Substances known to alter cyclic nucleotide levels in cells were applied to the isolated toad retina and effects on rod electrical and adaptive behavior were studied. The retina was continually superfused in control ringer’s or ringer’s containing one or a combination of drugs, and rod activity was recorded intracellularly. Superfusion with cGMP, Bu(2)GMP, isobutylmethylxanthine (IBMX; a phosphodiesterase inhibitor), or PGF(2α) (a prostaglandin) caused effects in rods that closely match those observed when extracellular Ca(2+) levels were lowered. For example, short exposures (up to 6 min) of the retina to these substances caused depolarization of the membrane potential, increase in response amplitudes, and some changes in waveform; but under dark-adapted or partially light-adapted conditions receptor sensitivity was virtually unaffected. That is, the position of the V-log I curve on the intensity axis was determined by the prevailing light level, not by drug level. These drugs, like lowered extracellular Ca(2+), also decreased the period of receptor saturation after a bright-adapting flash, resulting in an acceleration of the onset of membrane and sensitivity recovery during dark adaptation.
Long-term (6-15 min) exposure of a dark-adapted retina to 5 mM IBMX or a combination of IBMX and cGMP caused a loss of response amplitude and a desensitization of the rods that was similar to that observed in rods after a long-term low Ca(2+) (10(-9)M) treatment. Application of high (3.2 mM) Ca(2+) to the retina blocked the effects of applied Bu(2)cGMP. PGE(1) superfusion mimicked the effects of increasing extracellular Ca(2+). The results show that increased cGMP and lowered Ca(2+) produce similar alterations in the electrical activity of rods. These findings suggest that Ca(2+) and cGMP are interrelated messengers. We speculate that low Ca(2+) may lead to increased intracellular cGMP, and/or that applied cGMP, and/or that applied cGMP may lower cytosol Ca(2+), perhaps by stimulating Ca(2+)- ATPase pumps in the outer segment.
In etiolated pea and maize leaves illuminated after incubation at 38 degreesC, a new dark reaction was shown manifested in the bathochromic shift of spectral bands and accompanied by esterification of the product of protochlorophyllide photochemical reduction--Chld 684/676: Chld 684/676 --> Chl 688/680. After completion of the reaction a rapid (20-30 sec) quenching of the fluorescence of the reaction product (Chl 688/680) was observed. The reaction Chld 684/676 --> Chl 688/680 is inhibited under anaerobic conditions and in the presence of cyanide; the reaction accompanied by Chl 688/680 fluorescence quenching is not observed in pea mutants with impaired function of photosystem II reaction centers. The spectral properties of the formed Chl form with the absorption maximum at 680 nm, fluorescence quenching, and simultaneous synthesis of pheophytin suggest that the reaction is connected with the chlorophyll of photosystem II reaction center--P-680. 相似文献
Summary Immortal cells perpetuate the rises and falls of proliferation that are progressively damped in mortal long-term cultured
cells. For immortal rat hepatoma Fao cells, similar waves of proliferation occurred about every 3–4 wk. Under the same conditions,
embryonic human fibroblasts and transformed but not immortalized embryonic fibroblasts display similarly recurring proliferation
waves that progressively decrease in amplitude until senescence of the lines. In addition, strains of diploid normal human
skin fibroblasts cultured under different culture conditions display a similar time-pattern of proliferation. Although the
amplitude and baseline of these fluctuations are characteristic for each cell line, a common point was marked slow down in
proliferation after every sequence of about 25 population doublings for all cells. Renewed proliferation waves of Fao cells
allow about 22–23 additional population doublings each. Normal embryonic fibroblasts culture and its transformed counterpart
accumulate about 30 and 60 population doublings, respectively, before senescence. Normal fibroblast strains accumulate about
25 population doublings over their entire life spans. This halt in proliferation after every stretch of about 25 population
doublings may correspond to a structural or functional stop following attrition of telomeric DNA. This putative stop may be
bypassed once in transformed embryonic cells and repetitively in immortal cells. In support of this hypothesis, we observed
rapid telomere shortening, in two steps, during divisions of mortal embryonic cells, and maintenance of long telomeres in
immortal Fao cells, which may indicate episodic repair of telomeres. Alternatively, such maintenance of long telomeres may
reflect survival and successive clonal growth of rare cells with long telomeres. We suggest that the balance between telomere
attribution and repair processes regulates the waves of proliferation.
Equal contributors to these studies. 相似文献
Several fundamental questions remain enigmatic concerning human olfactory
sensitivity, including (i) whether detection threshold differences exist
between the two sides of the nose (and, if so, whether such differences are
influenced by handedness) and (ii) whether bilateral (i.e. binasal)
stimulation leads to lower thresholds than unilateral stimulation (and, if
so, whether the degree of facilitation is inversely related to general
olfactory ability). In this study, and well-validated single staircase
procedure was used to establish bilateral and unilateral detection
thresholds for the cranial nerve I stimulant phenyl ethyl alcohol in 130
right- and 33 left-handed subjects. No differences in sensitivity between
the left and right sides of the nose were observed in either group.
Bilateral thresholds were lower, on average, than unilateral thresholds
when the latter were categorized in terms of left and right nares. However,
the bilateral thresholds did not differ significantly from those of the
side of the nose with the lower threshold. Overall smell ability, as
measured by the University of Pennsylvania Smell Identification Test, did
not interact with any of the test measures. These data imply that (i) the
left and right sides of the nose do not systematically differ in detection
threshold sensitivity for either dextrals or sinistrals and (ii) if central
integration of left:right olfactory threshold sensitivity occurs, its
effects do not exceed the function of the better side of the nose.
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Ethyl isopropenyl ether reacts with D-glucose in N,N-dimethylformamide containing a trace of p-toluenesulfonic acid to give crystalline 4,6-O-isopropylidene-α,β-D-glucopyranose (2) in near-quantitative yield. The structure of 2 was established by n.m.r. spectroscopy of it and of its β-triacetate 3, and by conversion of 3 through deacetonation and subsequent acetylation into β-D-glucopyranose pentaacetate (5). The acetonation reagent operates under kinetic control, with favored attack at primary hydroxyl groups, instead of by the thermodynamic control associated with conventional acetonation methods. The reagents converts methyl α-D-glucopyranoside (7) into the 4,6-isopropylidene acetal 8, and D-mannitol (9) into a 2:1 mixture of the 1,2:5,6-di-isopropylidene acetal 10 and the 1,2:3,4:5,6-tri-isopropylidene acetal 11. 相似文献
Ophioglossum yongrenense Ching ex Z. R. He et W. M. Chu, sp. nov.Species nova aspectu inter O. kawamuram Tagawa et O. parvum Nishida et Kurita Japonicae, differt a priore frondibus pro parte laminis sterilibus praeditis (in illa frondibus omnino laminis sterilibus non praeditis), a posteriore frondibus pro parte laminis sterilibus non praeditis (in O. parvo Nishida et Kurita frondibus omino laminis sterilibus praeditis).…… 相似文献
The present study was conducted primarily to determine the occurrence of polypharmacy in patients with schizophrenia on risperidone. The secondary aim was to ascertain the incidence of inappropriate prescribing with anticholinergics. A retrospective review of the medical records of all patients who were being followed up at the out-patient clinic of a tertiary-care hospital in Malaysia was conducted. Only patients who were being prescribed risperidone between 1 June 2008 and 31 December 2008 were included in the study. Demographic data such as patient’s age, gender and race were obtained from the patient’s medical records. In total, 113 patients met the selection criteria. Polypharmacy was found to occur in 34 patients (30.09%), with the majority (76.47%) being on two antipsychotics. In total, 27 patients (34.18%) on monotherapy with risperidone were prescribed an anticholinergic on scheduled dosing, while 19 patients (24.05%) were prescribed it on an as-needed basis. Of the patients on polypharmacy, 26 (76.47%) were on scheduled dosing of anticholinergics, while three (8.82%) were taking the medication on an as-needed basis. Polypharmacy should be avoided, and the use of anticholinergics should be closely reviewed. By adopting more efficient prescribing practices, costs can be reduced and financial resources can instead be channelled towards more beneficial areas for the patients. 相似文献