-ATPase from the blue crab Callinectes danae: modulation of K+-phosphatase activity by potassium and ammonium ions

The kinetic properties of a microsomal gill (Na⁺,K⁺)-ATPase from the blue crab Callinectes danae were analyzed using the substrate p-nitrophenylphosphate. The (Na⁺,K⁺)-ATPase hydrolyzed PNPP obeying cooperative kinetics (n=1.5) at a rate of V=125.4±7.5 U mg⁻¹ with K_0.5=1.2±0.1 mmol l⁻¹; stimulation by potassium (V=121.0±6.1 U mg⁻¹; K_0.5=2.1±0.1 mmol l⁻¹) and magnesium ions (V=125.3±6.3 U mg⁻¹; K_0.5=1.0±0.1 mmol l⁻¹) was cooperative. Ammonium ions also stimulated the enzyme through site–site interactions (n_H=2.7) to a rate of V=126.1±4.8 U mg⁻¹ with K_0.5=13.7±0.5 mmol l⁻¹. However, K⁺-phosphatase activity was not stimulated further by K⁺ plus NH₄⁺ ions. Sodium ions (K_I=36.7±1.7 mmol l⁻¹), ouabain (K_I=830.3±42.5 μmol l⁻¹) and orthovanadate (K_I=34.0±1.4 nmol l⁻¹) completely inhibited K⁺-phosphatase activity. The competitive inhibition by ATP (K_I=57.2±2.6 μmol l⁻¹) of PNPPase activity suggests that both substrates are hydrolyzed at the same site on the enzyme. These data reveal that the K⁺-phosphatase activity corresponds strictly to a (Na⁺,K⁺)-ATPase in C. danae gill tissue. This is the first known kinetic characterization of K⁺-phosphatase activity in the portunid crab C. danae and should provide a useful tool for comparative studies.     

Effect of ionic liquid [BMIM][PF<Subscript>6</Subscript>] on asymmetric reduction of ethyl 2-oxo-4-phenylbutyrate by <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis>

The effect of ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF₆]) on the asymmetric reduction of ethyl 2-oxo-4-phenylbutyrate (EOPB) to synthesize optical active ethyl 2-hydroxy-4-phenylbutyrate (EHPB) catalyzed by Saccharomyces cerevisiae was investigated. (R)-EHPB [70.4%, e.e.(R)] is obtained using ethyl ether or benzene as the solvent. The main product is (S)-EHPB [27.7%, e.e.(S)] in [BMIM][PF₆]. However, in ionic liquid-water (10:1, v/v) biphasic system, the enantioselectivity of the reduction is shifted towards (R)-side, and e.e.(R) is increased from 6.6 to 82.5% with the addition of ethanol (1%, v/v). The effect of the use of [BMIM][PF₆] as an additive in relatively small amounts on the reduction was also studied. We find that there is a decline in the enantioselectivity of the reduction in benzene. In addition, a decrease in the conversion of EOPB and the yield of EHPB with increasing [BMIM][PF₆] concentrations occurs in either organic solvent–water biphasic systems or benzene.     

Roles of external and cellular Cl− ions on the activation of an apical electrodiffusional Cl− pathway in toad skin

Summary This study is concerned with the short-circuit current,I _sc, responses of the Cl^–-transporting cells of toad skin submitted to sudden changes of the external Cl^– concentration. [Cl]₀. Sudden changes of [Cl]₀, carried out under apical membrane depolarization, allowed comparison of the roles of [Cl]₀ and [Cl]_cell on the activation of the apical Cl^– pathways. Equilibration of shortcircuited skins symmetrically in K-Ringer's solutions of different Cl^– concentrations permitted adjustment of [Cl]_cell to different levels. For a given Cl^– concentration (in the range of 11.7 to 117mm) on both sides of a depolarized apical membrane, this structure exhibits a high Cl^– permeability,P _(Cl)apical. On the other hand, for the same range of [Cl]_cell but with [Cl]₀=0,P _(Cl)apical is reduced to negligible values. These observations indicate that when the apical membrane is depolarizedP _(Cl)apical is modulated by [Cl]₀; in the absence of external Cl^– ions, intracellular Cl^– is not sufficient to activateP _(Cl)apical. Computer simulation shows that the fast Cl^– currents induced across the apical membrane by sudden shifts of [Cl]₀ from a control equilibrium value strictly follow the laws of electrodiffusion. For each experimental group, the computer-generatedI _sc versus ([Cl]_cell–[Cl]₀) curve which best fits the experimental data can only be obtained by a unique pair ofP _(Cl)apical andR _b (resistance of the basolateral membrane), thus allowing the calculation of these parameters. The electrodiffusional behavior of the net Cl^– flux across the apical membrane supports the channel nature of the apical Cl^– pathways in the Cl^–-transporting cell. Cl^– ions contribute significantly to the overall conductance of the basolateral membrane even in the presence of a high K concentration in the internal solution.     

Computational study of enhanced excitability in Hermissenda: membrane conductances modulated by 5-HT

Serotonin (5-HT) applied to the exposed but otherwise intact nervous system results in enhanced excitability of Hermissenda type-B photoreceptors. Several ion currents in the type-B photoreceptors are modulated by 5-HT, including the A-type K⁺ current (I_K,A), sustained Ca²⁺ current (I_Ca,S), Ca-dependent K⁺ current (I_K,Ca), and a hyperpolarization-activated inward rectifier current (I_h). In this study, we developed a computational model that reproduces physiological characteristics of type B photoreceptors, e.g. resting membrane potential, dark-adapted spike activity, spike width, and the amplitude difference between somatic and axonal spikes. We then used the model to investigate the contribution of different ion currents modulated by 5-HT to the magnitudes of enhanced excitability produced by 5-HT. Ion currents were systematically varied within limits observed experimentally, both individually and in combinations. A reduction of I_K,A or I_K,Ca, or an increase in I_h enhanced excitability by 20–50%. Decreasing I_Ca,S produced a dramatic decrease in excitability. Reductions of I_K,V produced only minimal increases in excitability, suggesting that I_K,V probably plays a minor role in 5-HT induced enhanced excitability. Combinations of changes in I_K,A, I_K,Ca, I_h and I_Ca,S produced increases in excitability comparable to experimental observations. After 5-HT application, the cell's depolarization force is shifted from the I_h–I_Ca,S combination to predominantly I_h.     

Mechanism of stimulation by epinephrine of active transepithelial Cl transport in isolated frog cornea

Summary In the isolated frog cornea, the effects of 0.1mm epinephrine were measured on both the transepithelial and intracellular electrical parameters. Epinephrine increased the short-circuit current (I _sc) and transepithelial electrical conductance (g _t) by 176 and 96%, respectively. The effective electromotive driving force for active transepithelial Cl transport (E _Cl) was 45 mV and agrees with the value forE _Cl calculated by a different technique in the isolated rabbit corneal epithelium (Klyce, S.D., Wong, R.K.S., 1977,J. Physiol. (London) 266: 777). With respect to the tear-side bathing solution, epinephrine caused the intracellular potential difference of shortcircuited frog corneas to decrease from –54 to –50 mV (P>0.05). The fractional resistance of the apical membrane {F(R _o)=(R_o/R_o+R_i)} whereR _o andR _i represent the resistances of the apical and basolateral membranes, respectively, decreased from 0.38±0.06 to 0.23±0.03. Using these values ofF(R _o) and the cellular conductances, the calculated Cl resistances ofR _o andR _i decreased 4.3- and 2.3-fold, respectively. However, the value forE _Cl calculated from the intracellular electrical measurements (48 mV) did not appear to change since this value was in close agreement with the value forE _Cl calculated from the effects of epinephrine on the transepithelial electrical parameters. Thus, the effects of epinephrine onI _sc andg _t can be accounted for by increases in the Cl conductance of both the apical and basolateral membranes. Epinephrine caused the potential difference across the basolateral membrane to hyperpolarize by 9 mV. All of these results are consistent with the notion that the steps in transepithelial Cl transport include uphill movement into the cell across the basolateral membrane followed by downhill movement across the apical membrane into the tear-side bathing solution.     

Developmental changes in the calcium currents in embryonic chick ventricular myocytes

Summary Using the patch-clamp technique, we recorded whole-cell calcium current from isolated cardiac myocytes dissociated from the apical ventricles of 7-day and 14-day chick embryos. In 70% of 14-day cells after 24 hr in culture, two component currents could be separated from totalI _Ca activated from a holding potential (V _h) of –80 mV. L-type current (I _L) was activated by depolarizing steps fromV _h –30 or –40 mV. The difference current (I _T) was obtained by subtractingI _L, fromI _Ca.I _T could also be distinguished pharmacologically fromI _L in these cells.I _T was selectively blocked by 40–160 m Ni²⁺, whereasI _L was suppressed by 1 m D600 or 2 m nifedipine. The Ni²⁺-resistant and D600-resistant currents had activation thresholds and peak voltages that were near those ofI _T andI _L defined by voltage threshold, and resembled those in adult mammalian heart. In 7-day cells,I _T andI _L could be distinguished by voltage threshold in 45% (S cells), while an additional 45% of 7-day cells were nonseparable (NS) by activation voltage threshold. Nonetheless, in mostNS cells,I _Ca was partly blocked by Ni²⁺ and by D600 given separately, and the effects were additive when these agents were given together. Differences among the cells in the ability to separateI _T andI _L by voltage threshold resulted largely from differences in the position of the steady-state inactivation and activation curves along the voltage axis. In all cells at both ages in which the steady-state inactivation relation was determined with a double-pulse protocol, the half-inactivation potential (V _1/2) of the Ni²⁺-resistant currentI _L averaged –18 mV. In contrast,V _1/2 of the Ni²⁺-sensitiveI _T was –60 mV in 14-day cells, –52 mV in 7-dayS cells, and –43 mV in 7-day NS cells. The half-activation potential was near –2 mV forI _L at both ages, but that ofI _T was –38 mV in 14-day and –29 mV in 7-day cells. Maximal current density was highly variable from cell to cell, but showed no systematic differences between 7-day and 14-day cells. These results indicate that the main developmental change that occurs in the components ofI _Ca is a negative shift with, embryonic age in the activation and inactivation relationships ofI _T along the voltage axis.