Autonomic transmitter actions on cardiac pacemaker tissue: a brief review

Application of the voltage clamp technique to cardiac primary pacemaker tissue has yielded sufficiently detailed information that a qualitative model of the pacemaker response can now be formulated. One important difference between the generation of spontaneous activity in sinus tissue, and in the Purkinje fiber, appears to be the involvement of the slow inward current, Isi, in the sinus pacemaker depolarization. The voltage clamp results also demonstrate the importance of the Isi in the chronotropic responses of pacemaker tissue. Epinephrine has been shown to increase Isi in rabbit sinoatrial node, and there is indirect evidence that acetylcholine may reduce Isi in reptilian sinus venosus. Additional, more quantitative data are essential, however, before cardiac primary pacemaker activity and its modulation by the autonomic transmitters can be fully understood.     

Comparative bioenergetics of sulfate reduction in Desulfovibrio and Desulfotomaculum spp.

Extracts of Desulfotomaculum nigrificans, Desulfotomaculum orientis, and Desulfotomaculum ruminis exhibit low levels of inorganic pyrophosphatase but were found to have high levels of pyrophosphate:acetate phosphotransferase. Conversely, extracts of Desulfovibrio gigas, Desulfovibrio vulgaris, and Desulfovibrio desulfuricans Norway 4 were shown to have high levels of inorganic pyrophosphatase but negligible amounts of pyrophosphate:acetate phosphotransferase. Both enzymes are reductant activated and appear to have an analogous function in removing pyrophosphate formed during the activation of sulfate. Conservation of the bond energy of pyrophosphate in Desulfotomaculum eliminates the necessity for invoking electron-transfer-coupled phosphorylation to account for the growth of these bacteria on lactate plus sulfate. Relative growth yields of Desulfovibrio vulgaris and Desulfotomaculum orientis on lactate plus sulfate indicate that the latter does not carry out significant electron-transfer-coupled phosphorylation in this mode of growth.     

Lipid alterations in cell envelopes of polymyxin-resistant Pseudomonas aeruginosa isolates.

The lipid composition of cells of Pseudomonas aeruginosa strains resistant to polymyxin was compared with the lipid composition of cells of polymyxin-sensitive strains as to their content of readily extractable lipids (RELs), acid-extractable lipids, the fatty acid composition of RELs, and the contents of various phospholipids in the REL fraction. The polymyxin-resistant strains had an increased content of RELs, but a decreased phospholipid content. The REL fraction from the polymyxin-resistant strains had an increased content of unsaturated fatty acids accompanied by a decreased content of cyclopropane fatty acids as compared with the fatty acid composition of RELs from polymyxin-sensitive strains. The phosphatidylethanolamine content was greatly reduced in the polymyxin-resistant strains, whereas the content of an unidentified lipid, thought to be a neutral lipid lacking either a phosphate, free amino, or choline moiety, was greatly increased. Cell envelopes of the polymyxin-resistant strains contained reduced concentrations of Mg2+ and Ca2+ as compared with the cell envelopes of polymyxin-sensitive strains. It appears that polymyxin resistance in these strains is associated with a significant alteration in the lipid composition and divalent cation content of the cell envelope.     

Thiolation and 2-methylthio- modification of Bacillus subtilis transfer ribonucleic acids.

Six thionucleosides found in Bacillus subtilis transfer ribonucleic acids were investigated: N6-(delta 2-isopentenyl)-2-methylthioadenosine, 5-carboxymethylaminomethyl-2-thiouridine, 4-thiouridine, 2-methylthioadenosine, N-[(9-beta-D-ribofuranosyl-2-methylthiopurin-6-yl)carbamoyl]threonine, and one unknown (X1). The presence of N-[(9-beta-D-ribofuranosyl-2-methylthiopurin-6-yl)carbamoyl]threonine was demonstrated based on the affinity of the transfer ribonucleic acid containing it for an immunoadsorbent made with the antibody directed toward N-[9-(beta-D-ribofuranosyl)purin-6-ylcarbamoyl]-L-threonine. The existance of N-[(9-beta-D-ribofuranosyl-2-methylthiopurin-6-yl)carbamoyl]threonine in two species of lysine transfer ribonucleic acids was also confirmed by high-resolution mass spectrometry. Four of these thionucleosides--N6-(delta 2-isopenenyl)-2-methylthioadenosine, 2-methylthioadenosine, 5-carboxymethylaminomethyl-2-thiouridine, and the unknown designated X1--occurred only in specific areas in the elution profile of an RPC-5 column and probably affect the chromatographic properties of the transfer ribonucleic acids containing them. In contrast with Escherichia coli, where 4-thiouridine is the most frequent type of sulfur-containing modification, approximately one-third of the sulfur groups in B. subtilis transfer ribonucleic acid are present as thiomethyl groups on the 2 position of an adenosine or modified adenosine residue.     

Chemotaxis of Spirochaeta aurantia: involvement of membrane potential in chemosensory signal transduction.

The effects of valinomycin and nigericin on sugar chemotaxis in Spirochaeta aurantia were investigated by using a quantitative capillary assay, and the fluorescent cation, 3,3'-dipropyl-2,2'-thiodicarbocyanine iodide was used as a probe to study effects of chemoattractants on membrane potential. Addition of a chemoattractant, D-xylose, to cells in either potassium or sodium phosphate buffer resulted in a transient membrane depolarization. In the presence of valinomycin, the membrane potential of cells in potassium phosphate buffer was reduced, and the transient membrane depolarization that resulted from the addition of D-xylose was eliminated. Although there was no detectable effect of valinomycin on motility, D-xylose taxis of cells in potassium phosphate buffer was completely inhibited by valinomycin. In sodium phosphate buffer, valinomycin had little effect on membrane potential or D-xylose taxis. Nigericin is known to dissipate the transmembrane pH gradient of S. aurantia in potassium phosphate buffer. This compound did not dissipate the membrane potential or the transient membrane depolarization observed upon addition of D-xylose to cells in either potassium or sodium phosphate buffer. Nigericin did not inhibit D-xylose taxis in either potassium or sodium phosphate buffer. This study indicates that the membrane potential but not the transmembrane pH gradient of S. aurantia is somehow involved in chemosensory signal transduction.     

Transport of Ammonium and Methylammonium Ions by Azotobacter vinelandii

Ammonium and methylammonium are rapidly taken up by cultures of Azotobacter vinelandii respiring in the presence of succinate. The rate of methylamine uptake increased with external pH from 5.5 to 7.5 but increasing the pH further to 8.5 had little effect on activity, indicating that methylammonium cation rather than uncharged methylamine is the permeant species. The kinetics of methylammonium entry followed the Michaelis-Menten relationship, yielding a K_m of 25 μM and a V_max of 3.8 nmol/min per mg of cell protein. At saturating concentrations ammonium was taken up at rates 30-fold higher than those for methylammonium. Ammonium was a competitive inhibitor of methylammonium uptake and gave an inhibition constant of 1 μM. Ammonium derivatives were inhibitors of methylammonium entry in order of effectiveness: hydrazine > methylhydrazine > formamidine > guanidine > dimethylamine > ethylamine; amides and amino acids did not block uptake. Likewise, metal cations inhibited in the order Tl⁺ > Cs⁺ > Rb⁺, whereas Na⁺, K⁺, and Li⁺ produced no significant effect. Methylammonium uptake was blocked in cells exposed to an uncoupler, p-trifluorome-thoxycarbonyl cyanide-phenyl hydrazone or gramicidin D, but not with dicyclo-hexylcarbodiimide or arsenate. Valinomycin stimulated methylammonium entry into cells in a K⁺-free medium but prevented entry in the presence of 10 mM K⁺. Monensin and nigericin had little effect on transport. These results indicate that methylammonium and ammonium ions enter A. vinelandii electrogenically via a specific transporter.     

Identification of a major envelope protein in Chlamydia spp.

A major cell envelope protein of Chlamydia psittaci with a molecular weight of approximately 43,000 was identified and partially characterized. It was present at all stages of the C. psittaci developmental cycle. A major protein with a similar molecular weight was also observed in two Chlamydia trachomatis strains.     

Joint Segregation of Biochemical Loci in Salmonidae. II. Linkage Associations from a Hybridized Salvelinus Genome (S. NAMAYCUSHxS. FONTINALIS)

The results of more than 300 pairwise examinations of biochemical loci for joint segregation in brook trout (Salvelinus fontinalis) and in the hybridized genome of lake trout (S. namaycush) x brook trout are summarized. Nineteen loci have been assigned to the following eight linkage groupings on the basis of nonrandom assortment, including cases of both classical linkage and pseudolinkage: ODH with PMI with PGI-3, PGI-2 with SDH, ADA-1 with AGP-2, AAT-(1,2) with AGP-1 with MDH-1, MDH-3 with MDH-4, LDH-3 with LDH-4, IDH-3 with ME-2 and GUS with CPK-1. Pseudolinkage (an excess of nonparental progeny types) was observed only for male testcross parents. The results suggest that this phenomenon involves homeologous chromosome arms as evidenced by the de novo association of presumed duplicate loci in each case. Classical linkage has not been found for the five pairs of duplicate loci examined in Salvelinus, suggesting that not all of the eight metacentrics in the haploid complement involve fusions of homeologous chromosomes. Females consistently showed a greater degree of recombination.     

Fine control of adenylate cyclase by the phosphoenolpyruvate:sugar phosphotransferase systems in Escherichia coli and Salmonella typhimurium.

Inhibition of cellular adenylate cyclase activity by sugar substrates of the phosphoenolpyruvate-dependent phosphotransferase system was reliant on the activities of the protein components of this enzyme system and on a gene designated crrA. In bacterial strains containing very low enzyme I activity, inhibition could be elicited by nanomolar concentrations of sugar. An antagonistic effect between methyl alpha-glucoside and phosphoenolpyruvate was observed in permeabilized Escherichia coli cells containing normal activities of the phosphotransferase system enzymes. In contrast, phosphoenolpyruvate could not overcome the inhibitory effect of this sugar in strains deficient for enzyme I or HPr. Although the in vivo sensitivity of adenylate cyclase to inhibition correlated with sensitivity of carbohydrate permease function to inhibition in most strains studied, a few mutant strains were isolated in which sensitivity of carbohydrate uptake to inhibition was lost and sensitivity of adenylate cyclase to regulation was retained. These results are consistent with the conclusions that adenylate cyclase and the carbohydrate permeases were regulated by a common mechanism involving phosphorylation of a cellular constituent by the phosphotransferase system, but that bacterial cells possess mechanisms for selectively uncoupling carbohydrate transport from regulation.     

Influences of growth substrates and oxygen on the electron transport system in Acinetobacter sp. HO1-N.

The electron transport system of Acinetobacter sp. HO1-N was studied to determine the specific cytochromes and to measure changes in the composition of the respiratory system due to growth in various concentrations of oxygen or types of growth substrates. Spectrophotometric analysis revealed that the quantity and types of cytochromes changed in response to growth under various concentrations of oxygen. Growth on alkane and nonalkane substrates resulted in only minor differences in cytochrome composition or oxidase activities. Membranes prepared from cells grown under oxygen-limiting conditions contained at least one b-type cytochrome, cytochrome o, cytochrome d, and slight traces of cytochrome a1, whereas membranes prepared from cells grown in the presence of high oxygen concentrations contained only low levels of cytochromes b and o. Polarographic measurements, electron transport inhibitor studies, and photoaction spectrum analyses indicated that cytochromes o, a1, and d were potentially capable of functioning as terminal oxidases in this organism. These experiments also revealed that all three cytochromes may be involved in the oxidation of reduced nicotinamide adenine dinucleotide, succinate, or N,N,N',N'-tetramethyl-p-phenylenediamine.