UV Effects in Aquatic Organisms and Ecosystems. Edited by Helbling, E. W. and Zagarese, H. (2003) The Royal Society of Chemistry, Springer Verlag, Cambridge, UK. $290.00. ISBN 0-854-04301-2.

All Earthly events can be eventually blamed on the Sun. Is itnot? There is no doubt that solar radiation is a fundamental forcein nature. At least two earlier books have dealt with the premisethat solar ultraviolet radiation (UVR) penetrating our wateryplanet profoundly impacts both the living as well as the non-livingcomponents (Calkins, 1982; De Mora et al., 2000). Today, inan     

Modulation of K+ channels by arachidonic acid in T84 cells. II. Activation of a Ca2+-independent K+ channel

We usedsingle-channel recording techniques to identify and characterize alarge-conductance,Ca²⁺-independentK⁺ channel in the colonicsecretory cell line T84. In symmetric potassium gluconate, this channelhad a linear current-voltage relationship with a single-channelconductance of 161 pS. Channel open probability(P_o) wasincreased at depolarizing potentials. Partial substitution of bathK⁺ withNa⁺ indicated a permeability ratioof K⁺ toNa⁺ of 25:1. ChannelP_o was reduced byextracellular Ba²⁺. Event-durationanalysis suggested a linear kinetic model for channel gating having asingle open state and three closed states: C₃C₂C₁O.Arachidonic acid (AA) increased theP_o of thechannel, with an apparent stimulatory constant(K_s)of 1.39 µM. Neither channel open time (O) nor the fast closed time(C₁) was affected by AA. Incontrast, AA dramatically reduced mean closed time by decreasing bothC₃ andC₂. Thecis-unsaturated fatty acid linoleate increased P_oalso, whereas the saturated fatty acid myristate and thetrans-unsaturated fatty acid elaidatedid not affectP_o. This channelis activated also by negative pressure applied to the pipette duringinside-out recording. Thus we determined the effect of thestretch-activated channel blockers amiloride and Gd³⁺ on theK⁺ channel after activation by AA.Amiloride (2 mM) on the extracellular side reduced single-channelamplitude in a voltage-dependent manner, whereasGd³⁺ (100 µM) had no effect onchannel activity. Activation of this K⁺ channel may be important duringstimulation of Cl secretionby agonists that use AA as a second messenger (e.g., vasoactiveintestinal polypeptide, adenosine) or during the volume regulatoryresponse to cell swelling.

     

Vegetable Plant Part Relationships. II. A Quantitative Hypothesis for Shoot/Storage Root Development

A simple quantitative formulation of the concept of the controlof partitioning of assimilated carbon by the behaviour of plantcomponents as competing sinks is developed. An equation, In s = + In r—t, relating shoot (s) and storage root (r) d. wts, and the lengthof growth period (t), is constructed by considering possiblefates of imported assimilates into different plant parts. Thevalues of the equations' parameters depend on the relative sinkactivities of the plant parts, tissue respiration rates andinitial weights of plant components. The equation closely fitteddata collected from a number of carrot and beet experimentsin which planting density had been varied. Estimates of shootand storage root maintenance respiration rates, derived fromthe parameter , were of the correct order of magnitude. Othersets of experimental data are also discussed in the light ofpredictions of the theory and possible uses and extensions ofthis approach to assimilate partitioning are briefly discussed. Daucus carota L., Beta vulgaris, carrot, red beet, partition of assimilated carbon, maintenance respiration, storage root     

Contractile Proteins of a Benthic Fish.I. Effects of Temperature and Pressure on Myosin ATPase.

SYNOPSIS. Studies are described on the adenosine triphosphatase(ATPase) properties of myosin isolated from skeletal muscleof Coryphaenoides, a benthic fish captured at 2,200 meters depth.Ca²⁺-ATPase and EDTA-ATPase of Coryphaenoides myosin show thesame pH dependence as ATPase of mammalian myosin; however, ratesof ATP hydrolysis by Coryphaenoides myosin are only 5–10%of rates of ATP hydrolysis by rabbit skeletal myosin. Coryphaenoidesmyosin ATPase shows a decrease from Q₁₀ of 2.0 at 25°C toQ₁₀ of 1.4 a t 2°C, and undergoes irreversible denaturationat temperatures above 25°C. At pH 6.8 to pH 8.5, Coryphaenoidesmyosin ATPase undergoes activation by pressure at 25°C,but at 2°C shows negligible effect of pressure at valuesbelow 3,000 psi. The kinetic data on Ca²⁺-ATPase indicate valuesof 11 kcal/mole for H, –7.5 kcal/mole for TS, and –5.7cc/mole for V at 25°C, pH 7.6. Comparable data at 2°Cindicate values of 5 kcal/mole for H. –13 kcal/mole forTS, and negligible V. According to the results of 25°C,Ca²⁺-activatkm of myosin-ATP may involve disruption of fouror five hydrophobic or polar groups, presumably due to an "opening-up"of the myosin molecule at or near the site for ATP binding.It would also appear that Coryphaenoides myosin has undergonean adaptive change in the enzyme mechanism for ATPase such thatthe rate of ATP hydrolysis is relatively insensitive to pressureand temperature under conditions encountered by the living fish.     

Mechanisms of Inhibition of Water Movement in Anaerobically Treated Roots of Zea mays L.

Changes in water flux (Jv) across detopped, 7-d-old, maize rootswere characterized during the initial 24 h of being made anoxicby exposure to an anaerobic nutrient solution. Suction (50 kPa)was applied to the xylem and samples of the xylem sap were collectedat intervals and the osmolality and ionic content were measured. Values of Jv through anoxic roots fell below those of aerobiccontrols 1 h after the equilibrium oxygen partial pressure inthe bathing medium dropped below 20 kPa (air = 20.6 kPa). Thereduction in Jv was due primarily to a nullification of thediurnal rhythm in hydraulic conductivity (Lp) that was measuredin aerobic roots. However, about one-quarter of the reductionin Jv could be accounted for by a smaller osmotic componentof the driving force () on water movement. The significance of changes in Jv in anoxic roots is discussedin terms of the reliability of estimates of Lp, the reflectioncoefficient () and . Key words: Anaerobiosis, hydraulic conductivity, osmotic potential, water     

Protein kinase C{varepsilon} contributes to regulation of the sarcolemmal Na+-K+ pump

A reduction in angiotensinII (ANG II) in vivo by treatment of rabbits with theangiotensin-converting enzyme inhibitor, captopril, increasesNa⁺-K⁺ pump current (I_p)of cardiac myocytes. This increase is abolished by exposure of myocytesto ANG II in vitro. Because ANG II induces translocation of the-isoform of protein kinase C (PKC), we examined whether thisisozyme regulates the pump. We treated rabbits with captopril, isolatedmyocytes, and measured I_p of myocytes voltageclamped with wide-tipped patch pipettes. I_p ofmyocytes from captopril-treated rabbits was larger thanI_p of myocytes from controls. ANG II superfusionof myocytes from captopril-treated rabbits decreasedI_p to levels similar to controls. Inclusion ofPKC-specific blocking peptide in pipette solutions used to perfusethe intracellular compartment abolished the effect of ANG II. Inclusionof RACK, a PKC-specific activating peptide, in pipettesolutions had an effect on I_p that was similarto that of ANG II. There was no additive effect of ANG II andRACK. We conclude that PKC regulates the sarcolemmalNa⁺-K⁺ pump.

     

Sex-ratio differences in Mnium hornum Hedw. and M. undulatum Sw. in relation to spore germination and vegetative regeneration

As Mnium undulatum was shown to be homosporous, it was concludedthat neither male nor female derived an advantage from sporesize that might be related to the observed excess of femaleplants. The rate of germination was greater at 20 °C thanat 10 °C in M. hornum and M. undulatum, and was also reducedin short days (7.25 hours) at both temperatures. Spores of M.undulatum germinated more slowly than those of M. hornum undereach of the environmental regimes used. Isolated spores of M.undulatum showed a ratio of 1: 4.1 compared with 1: 0.89 inM. hornum. The excess of female plants of M. undulatum thathad been established by the end of germination, was maintainedamongst the first protonemal buds produced (1: 3.5), whereasan excess of male M. hornum was observed in the first protonemalbuds (1: 0.45). Frost reduced the rate of germination in M.undulatum, but unlike desiccation did not affect the final percentage.Male and female were amongst the spores which survived desiccationat 10 °C. Regeneration of detached leaves occurred more rapidly in M.undulatum than in M. hornum, and no difference between maleand female was detected. It was found that frost prior to orduring regeneration did not produce long-term harmful effectsin M. undulatum. None of the young male gametophytes producedby regeneration from leaves survived desiccation, compared with77 per cent of similarly produced female gametophytes.     

Quantitative analysis of transpleural flux in the isolated lung

Li, M. H., J. Hildebrandt, and M. P. Hlastala.Quantitative analysis of transpleural flux in the isolated lung.J. Appl. Physiol. 82(2): 545-551, 1997.In this study, the loss of inert gas through the pleura of anisolated ventilated and perfused rabbit lung was assessed theoreticallyand experimentally. A mathematical model was used to represent an idealhomogeneous lung placed within a box with gas flow(box) surrounding the lung. Thealveoli are assumed to be ventilated with room air(A) andperfused at constant flow () containinginert gases (x) with various perfusate-air partition coefficients(_p,x).The ratio of transpleural flux of gas(pl_x)to its total delivery to the lung via pulmonary artery( ),representing fractional losses across the pleura, can be shown todepend on four dimensionless ratios:1)_p,x,2) the ratio of alveolar ventilation to perfusion(A/), 3) the ratioof the pleural diffusing capacity(Dpl_x) to the conductance ofthe alveolar ventilation (Dpl_x /A_g,where _g is the capacitancecoefficient of gas), and 4) theratio of extrapleural (box) ventilation to alveolar ventilation(box/A).Experiments were performed in isolated perfused and ventilated rabbitlungs. The perfusate was a buffer solution containing six dissolvedinert gases covering the entire 10⁵-fold range of_p,x usedin the multiple inert gas elimination technique. Steady-state inert gasconcentrations were measured in the pulmonary arterial perfusate,pulmonary venous effluent, exhaled gas, and box effluent gas. Theexperimental data could be described satisfactorily by thesingle-compartment model. It is concluded that a simple theoreticalmodel is a useful tool for predicting transpleural flux from isolatedlung preparations, with known ventilation and perfusion, for inertgases within a wide range of .

     

Enhancement of L-type Ca(2+) current from neonatal mouse ventricular myocytes by constitutively active PKC-betaII

The cardiacL-type calcium current (I_Ca) can be modified byactivation of protein kinase C (PKC). However, the effect of PKC activation on I_Ca is still controversial. Somestudies have shown a decrease in current, whereas other studies havereported a biphasic effect (an increase followed by a decrease incurrent or vice versa). A possible explanation for the conflictingresults is that several isoforms of PKC with opposing effects onI_Ca were activated simultaneously. Here, weexamined the influence of a single PKC isoform (PKC-II) on L-typecalcium channels in isolation from other cardiac isoforms, using atransgenic mouse that conditionally expresses PKC-II. Ventricularcardiac myocytes were isolated from newborn mice and examined forexpression of the transgene using single cell RT-PCR afterI_Ca recording. Cells expressing PKC-II showeda twofold increase in nifedipine-sensitive I_Ca. The PKC-II antagonist LY-379196 returned I_Caamplitude to levels found in non-PKC-II-expressing myocytes. Theincrease in I_Ca was independent ofCa_v1.2-subunit mRNA levels as determined by quantitativeRT-PCR. Thus these data demonstrate that PKC- is a potent modulatorof cardiac L-type calcium channels and that this specific isoformincreases I_Ca in neonatal ventricular myocytes.

     

Shear stress induces a time- and position-dependent increase in endothelial cell membrane fluidity

Blood flow-associatedshear stress may modulate cellular processes through its action on theplasma membrane. We quantified the spatial and temporal aspects of theeffects of shear stress () on the lipid fluidity of1,1'-dihexadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate [DiIC₁₆(13)]-stained plasma membranesof bovine aortic endothelial cells in a flow chamber. A confocalmicroscope was used to determine the DiI diffusion coefficient(D) by fluorescence recovery after photobleaching on cellsunder static conditions, after a step- of 10 or 20 dyn/cm², and after the cessation of . The methodallowed the measurements of D on the upstream and downstreamsides of the cell taken midway between the respective cell borders andthe nucleus. In <10 s after a step- of 10 dyn/cm²,D showed an upstream increase and a downstream decrease, and both changes disappeared rapidly. There was a secondary, larger increase in upstream D, which reached a peak at 7 min and decreased thereafter, despite the maintenance of .D returned to near control values within 5 s aftercessation of . Downstream D showed little secondarychanges throughout the 10-min shearing, as well as after its cessation.Further investigations into the early phase, with simultaneousmeasurements of upstream and downstream D, confirmed that astep- of 10 dyn/cm² elicited a rapid (5-s) but transientincrease in upstream D and a concurrent decrease indownstream D, yielding a significant difference between thetwo sites. A step- of 20 dyn/cm² caused D toincrease at both sites at 5 s, but by 30 s and 1 min theupstream D became significantly higher than the downstream D. These results demonstrate shear-induced changes inmembrane fluidity that are time dependent and spatially heterogeneous. These changes in membrane fluidity may have important implications inshear-induced membrane protein modulation.

     

Mechanical and metabolic determination of VO2 and fatigue during repetitive isometric contractions in situ

Repetitiveisometric tetanic contractions (1/s) of the caninegastrocnemius-plantaris muscle were studied either at optimal length(L_o) or shortlength (L_s;~0.9 · L_o),to determine the effects of initial length on mechanical and metabolicperformance in situ. Respective averages of mechanical and metabolicvariables were(L_o vs.L_s, allP < 0.05) passive tension (preload) = 55 vs. 6 g/g, maximal active tetanic tension(P_o) = 544 vs. 174 (0.38 · P_o)g/g, maximal blood flow () = 2.0 vs. 1.4 ml · min¹ · g¹,and maximal oxygen uptake(O₂) = 12 vs. 9 µmol · min¹ · g¹.Tension at L_odecreased to0.64 · P_o over20 min of repetitive contractions, demonstrating fatigue; there were nosignificant changes in tension atL_s. In separatemuscles contracting atL_o, was set to that measured atL_s (1.1 ml · min¹ · g¹),resulting in decreased O₂(7 µmol · min¹ · g¹),and rapid fatigue, to0.44 · P_o. Thesedata demonstrate that 1)muscles at L_ohave higher andO₂ values than those at L_s;2) fatigue occurs atL_o with highO₂, adjusting metabolic demand (tension output) to match supply; and3) the lack of fatigue atL_s with lowertension, , andO₂ suggestsadequate matching of metabolic demand, set low by shortmuscle length, with supply optimized by low preload. Thesedifferences in tension andO₂ betweenL_o andL_s groupsindicate that muscles contracting isometrically at initial lengthsshorter than L_oare working under submaximal conditions.

     

亮叶中南鱼藤的杀虫活性及有效成分

研究了亮叶中南鱼藤Derris fordii var. lucida的杀虫活性及有效成分。通过生物测定确定了该植物提取物对几种害虫的杀虫活性及其作用方式,并在活性跟踪的基础上,通过萃取、柱层析、薄层制备、重结晶、核磁共振和质谱等方法对其有效成分进行了分离和鉴定。结果表明,亮叶中南鱼藤不同部位甲醇提取物中仅根部提取物表现出杀虫活性。根甲醇提取物对白纹伊蚊Aedes albopictus 4龄幼虫、棉蚜Aphis gossypii Glover 无翅成蚜、豆蚜Aphis craccivora Koch无翅成蚜、桃蚜Myzus persicae (Sulzer)无翅成蚜、甘薯天蛾Herse convolvuli L. 2龄幼虫、三化螟Scirpophaga incertulas (Walker)初孵幼虫、菜青虫Pieris rapae (L.) 2龄幼虫和黄曲条跳甲Phyllotreta striolata (Fabricius)成虫都有毒杀效果,其LC₅₀值分别是260.3 mg/L、234.6 mg/L、141.3 mg/L、16.4 mg/L、233.4 mg/L、20.8 mg/L、11.7 mg/L和148.4 mg/L。根甲醇提取物对甘薯天蛾3龄幼虫24 h的触杀和胃毒毒力LC₅₀值分别为101.6 mg/L和234.9 mg/L。此外,亚致死剂量的根甲醇提取物对甘薯天蛾3龄幼虫还有拒食和抑制生长发育的作用。从根中分离和鉴定了3个化合物,即鱼藤酮、6a,12a-脱氢鱼藤素和β-谷甾醇。鱼藤酮和6a,12a-脱氢鱼藤素对三化螟初孵幼虫表现出毒杀活性,24 h的LC₅₀值分别是2.6 mg/L和5.3 mg/L。结论为：亮叶中南鱼藤仅根为活性部位,该甲醇提取物对棉蚜等多种害虫有活性,其主要作用方式为触杀和胃毒,鱼藤酮和6a,12a-脱氢鱼藤素是根中的主要杀虫活性成分。     

Aerosolized manganese SOD decreases hyperoxic pulmonary injury in primates. II. Morphometric analysis

Welty-Wolf, Karen E., Steven G. Simonson, Yuh-Chin T. Huang,Stephen P. Kantrow, Martha S. Carraway, Ling-Yi Chang, James D. Crapo, and Claude A. Piantadosi. Aerosolizedmanganese SOD decreases hyperoxic pulmonary injury in primates. II.Morphometric analysis. J. Appl.Physiol. 83(2): 559-568, 1997.Hyperoxia damages lung parenchyma via increased cellular production of reactive oxygenspecies that exceeds antioxidant defenses. We hypothesized thataerosolized human recombinant manganese superoxide dismutase (rhMnSOD)would augment extracellular antioxidant defenses and attenuateepithelial injury in the lung during hyperoxia in primates. Twenty-fouradult male baboons were anesthetized and mechanically ventilated with100% oxygen for 96 h. The baboons were divided equally into fourgroups. Oxygen alone and oxygen plus rhMnSOD given at 3 mg · kg¹ · day¹were compared to assess efficacy of the drug. Subsequently, aerosolized rhMnSOD was given at 1 or 10 mg · kg¹ · day¹to study dose effects and toxicity. Quantitative morphometry showedprotection of alveolar epithelium from hyperoxia by 3 mg · kg¹ · day¹rhMnSOD (P < 0.05). In addition,interstitial fibroblast volumes were increased in the treatment group(P = 0.06). This effect appearedgreater at the two higher doses of the rhMnSOD. The aerosolized drugwas localized to the surface of airways and air spaces and macrophagesby immunolabeling studies, suggesting efficacy via physicochemicalproperties that localize it to cell surfaces or by effects on alveolarmacrophage function.

     

NO does not mediate inhibitory neural responses in sheep airway and bronchial vascular smooth muscle

Endogenous nitric oxide (NO) influences acetylcholine-inducedbronchovascular dilation in sheep and is a mediator of the airway smooth muscle inhibitory nonadrenergic, noncholinergic neural responsein several species. This study was designed to determine the importanceof NO as a neurally derived modulator of ovine airway and bronchialvascular smooth muscle. We measured the response of pulmonaryresistance (RL) and bronchialblood flow (br) to vagal stimulationin 14 anesthetized, ventilated, open-chest sheep duringthe following conditions: 1)control; 2) infusion of the -agonist phenylephrine to reduce baseline br bythe same amount as would be produced by infusion ofN-nitro-L-arginine(L-NNA), a NO synthaseinhibitor; 3) infusion ofL-NNA(10² M); and4) after administration of atropine(1.5 mg/kg). The results showed that vagal stimulation produced anincrease in RL andbr in periods 1, 2, and 3 (P < 0.01) that was not affected byL-NNA. Afteratropine was administered, there was no increase inbr or RL. Invitro experiments on trachealis smooth muscle contracted with carbachol showed no effect ofL-NNA on neural relaxation butshowed a complete blockade with propranolol(P < 0.01). In conclusion, thevagally induced airway smooth muscle contraction and bronchial vasculardilation are not influenced by NO, and the sheep's trachealis muscle,unlike that in several other species, does not have inhibitorynonadrenergic, noncholinergic innervation.

     

Interaction of cross-sectional area, driving pressure, and airflow of passive velopharynx

Isono, Shiroh, Thom R. Feroah, Eric A. Hajduk, Rollin Brant,William A. Whitelaw, and John E. Remmers. Interaction ofcross-sectional area, driving pressure, and airflow of passive velopharynx. J. Appl. Physiol. 83(3):851-859, 1997.Previous studies have shown that, when thepharyngeal muscles are relaxed, the velopharynx is a highly compliantsegment of the pharynx. Thus, under these circumstances,cross-sectional area of the velopharynx (A_VP), drivingpressure across the velopharynx (P), and inspiratory airflow(I) willbe mutually interdependent variables. The purpose of the presentinvestigation was to describe the interrelation among these threevariables during inspiration. We studied 15 sleeping patients withobstructive sleep apnea/hypopnea when the pharyngeal muscles wererendered hypotonic by applying continuous positive airway pressure tothe nasal airway.A_VP, determined by endoscopic imaging, was significantly greater at onset ofI limitationthan at minimum oropharyngeal pressure(P < 0.01). Snoring was neverobserved duringIlimitation. In a subgroup of six patients, values for P,I, andA_VP were obtainedat 0.1-s intervals at various levels of mask pressure. For these sixpatients, the mathematical expressionI = 0.657(A_VP/A_max) · P^0.332,where A_max ismaximal A_VP,described the relationship among the three variables(R² = 0.962) forflow-limited and non-flow-limited inspirations. The impedance of thepassive velopharynx, defined asP^0.33/,was inversely related toA_VP and increaseddramatically when A_VP was <0.3cm². In summary, we observed aprogressive decrease inA_VP during flow-limited inspiration in patients with obstructive sleep apnea. Thisconstriction of the velopharynx contributes to an increase invelopharyngeal impedance that, in turn, counterbalances the increase inP during flow limitation.

     

Synchrony, asynchrony, and temporally random mating: a new method for analyzing breeding synchrony

Much research has focused on understanding breeding synchronyin animals and its relationship to such issues as mating systemsand extrapair copulations in birds (Birkhead and Biggins, 1987;Emlen and Oring, 1977; Knowlton, 1979; Stutchbury and Morton,1995). To empirically examine synchrony, one needs an appropriatemeasure of the degree of synchrony in a population. Kempenaers(1993) presented an index of breeding synchrony (modified fromBjörklund and Westman, 1986) that has gained wide use asa simple representation of the degree to which breeding is synchronizedamong animals. This synchrony index (SI) is calculated as follows:

where F is the totalnumber of breeding females in the population; f_i,p is the numberof fertile females, excluding     

Role of gelsolin in the actin filament regulation of cardiac L-type calcium channels

The actin cytoskeleton is an important contributor to themodulation of the cell function. However, little is known about theregulatory role of this supermolecular structure in the membrane eventsthat take place in the heart. In this report, the regulation of cardiacmyocyte function by actin filament organization was investigated inneonatal mouse cardiac myocytes (NMCM) from both wild-type mice andmice genetically devoid of the actin filament severing protein gelsolin(Gsn/). Cardiac L-type calcium channel currents(I_Ca) wereassessed using the whole cell voltage-clamp technique. Addition of theactin filament stabilizer phalloidin to wild-type NMCM increasedI_Ca by 227% overcontrol conditions. The basalI_Ca ofGsn/ NMCM was 300% higher than wild-type controls. Thisincrease was completely reversed by intracellular perfusion of theGsn/ NMCM with exogenous gelsolin. Further, cytoskeletal disruption of either Gsn/ or phalloidin-dialyzedwild-type NMCM with cytochalasin D (CD) decreased the enhancedI_Ca by 84% and 87%, respectively. The data indicate that actin filament stabilization by either a lack of gelsolin or intracellular dialysis with phalloidin increase I_Ca,whereas actin filament disruption with CD or dialysis ofGsn/ NMCM with gelsolin decreaseI_Ca. We concludethat cardiac L-type calcium channel regulation is tightly controlled byactin filament organization. Actin filament rearrangement mediated by gelsolin may contribute to calcium channel inactivation.

     

Calcium dependence of C-type natriuretic peptide-formed fast K+ channel

The lipid bilayertechnique was used to characterize theCa²⁺ dependence of a fastK⁺ channel formed by a synthetic17-amino acid segment [OaCNP-39-(1-17)] ofa 39-amino acid C-type natriuretic peptide (OaCNP-39) found in platypus (Ornithorhynchusanatinus) venom (OaV). TheOaCNP-39-(1-17)-formed K⁺ channel was reversiblydependent on1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-buffered cis (cytoplasmic)Ca²⁺ concentration([Ca²⁺]_cis).The channel was fully active when[Ca²⁺]_ciswas >10⁴ M andtrans (luminal)Ca²⁺ concentration was 1.0 mM, butnot at low[Ca²⁺]_cis.The open probability of single channels increased from zero at1 × 10⁶ McisCa²⁺ to 0.73 ± 0.17 (n = 22) at10³ McisCa²⁺. Channel openings to themaximum conductance of 38 pS were rapidly and reversibly activated when[Ca²⁺]_cis,but not transCa²⁺ concentration(n = 5), was increased to >5 × 10⁴ M(n = 14). Channel openings to thesubmaximal conductance of 10.5 pS were dominant at5 × 10⁴ MCa²⁺.K⁺ channels did not open whencisMg²⁺ orSr²⁺ concentrations were increasedfrom zero to 10³ M or when[Ca²⁺]_ciswas maintained at 10⁶ M(n = 3 and 2). The Hill coefficientand the inhibition constant were 1 and 0.8 × 10⁴ McisCa²⁺, respectively. Thisdependence of the channel on high[Ca²⁺]_cissuggests that it may become active under1) physiological conditions whereCa²⁺ levels are high, e.g., duringcardiac and skeletal muscle contractions, and2) pathological conditions that leadto a Ca²⁺ overload, e.g., ischemicheart and muscle fatigue. The channel could modify a cascade ofphysiological functions that are dependent on theCa²⁺-activatedK⁺ channels, e.g., vasodilationand salt secretion.

     

Phosphate and Membrane Electropotentials in Trifolium repens L.

In Trifolium repens L. there were immediate transient depolarizationsof the membrane electropotential (E_vo) when KH₂PO₄ was addedto phosphate-free media, but these were of the same magnitudeas the controls (K²SO⁴ and KCI). Furthermore, the extents ofdepolarization were the same as the expected effect of the addedK⁺ calculated using the Goldman equation. There was no significantdepolarization on adding H₃PO₄ to buffered media. Consequently,there was no evidence for a depolarization caused by phosphate.This result provides evidence that the H⁺–H₂PO₄ symportin roots of T. repens operates with a stoichiometry of 1: 1. In a group of control plants ( + P plants) and a group whichwere stressed by reducing the supply of phosphate (– Pplants), the – P plants had lower values for E_vo than+P plants (– 118 mV and – 130 mV, respectively).The absence of phosphate from the measurement media also reducedE_vo (mean effect = 9 mV). A significant difference in E_vo between– P and + P plants persisted when phosphate was addedto – P plants. The electropotential difference acrossthe tonoplast (E_vo) in – P plants became more positivewith time. Key words: White clover, membrane transport, roots, tonoplast, symport     

A second enzyme protecting mineralocorticoid receptors from glucocorticoid occupancy

We have confirmed that A6 cells (derived fromkidney of Xenopus laevis), whichcontain both mineralocorticoid and glucocorticoid receptors, do notnormally possess 11-hydroxysteroid dehydroxgenase (11-HSD1 or11-HSD2) enzymatic activity and so are without apparent "protective" enzymes. A6 cells do not convert the glucocorticoid corticosterone to 11-dehydrocorticosterone but do, however, possess steroid 6-hydroxylase that transforms corticosterone to6-hydroxycorticosterone. This hydroxylase is cytochromeP-450 3A (CYP3A). We have nowdetermined the effects of 3,5-tetrahydroprogesterone andchenodeoxycholic acid (both inhibitors of 11-HSD1) and11-dehydrocorticosterone and11-hydroxy-3,5-tetrahydroprogesterone (inhibitors of11-HSD2) and carbenoxalone, which inhibits both 11-HSD1 and11-HSD2, on the actions and metabolism of corticosterone and activeNa⁺ transport [short-circuitcurrent(I_sc)] inA6 cells. All of these 11-HSD inhibitory substances induced asignificant increment in corticosterone-inducedI_sc, which wasdetectable within 2 h. However, none of these agents caused an increasein I_sc whenincubated by themselves with A6 cells. In all cases, the additionalI_sc was inhibitedby the mineralocorticoid receptor (MR) antagonist, RU-28318, whereasthe original I_scelicited by corticosterone alone was inhibited by the glucocorticoidreceptor antagonist, RU-38486. In separate experiments, each agent wasshown to significantly inhibit metabolism of corticosterone to6-hydroxycorticosterone in A6 cells, and a linear relationshipexisted between 6-hydroxylase inhibition and the MR-mediatedincrease in I_scin the one inhibitor tested. Troleandomycin, a selective inhibitor ofCYP3A, inhibited 6-hydroxylase and also significantly enhancedcorticosterone-induced I_sc at 2 h. Theseexperiments indicate that the enhanced MR-mediated I_sc in A6 cellsmay be related to inhibition of 6-hydroxylase activity in thesecells and that this 6-hydroxylase (CYP3A) may be protecting theexpression of corticosterone-induced active Na⁺ transport in A6 cells byMR-mediated mechanism(s).