Hystereses in the force-length relation and regulation of cross-bridge recruitment in tetanized rat trabeculae

Various mechanisms have been suggested to explain cardiac force-length Ca2+ relations. The existence of a cooperativity mechanism, whereby cross-bridge (XB) recruitment is affected by the number of active XBs, suggests that the force response to length oscillations should lag length oscillations. Consequently, the oscillatory force response should be larger during shortening than during lengthening. To test this prediction, force responses to large-sarcomere length (SL) oscillations (36.7 +/- 16.0 nm) at different SLs (n = 6) and frequencies (n = 7) were studied in intact tetanized trabeculae dissected from rat right ventricle (n = 13). Stable tetani were obtained by utilizing 30 microM cyclopiazonic acid in Krebs-Henseleit solution containing 6 mM extracellular Ca(2+) at 25 degrees C. SL was measured by laser diffraction techniques (Dalsa). Force was measured by silicone strain gauge. Instantaneous dynamic stiffness during large oscillations was measured by superimposing additional fast (50 or 200 Hz) and small-amplitude (2.25 +/- 0.25 nm) oscillations. The force responses lagged the SL oscillations at slow frequencies (112 +/- 41 ms at 1 Hz), and counterclockwise hystereses were obtained in the force-length plane: the force was higher during shortening than during lengthening. The delay in the force response decreased as the frequency of the SL oscillation was increased. Clockwise hysteresis, where the force preceded the SL, was obtained at frequencies >4 Hz. Similar hysteresis characteristics were obtained in the force-SL and stiffness-SL planes. Maximal lag was observed at the shortest SL, and the delay decreased with sarcomere elongation: 131.1 +/- 31.7 ms at 1.78 +/- 0.03 microm vs. 14.7 +/- 18.5 ms at 1.99 +/- 0.015 microm. The results establish the ability of cardiac fiber to adapt XB recruitment to changes in prevailing loading conditions. This study supports the stipulated existence of a cooperativity mechanism that regulates XB recruitment and highlights an additional method to characterize regulation of the force-length relation.     

Follicular dynamics and concentrations of steroids and gonadotropins in lactating cows and nulliparous heifers

Differences in follicular development and circulating hormone concentrations, between lactating cows and nulliparous heifers, that may relate to differences in fertility between the groups, were examined. Multiparous, cyclic, lactating Holstein cows (n=19) and cyclic heifers (n=20) were examined in the winter, during one estrous cycle. The examinations included ultrasound monitoring and daily blood sampling. Distributions of two-wave and three-wave cycles were similar in the two groups: 79 and 21% in cows, 70 and 30% in heifers, respectively. Cycle lengths were shorter by 2.6 days in heifers than in cows, and in two-wave than in three-wave cycles. The ovulatory follicle was smaller in heifers than in cows (13.0+/-0.3 mm versus 16.5+/-0.05 mm). The greater numbers of large follicles in cows than in heifers corresponded well to the higher concentrations of FSH in cows. The duration of dominance of the ovulatory follicle tended to be longer in cows than in heifers. Estradiol concentrations around estrus and the preovulatory LH surge were higher in heifers than in cows (20 versus 9 ng/ml). Progesterone concentrations were higher in heifers than in cows from Day 3 to Day 16 of the cycle. Circulating progesterone did not differ between two-wave and three-wave cycles. The results revealed differences in ovarian follicular dynamics, and in plasma concentrations of steroids and gonadotropins; these may account for the differences in fertility between nulliparous heifers and multiparous lactating cows.     

cAMP regulation of "pathogenic" and "saprophytic" fungal spore germination

We report on the elucidation of two separate pathways of spore germination in a plant pathogenic fungus Colletotrichum gloeosporioides f. sp. aeschynomene. Conidia of the fungus can germinate either from one side or from both sides, depending on external conditions. In shake culture that includes an extract made up from fresh peas, the unicellular conidium divides and one of the two cells develops a germ tube. On a solid surface this germ tube differentiates an appressorium. In rich medium without pea extract, germination is highly similar to Aspergillus spore germination: the conidium swells, forms a single germ tube and then divides and forms a second germ tube. Conidia that germinate in a rich medium do not form appressoria even on a solid surface and are non-pathogenic. In rich medium, cAMP stimulates germination in rich liquid cultures and induces appressoria formation on a hard surface. In pea extract cAMP induces swelling and formation of irregular germ tubes and appressoria. Our results suggest that plant surface signals induce pathogenic-specific spore germination in a cAMP-independent manner. cAMP is required for saprophytic germination and for appressorium formation.     

On the site of action of plastocyanin in isolated chloroplasts

Adding plastocyanin to plastocyanin-depleted chloroplast particles, restored both their ability to catalyse the photoinduced electron transfer from ascorbate-DCIP to NADP, and to induce the photooxidation of cytochrome f. It is concluded, therefore, that plastocyanin mediates the photoinduced oxidation of cytochrome f, as previously suggested.     

TRACTS: A program to map oligopurine.oligopyrimidine and other binary DNA tracts

A program to map the locations and frequencies of DNA tracts composed of only two bases ('Binary DNA') is described. The program, TRACTS (URL http://bioportal.weizmann.ac.il/tracts/tracts.html and/or http://bip.weizmann.ac.il/miwbin/servers/tracts) is of interest because long tracts composed of only two bases are highly over-represented in most genomes. In eukaryotes, oligopurine.oligopyrimidine tracts ('R.Y tracts') are found in the highest excess. In prokaryotes, W tracts predominate (A,T 'rich'). A pre-program, ANEX, parses database annotation files of GenBank and EMBL, to produce a convenient one-line list of every gene (exon, intron) in a genome. The main unit lists and analyzes tracts of the three possible binary pairs (R.Y, K.M and S;W). As an example, the results of R.Y tract mapping of mammalian gene p53 is described.     

Sequence,time, or state representation: how does the motor control system adapt to variable environments?

Does the observation of well-timed movements imply the existence of some internal representation of time, such as a hypothetical neural clock? Here we report the results of experiments designed to investigate whether subjects form a correct adaptive representation of mechanical environments that change in a very predictable manner. In these experiments, subjects were asked to execute arm movements over a two-dimensional workspace while experiencing time-dependent disturbing forces. We provide a formal definition for time representation and conclude that our subjects didn't use time representation for motor adaptation under the tested conditions. Subjects performed arm-reaching movements in the following experiments: (1) six experiments in a sinusoidal time-varying force field; (2) six experiments in a simple sequence of alternating viscous force fields, in which the number of targets allowed for the approximation of the force by a complex state-dependent force field; and (3) six experiments in the same simple sequence of alternating viscous force fields, in which no state-dependent force field approximation was possible. We found that the subjects did not adapt to the time-varying force field and were unable to form an adequate representation of the simple sequence of force fields. In the latter case, whenever possible, they adapted to a single state-dependent field that produced forces similar to the two alternating fields. This state-dependent field produced the same forces as the applied sequence of fields only over the trajectories that subjects executed during the training phase. However, the state-dependent field was inadequate to produce the correct forces generated by the field sequence over a new set of trajectories.These results are not consistent with the hypothesis that subjects would develop a correct representation of time-dependent forces, at least under the tested circumstances. We speculate that the system responsible for adaptation of movements to external forces may be unable to employ temporal representation. While it is possible that such a representation may emerge in a more prolonged and/or intense training, our findings indicate a preference by the adaptive system to generalize based on representing dependence of external forces upon state rather than upon time.     

Molecular movement of the voltage sensor in a K channel

The X-ray crystallographic structure of KvAP, a voltage-gated bacterial K channel, was recently published. However, the position and the molecular movement of the voltage sensor, S4, are still controversial. For example, in the crystallographic structure, S4 is located far away (>30 A) from the pore domain, whereas electrostatic experiments have suggested that S4 is located close (<8 A) to the pore domain in open channels. To test the proposed location and motion of S4 relative to the pore domain, we induced disulphide bonds between pairs of introduced cysteines: one in S4 and one in the pore domain. Several residues in S4 formed a state-dependent disulphide bond with a residue in the pore domain. Our data suggest that S4 is located close to the pore domain in a neighboring subunit. Our data also place constraints on possible models for S4 movement and are not compatible with a recently proposed KvAP model.