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31.
Summary In the isolated bullfrog cornea, three calcium channel antagonists had dose-dependent inhibitory effects on the Cl-originated short-circuit current (SCC). Their order of decreasing potency was bepridil, verapamil and diltiazem. One millimolar diltiazem inhibited the SCC by 98% and subsequent incubation with the calcium ionophore A23187 had no restorative effect. Increasing the bathing solution Ca concentration from 0.05 to 15mm, however, decreased diltiazem's inhibitory efficacy. This antagonist depolarized the intracellular potential differenceV
m
from –54 to –18 mV (tear: reference) and the voltage divider ratioFR
0 decreased from 0.58 to 0.30, suggesting an increase in basolateral membrane electrical resistance. Additional indication of a basolateral membrane effect by the drug was that preincubation with 105
m amphotericin B in Cl-free Ringer's did not eliminate the inhibitory effect of the drug on the Na- and K-elicited SCC. In the absence of amphotericin B in Cl-free Ringer's (SCC=0), 1 ×103
m diltiazem depolarized theV
m
from –78 to –9 mV suggesting that the increase in basolateral membrane resistance was due to K channel blockade. Diltiazem (1×103
m) significantly decreased cyclic AMP content; however, isoproterenol in the presence of the drug increased cyclic AMP fourfold without having any restorative effect on the inhibited SCC. Therefore, the inhibition of the Cl-originated SCC resulting from an increase in basolateral membrane K resistance is not caused by a decline in cyclic AMP content. In plasma membrane-enriched fractions prepared from broken cell preparations of bovine corneal epithelium, 1×103
m diltiazem had no inhibitory effects on either Na,K-ATPase or Ca,Mg-ATPase activities. These latter effects further point to the selectivity of diltiazem as an inhibitor of K-channel activity, but do not preclude a Ca-channel blocker effect by the drug in the micromolar range. 相似文献
32.
DIRECT FOLIAR EFFECTS OF SIMULATED ACID RAIN 总被引:4,自引:3,他引:1
33.
(±)-Abscisic acid (ABA) at 10-5 M was added to the transpiration stream of leaves of 16 species (C3 and C4, monocotyledons and dicotyledons). Stomatal responses followed one of three patterns: i) stomata that were wide and insensitive to CO2 initially, closed partially and became sensitive to CO2; ii) for stomata that were sensitive to CO2 before the application of ABA, the range of highest sensitivity to CO2 shifted from high to low intercellular partial pressures of CO2, for instance in leaves of Zea mays from 170–350 to 70–140 bar; iii) when stomata responded strongly to ABA, their conductance was reduced to a small fraction of the initial conductance, and sensitivity to CO2 was lost. The photosynthetic apparatus was affected by applications of ABA to various degrees, from no response at all (in agreement with several previous reports on the absence of effects of ABA on photosynthesis) through a temporary decrease of its activity to a lasting reduction. Saturation curves of photosynthesis with respect to the partial pressure of CO2 in the intercellular spaces indicated that application of ABA could produce three phenomena: i) a reduction of the initial slope of the saturation curve (which indicates a diminished carboxylation efficiency); ii) a reduction of the level of the CO2-saturated rate of assimilation (which indicates a reduction of the ribulose-1,5-bisphosphate regeneration capacity); and iii) an increase of the CO2 compensation point. Photosynthesis of isolated mesophyll cells was not affected by ABA treatments. Responses of the stomatal and photosynthetic apparatus were usually synchronous and often proportional to each other, with the result that the partial pressure of CO2 in the intercellular spaces frequently remained constant in spite of large changes in conductance and assimilation rate. Guard cells and the photosynthetic apparatus were able to recover from effects of ABA applications while the ABA supply continued. Recovery was usually partial, in the case of the photosynthetic apparatus occasionally complete. Abscisic acid did not cause stomatal closure or decreases in the rate of photosynthesis when it was applied during a phase of stomatal opening and induction of photosynthesis that followed a transition from darkness to light.Abbreviations and symbols
A
rate of CO2 assimilation
- ABA
(±)-abscisic acid
-
c
a
partial pressure of CO2 in the ambient air or in the gas supplied to the leaf chambers
-
c
i
partial pressure of CO2 in the intercellular spaces of a leaf
-
e
a
partial pressure of H2O in the air
-
g
conductance for water vapor
-
J
quantum flux
-
T
1
leaf temperature 相似文献
34.
Stomatal response to air humidity and its relation to stomatal density in a wide range of warm climate species 总被引:1,自引:0,他引:1
Mabrouk A. El-Sharkawy James H. Cock Ana Del Pilar Hernandez 《Photosynthesis research》1985,7(2):137-149
The gas exchange of 19 widely different warm climate species was observed at different leaf to air vapour pressure deficits (VPD). In all species stomata tended to close as VPD increased resulting in a decrease in net photosynthesis. The absolute reduction in leaf conductance per unit increase in VPD was greatest in those species which had a large leaf conductance at low VPDs. This would be expected even if stomata of all species were equally sensitive. However the percentage reduction in net photosynthesis (used as a measure of the relative sensitivity of stomata of the different species) was also closely related to the maximal conductance at low VPD. Similarily the relative sensitivity of stomata to changes in VPD was closely related to the weighted stomatal density or crowding index.The hypothesis is presented that stomatal closure at different VPDs is related to peristomatal evaporation coupled with a high resistance between the epidermis and the mesophyll and low resistance between the stomatal apparatus and the epidermal cells. This hypothesis is consistent with the greater relative sensitivity of stomata on leaves with a high crowding index.The results and the hypothesis are discussed in the light of selection, for optimal productivity under differing conditions of relative humidity and soil water availablility, by observation of stomatal density and distribution on the two sides of the leaf.Visiting scientist, plant physiologist and research assitant of the Cassava Program 相似文献
35.
J. A. BUNCE 《Plant, cell & environment》1985,8(1):55-57
Abstract. Leaf conductance responses to leaf to air water vapour partial pressure difference (VPD) have been measured at air speeds of 0.5 and 3.0 ms−1 in single attached leaves of three species in order to test the hypothesis that leaf conductance response to VPD is controlled by evaporation from the outer surface of the epidermis, rather than by evaporation through stomata. Total conductance decreased linearly with increassing VPD at both air speeds, but was decreased 1.6 3.0 times as much as by a given incrase in VPD at high than at low air speed. depending on species. In all species the relationship between leaf conductance and the gradient for evaporation from the epidermis was the same at both values of boundary layer conductance, supporting the hypothesis that direct epidermal evaporation controls stomatal guard cell behaviour in responses of stomata to VPD in these species. 相似文献
36.
37.
Peter B. Reich 《Physiologia plantarum》1984,61(4):541-548
Cycling of stomatal conductance in three hybrid poplar ( Populus sp.) cultivars was observed under a variety of conditions. Illumination of plants kept previously in the dark induced very large oscillations with a period of about 40 min and large oscillations with a shorter period (< 10 min) were superimposed on the longer cycles. During these oscillations, large changes in conductance could occur very rapidly (1.0 cm s−1 in 3 min). Plants in constant light also displayed both long and short term cycles in conductance, but these were smaller in amplitude than those induced by sudden illumination. Stomatal oscillations were also observed in darkness and after darkening of previously illuminated plants. These oscillations had shorter (< 30 min) and less regular periods than those observed in the light. Such cycling in the dark is rare. Cycling of the two leaf surfaces was sometimes in synchrony in the light, and more so after a perturbation. Little synchrony between the two surfaces was observed in the dark. Stomatal movements of different leaves on a plant were usually relatively independent. Transient stomatal opening occurred following leaf excision in the light or dark, and often after sudden darkening of intact leaves. Also, stomata of intact leaves sometimes transiently closed following illumination. 相似文献
38.
Summary Cellular impalements were used in combination with standard transepithelial electrical measurements to evaluate some of the determinants of the spontaneous lumen-positive voltage,V
e
, which attends net Cl– absorption,J
Cl
net
, and to assess how ADH might augment bothJ
Cl
met
andV
e
in the mouse medullary thick ascending limb of Henle microperfusedin vitro. Substituting luminal 5mm Ba++ for 5mm K+ resulted in a tenfold increase in the apical-to-basal membrane resistance ratio,R
c
/R
bl
, and increasing luminal K+ from 5 to 50mm in the presence of luminal 10–4
m furosemide resulted in a 53-mV depolarization of apical membrane voltage,V
a
. Thus K+ accounted for at least 85% of apical membrane conductance. Either with or without ADH. 10–4
m luminal furosemide reducedV
e
andJ
Cl
net
to near zero values and hyperpolarized bothV
a
andV
bl
, the voltage across basolateral membranes; however, the depolarization ofV
bl
was greater in the presence than in the absence of hormone while the hormone had no significant effect on the depolarization ofV
a
, Thus ADH-dependent increases inV
b
were referable to greater depolarizations ofV
bl
in the presence of ADH than in the absence of ADH 68% of the furosemide-induced hyperpolarization ofV
a
was referable to a decrease in the K+ current across apical membranes, but, at a minimum, only 19% of the hyperpolarization ofV
bl
could be accounted for by a furosemide-induced reduction in basolateral membrane Cl– current. Thus an increase in intracellular Cl– activity may have contributed to the depolarization ofV
bl
during net Cl– absorption, and the intracellular Cl– activity was likely greater with ADH than without hormone. Since ADH increases apical K+ conductance and since the chemical driving force for electroneutral Na+,K+,2Cl– cotransport from lumen to cell may have been less in the presence of ADH than in the absence of hormone, the cardinal effects of ADH may have been to increase the functional number of both Ba++-sensitive conductance K+ channels and electroneutral Na+,K+,2Cl– cotransport units in apical plasma membranes. 相似文献
39.
Summary Two methods, the measurement of the response of the basolateral membrane potential (V
bl) of proximal tubule cells ofNecturus to step changes in basolateral K+ concentration, and cellular cable analysis, were used to assess the changes in basolateral potassium conductance (G
K) caused by a variety of maneuvers. The effects of some of these maneuvers on intracellular K+ activity (a
K
i
) were also evaluated using double-barreled ion-selective electrodes. Perfusion with 0mm K+ basolateral solution for 15 min followed by 45 min of 1mm K+ solution resulted in a fall in basolateral potassium (apparent) transference number (t
K),V
bl anda
K
i
. Results of cable analysis showed that total basolateral resistance,R
b
, rose. The electrophysiological effects of additional manipulations, known to inhibit net sodium reabsorption across the proximal tubular epithelium ofNecturus, were also investigated. Ouabain caused a fall int
K accompanied by large decreases ina
K
i
andV
bl. Lowering luminal sodium caused a fall int
K and a small reduction inV
bl. Selective reduction of peritubular sodium, a maneuver that has been shown to block sodium transport from lumen to peritubular fluid, also resulted in a significant decrease int
K. These results suggest thatG
K varies directly with rate of transport of the sodium pump, irrespective of the mechanism of change in pump turnover.Part of this material has been presented at the 10th International Conference on Biological Membranes (Cohen & Giebisch, 1984). 相似文献
40.
Peter H. Barry 《The Journal of membrane biology》1984,82(3):221-239
Summary Many neurones are extremely invaginated and possess branching processes, axons and dendrites. In general, they are surrounded by a restricted diffusion space. Many of these cells exhibit large, slow potential changes during the passage of current across their membranes. Whenever currents cross membranes separating aqueous solutions, differences in transport numbers of the major permeant ions give rise to local concentration changes of these ions adjacent to the membranes, which will result in various electrical and osmotic effects. These transport number effects are expected to be enhanced by the presence of membrane invaginations. Dendrites are equivalent to reversed invaginations and there should be significant changes in concentrations of permeant ions within them. In general, the effects of such changes on the electrical response of a cell will be greater when the concentration of a major permeant ion is low. The effects have been modelled in terms of two nondimensional parameters: the invagination transport number parameter and the relative area occupied by the invaginations A. If these two parameters are known, the magnitudes and time course of the slow potential changes can immediately be estimated and the time course converted to real time, if the length of the invaginations (l) and ionic diffusion coefficient (D) within them are also known. Both analytical and numerical solutions have been given and predictions compared. It is shown that in the case of large currents and potentials the analytical solution predictions will underestimate the magnitudes and rates of onset of the voltage responses. The relative magnitude of the transport number effect within the invaginations (or dendrites) and other transport number contributions to slow potential changes have also been assessed and order-of-magnitude values of these are estimated for some biological data. 相似文献