Ion permeation through 5-hydroxytryptamine-gated channels in neuroblastoma N18 cells

Ionic currents induced by 5-hydroxytryptamine (5-HT) in cultured neuroblastoma N18 cells were studied using whole-cell voltage clamp. The response was blocked by 1-10 nM 5-HT3 receptor-specific antagonists MDL 7222 or ICS 205-930, but not by 1 microM 5-HT1/5-HT2 receptor antagonist spiperone or 5-HT2 receptor-specific antagonist ketanserin. These 5-HT3 receptors seem to be ligand-gated channels because the response (a) did not require internal ATP or GTP, (b) persisted with long internal dialysis of CsF (90 mM), A1F4- (100 microM), or GTP gamma S (100 microM), and (c) with ionophoretic delivery of 5-HT developed with a delay of less than 10 ms and rose to a peak in 34-130 ms. Fluctuation analysis yielded an apparent single-channel conductance of 593 fS. The relative permeabilities of the channel for a variety of ions were determined from reversal potentials. The channel was only weakly selective among small cations, with permeability ratios PX/PNa of 1.22, 1.10, 1.01, 1.00, and 0.99 for Cs+, K+, Li+, Na+, and Rb+, and 1.12, 0.79, and 0.73 for Ca2+, Ba2+, and Mg2+ (when studied in mixtures of 20 mM divalent ions and 120 mM N-methyl-D-glucamine). Apparent permeability ratios for the divalent ions decreased as the concentration of divalent ions was increased. Small monovalent organic cations were highly permeant. Large organic cations such as Tris and glucosamine were measurably permeant with permeability ratios of 0.20 and 0.08, and N-methyl-D-glucamine was almost impermeant. Small anions, NO3-, Cl-, and F-, were slightly permeant with permeability ratios of 0.08, 0.04, and 0.03. The results indicate that the open 5-HT3 receptor channel has an effective minimum circular pore size of 7.6 A and that ionic interactions in the channel may involve negative charges near the pore mouth.     

Elevated potassium shortens action potential duration by altering outward currents in chick dorsal root ganglia neurons

Dissociated embryonic chick dorsal root ganglion (DRG) neurons maintained in culture exhibit a mixed Na+/Ca2+ action potential. The characteristic "shoulder" on the repolarizing phase is due to the relatively prolonged inward Ca2+ current. DRG neurons grown in an elevated K+ medium (25 versus. 5 mM) lack the plateau phase of the action potential. Voltage-clamp analysis showed that this plastic change in action potential duration is not due to the loss of the inward Ca2+ current but is partly due to the appearance of a Ca2(+)-dependent, 4-aminopyridine-(4-AP)-sensitive transient outward current. Faster activation of the purely voltage-dependent delayed rectifier outward current also contributes to the rapid repolarization observed in neurons cultured in elevated K+ medium.     

Combined in-beam electron impact-B/E-linked scan mass spectrometry of oxazoline derivatives for the structure determination of long-chain unsaturated fatty acids

Long-chain unsaturated fatty acids (UFA) having up to six double bonds are derivatized to 2-substituted 4,4-dimethyloxazolines (DMOX) and then analyzed by combined in-beam electron impact (IBEI)-B/E-linked scan mass spectrometry. This technique provides highly characteristic mass spectra and may serve as an auxiliary means for direct structure determination of individual UFA in mixtures.     

大肠杆菌青霉素G酰化酶基因及其邻近区域的核苷酸全序列

Some of microorganisms have been known to possess penicillin G acylase activity. The E. coli derived penicillin G acylase (PGA) can catalyze the conversion of penicillin G into phenylacetic acid and 6-amino-penicillanic acid, the latter is used as the starting compound for the industrial formation of semi-synthetic penicillins. Apart from its industrial importance, the enzyme PGA displays a number of interesting properties. Catalytically active enzyme is localized in the periplasmic space of E. coli cells and composed of two dissimilar subunits. The two subunits are apparently produced from a precursor protein, via a processing pathway hitherto unique in its features for a prokaryotic enzyme. The studies on processing of the precursor and on the relationship between structure and function of the mature enzyme are important theoretically. Previously we cloned a 3.5 kb DNA fragment from a strain (E. coli AS 1.76), which displays PGA activity. In this paper, we report a nucleotide sequence of the 3.5 kb DNA fragment containing PGA gene. After insertion of the DNA fragment into EcoR I and Hind III sites in pWR 13, pPGA 20 had been obtained. We subcloned the Hind III and Bg1 II treated fragment of 1.6 kb in length from pPGA 20 into Hind III and BamH I sites of pWR 13 to get a pPGA 1.6, and Bg1 II and EcoR I treated fragment of 1.9 kb in length into BamH I and EcoR I sites of pWR 13 to get a pPGA 1.9. The linearized pPGA 1.9 which were digested with appropriate restriction enzymes were progressively shortened from both ends respectively by digestion with Bal 31 nuclease, followed by cleavage of shortened target DNA off vector DNA molecules with appropriate restriction enzymes. The series of the DNA fragments shortened from EcoR I end were then cloned into plasmid pWR 13 which had previously digested with Hind III and Sma I enzymes (Fig. 1). The DNA fragment cloned in pWR 13 were directly sequenced on the resulted plasmids by using primer I and primer II. Thus we have obtained the complete nucleotide sequence of the 3.5 kb DNA fragment. The 3.5 kb fragment contains an intact PGA gene which is 2.6 kb.(ABSTRACT TRUNCATED AT 400 WORDS)     

血管紧张素Ⅱ中枢加压作用的机制

血管紧张素II(AII)具有中枢加压作用。中枢产生的AII在脑内的作用部位广泛,而外周产生的AII主要通过脑内某些特殊区域起作用。AII的中枢加压作用主要通过以下三条途径实现:(1)中枢性兴奋交感神经;(2)抑制迷走中枢;(3)促进加压素释放。     

向日葵离体孤雌生殖的超微结构研究

本文是研究未受精胚珠培养诱导的孤雌生殖过程超微结构变化的首次报道。向日葵(Heliaanthus annuus L.)的卵细胞在离体条件下被激活,发生细胞核移位、极性丧失、细胞器增多并转变成活动状态、液泡化程度增大、合点端形成细胞壁等一系列变化,预示即将启动孤雌生殖。孤雌生殖的原胚具有若干显著特征,如极性颠倒、有自体吞噬活动、壁的自由生长、游离核分裂等。对这些现象作了初步的讨论。     

气孔下腔间的水汽扩散

本文把气孔及其下腔看作截面为椭圆形的柱形区域,提出一个水汽从气孔下腔内所有细胞表面扩散到气孔外端的三维扩散模型。根据 Fick 定律和质量守恒定律建立了支配该模型的水汽扩散方程。用有限差分法,借助于计算机求得水汽从气孔下腔的所有细胞表面扩散到气孔内端所遇到的阻力及其近似表达式。并从理论上对该阻力的倒数——导度随气孔面积而变化的方式做了分析和解释。通过将本模型求得的气孔下腔阻力计算公式与 Brown 等以及 Cooke 的公式比较,发现在气孔开度变化相当大的范围内用后面两公式计算的阻力偏大0.5—1倍左右。此外,计算结果还表明:在气孔下腔水散失总量中,腔内表皮细胞表面上的水散失量占86—96%,而保卫细胞表面上的水散失量又占后者的88—93%,副卫细胞表面上的水散失量仅7—12%。     

甘蔗幼叶切段培养中的体细胞胚胎发生

本文从形态学和组织学方面研究了甘蔗幼叶胚性愈伤组织发生及体细胞胚胎的形成过程。甘蔗幼叶片切段培养于含2.4—D1.5mg/1的MS培养基上,4—6天后切段开始形成愈伤组织,约10天后愈伤组织表面出现白色颗粒状结构。将含有白色颗粒状结构的愈伤组织转移至不含激素的培养基中,7—10天后可见有小植株长出。组织学和形态学观察表明,甘蔗离体再生植株是通过体细胞胚胎发生途径。