Guanidine block of single channel currents activated by acetylcholine

The acetylcholine-activated channel of chick myotube was studied using the patch-clamp method. Single channel current amplitudes were measured between -300 and +250 mV in solutions containing the permeant ions Cs+ and guanidine (G+). G+ has a relative permeability, PG/PCs, of 1.6, but carries no more than half the current that Cs+ does, with an equivalent electrochemical driving force. Experiments using G+ revealed an asymmetry of the acetylcholine-activated channel, with G+ being more effective at reducing Cs+ currents when added to the outside than when added to the inside. The block caused by outside, but not inside, G+ was evident for both inward and outward currents. The block caused by outside G+ was voltage dependent, first increasing and then being partially relieved when the driving force was made more negative. Experiments with mixtures of Cs+ and G+ revealed anomalously low magnitudes for reversal potentials, relative to predictions based on the Goldman-Hodgkin-Katz equation. These findings are consistent with a two-well, three-barrier Eyring rate model for ion flow, and demonstrate that a highly permeant ion, guanidine, can block asymmetrically by acting from within the voltage field of the acetylcholine-activated channel.     

Interleukin 1 increases the binding of human B and T lymphocytes to endothelial cell monolayers

Lymphocyte binding to specialized high-endothelial venules (HEV) in lymph nodes and Peyer's patches is the first step in normal lymphocyte emigration and recirculation. The development and maintenance of HEV in these lymphoid organs are thought to be immunologically controlled. Because postcapillary venules in chronic inflammatory tissue often resemble the HEV of lymphoid tissue and may also be a site of lymphocyte emigration, examination of the effects of immunologic and inflammatory mediators on endothelial cells (EC) may provide important information about the physiology of both normal lymphocyte recirculation and chronic inflammation. It is reported here that treatment of human umbilical vein EC monolayers in vitro with affinity-purified human interleukin 1 (IL 1) markedly enhances the binding of both B and T lymphocytes. Increased binding was observed within 1 h of treatment of EC with as little as 0.04 U/ml IL 1. This effect of IL 1 was EC-specific, because pretreatment of T cells or human skin fibroblasts with IL 1 did not increase the binding of lymphocytes. Stimulation of binding required active EC metabolism because incubation of EC with IL 1 at 4 degrees C, or prior fixation of EC, prevented enhanced binding. The action of IL 1 was not associated with EC damage. The secretion of IL 1 by macrophages and perhaps other cells in inflammatory lesions may exert a positive feedback signal on EC to enhance further emigration of lymphocytes into the inflammatory focus.     

Involvement of Calcium in the Regulation of Serotonin N-Acetyltransferase in Retina

The possible involvement of calcium in the regulation of retinal serotonin N-acetyltransferase (NAT) activity was investigated using eye cups of Xenopus laevis cultured in defined medium. Omitting CaCl2 from the culture medium completely inhibited the dark-dependent increase of NAT activity at night. Approximately 10(-4)-10(-3) M free Ca2+ was found to be required for the maximal increase of NAT activity in the dark. Other divalent cations--Ba2+, Sr2+, and Mn2+--did not substitute for Ca2+. Antagonists of voltage-sensitive calcium channels, including nifedipine, methoxyverapamil (D600), Co2+, and Mg2+, were found to be effective inhibitors of the dark-dependent increase of retinal NAT activity. Trifluoperazine also decreased retinal NAT activity. These studies indicate that the increase of retinal NAT activity in the dark is mediated by a specific Ca2+-dependent process and that Ca2+ influx through voltage-sensitive calcium channels is involved.     

The nucleotide sequence of the mercuric resistance operons of plasmid R100 and transposon Tn501: further evidence for mer genes which enhance the activity of the mercuric ion detoxification system

Summary The DNA sequences of the mercuric resistance determinants of plasmid R100 and transposon Tn501 distal to the gene (merA) coding for mercuric reductase have been determined. These 1.4 kilobase (kb) regions show 79% identity in their nucleotide sequence and in both sequences two common potential coding sequences have been identified. In R100, the end of the homologous sequence is disrupted by an 11.2 kb segment of DNA which encodes the sulfonamide and streptomycin resistance determinants of Tn21. This insert contains terminal inverted repeat sequences and is flanked by a 5 base pair (bp) direct repeat. The first of the common potential coding sequences is likely to be that of the merD gene. Induction experiments and mercury volatilization studies demonstrate an enhancing but non-essential role for these merA-distal coding sequences in mercury resistance and volatilization. The potential coding sequences have predicted codon usages similar to those found in other Tn501 and R100 mer genes.     

Secretion of metalloproteinases by stimulated capillary endothelial cells. I. Production of procollagenase and prostromelysin exceeds expression of proteolytic activity

We have characterized the biosynthesis of two metalloproteinases, procollagenase and prostromelysin, by rabbit brain capillary endothelial cells (RBCE) by means of immunochemical, biosynthetic, and functional assays. Unstimulated RBCE secreted no detectable metalloproteinases. Secretion of both procollagenase and prostromelysin was induced within 6 h by treating the cells with 50 ng/ml 12-O-tetradecanoylphorbol-13-acetate. In treated cells, the two proenzymes accounted for up to 20% of the [35S]methionine-labeled secreted proteins; about 15 micrograms of each protein was secreted in 48 h by 10(6) RBCE. Although RBCE secreted approximately as much procollagenase and prostromelysin as did rabbit fibroblasts, virtually no enzyme activity could be measured in RBCE-conditioned medium, even after activation of the proenzymes by trypsin or an organomercurial agent.     

Superoxide Radical-Mediated Alteration of Synaptosome Membrane Structure and High-Affinity γ-[14C]Aminobutyric Acid Uptake

Mouse cortical synaptosomal structure and function are altered when exposed to hypoxanthine/xanthine oxidase (HPX/XOD)-generated active oxygen/free radical species. The structure of both the synaptic vesicle and plasma membrane systems are altered by HPX/XOD treatment. The alteration of synaptic vesicle structure is exhibited by a significant increase in the cumulative length of nonsynaptic vesicle membrane per nerve terminal. With respect to the nerve terminal plasma membrane, the length of the perimeter of the synaptosome is increased as the membrane pulls away from portions of the terminal in blebs. The functional lesion generated by HPX/XOD treatment results in a reduction in selective high-affinity gamma-[14C]aminobutyric acid (GABA) uptake. Kinetic analysis of the reduction in high-affinity uptake reveals that the Vmax is significantly altered whereas the Km is not. Preincubation with specific active oxygen/free radical scavengers indicates that the super-oxide radical is directly involved. This radical, most probably in the protonated perhydroxyl form, initiates lipid peroxidative damage of the synaptosomal membrane systems. Low-affinity [14C]GABA transport is unaltered by the HPX/XOD treatment. The apparent ineffectiveness of free radical exposure on low-affinity [14C]GABA transport coupled with its effectiveness in reducing high-affinity transport supports the idea that two separate and different amino acid uptake systems exist in CNS tissue, with the high-affinity being more sensitive (lipid-dependent) and/or more energy-dependent (Na+,K+-ATPase) than the low-affinity system.     

Characteristics of the β-Adrenoreceptor from Neuronal and Glial Cells in Primary Cultures of Rat Brain

The cellular characteristics of the beta-adrenoreceptor in glial and neuronal cells from the newborn rat brain were determined by (-)-[125I]iodocyanopindolol binding. In membranes from both cell types, the binding was saturable and from competition assays the potency series of (-)-isoproterenol greater than (-)-epinephrine = (-)-norepinephrine greater than (+)-isoproterenol was observed. 5'-Guanylyl-imidodiphosphate reduced the affinity of (-)-isoproterenol for the beta-adrenoreceptor from glial cells but had no effect on agonist affinity in neuronal cells. Chronic treatment of both cell types with (-)-isoproterenol reduced the receptor content and the capacity of the agonist to increase the cellular cyclic AMP content. However, the receptor recovery after chronic agonist treatment was faster in glial cells (72 h) than neuronal cells (120 h) and was blocked by cycloheximide. Treatment of both types with the irreversible beta-blocker bromoacetylalprenololmentane (2 microM) reduced the receptor content by 78% but no receptor recovery was observed for 120 h after the initial receptor loss. The data indicated that the majority of beta-adrenoreceptors in both cell types are the beta-1 subtype, but show some differences in receptor-agonist interactions. Furthermore, these CNS cells may be useful models for regulatory studies on the beta-adrenoreceptor.     

Mechanism of inhibition of net ion transport across frog corneal epithelium by calcium channel antagonists

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.