Maximal dynamic expiratory pressures with fast and slow inspirations

Maximal dynamic expiratory pressures are higher when forced expiration is preceded by a fast inspiration to total lung capacity (TLC) than when preceded by a slow inspiration and a few seconds pause at TLC. We hypothesized that these pressure differences are due to the stretch-shorten cycle (SSC), which refers to enhancement of muscle force when a concentric muscle contraction is immediately preceded by an eccentric contraction. Seven volunteers [36 (2) years; mean (SEM)] performed maximal forced expirations against minimal resistance with fast (F) or slow (S) maneuvers. F maneuvers consisted of a fast inspiration to TLC followed immediately by a fast expiration, whereas S consisted of a slow inspiration to TLC and a 4- to 5-s pause at TLC prior to forced expiration. We measured esophageal pressure (P _es), peak expiratory flow rate (PEFR), and the EMG activity of the transversus abdominis (Tr) by means of intramuscular fine-wire electrodes. The subjects performed several runs of each maneuver in a random order, and runs with the greatest expiratory P _es were analyzed. In comparison with S, F yielded greater P _es [182 (15) versus 167 (15) cmH₂O; P=0.003)] but similar PEFR [9.8 (0.7) versus 9.6 (0.7) l/s, P>0.05] and EMG activity of the Tr during forced expiration [221 (31) versus 208 (34) a.u., P>0.05]. Further analysis revealed significant EMG activity of Tr during end-inspiration (eccentric contraction) with F maneuvers only [73 (22) versus 32 (17) a.u., P<0.05]. We conclude that the ability of expiratory muscles to generate greater P _es with F maneuvers is related to the sequence of an eccentric contraction, which is followed immediately by concentric contraction in a manner analogous to SSC described in skeletal muscles. Electronic Publication     

Regulation of phosphatidylinositol kinases by arachidonic acid in rat submandibular gland cells

Phosphoinositide kinases were characterized in membrane extracts of rat submandibular gland cells. Both phosphatidylinositol (PI) 4-kinase and phosphatidylinositol-4-phosphate (PI(4)P) 5-kinase phosphorylated endogenous substrates in reactions that were linear for up to 5 min, were activated by Mg²⁺ and showed maximal activity around neutral pH. PI 4-kinase was stimulated by Triton X-100 at an optimal concentration of 0.22%, but the detergent had an inhibitory effect on PI(4)P 5-kinase. Arachidonic acid (AA), at concentrations greater than 100 M, inhibited the activity of both enzymes in a dose-dependent manner. The inhibitory effect was replicated by other unsaturated fatty acids, but not by a saturated fatty acid of the sn-20 series. The nature of AA inhibition of the kinases was examined in enzyme kinetic studies with exogenous phosphoinositide and adenosine 5-triphosphate (ATP) substrates. Lineweaver-Burk plots of PI 4-kinase activity showed that AA had no effect on the apparent K _m for either PI or ATP, but that the fatty acid significantly reduced V _max (PI) from 331 to 177 pmol.mg^–1.min^–1 and V _max (ATP) from 173 to 59 pmol.mg^–1.min^–1. This inhibitory action was consistent for PI(4)P 5-kinase kinetics, where again, AA did not alter apparent K _m values, but lowered V _max for both PI(4)P and ATP by around 50%. Since the combination of a reduced V _max and an unchanged K _m value indicates noncompetitive enzyme inhibition, it is proposed that AA regulates phosphoinositide cycle activity in submandibular gland cells by acting as a noncompetitive inhibitor of PI 4-kinase and PI(4)P 5-kinase.     

Neuromuscular function and mechanical efficiency of human leg extensor muscles during jumping exercises

The influence of prestretch amplitude on the mechanical efficiency was examined with 5 subjects, who performed 5 different series of vertical jumps, each of which differed with respect to the mechanics of the knee joint action during the prestretch (eccentric) phase of the contact on the floor. Electromyographic activity was recorded from the major extensor muscles during the entire work period of 1 min per series. In addition, expired air was collected during the test and recovery for determination of energy expenditure. Mechanical work was calculated from the vertical displacement of the body during the jumps. The results indicated that high net efficiency of 38.7% was observed in condition where amplitude of knee bending in eccentric phase was small. In large range motion the corresponding net efficiency was 30.1%. In jumps where no prestretching of extensor muscles ocurred the net efficiency was 19.7%. The high efficiency of small amplitude jumps was characterized by low myoelectrical activity of the leg extensor muscles during the positive (concentric) work phase. In addition, the small amplitude jumps had shorter transition time in the stretch-shortening cycle, high average eccentric force and high stretching speed. Therefore the results suggest that the restitution of elastic energy, which was also related to the length change and stiffness of the muscles during stretch, plays an important role in regulating the mechanical efficiency of work.     

QUANTIFICATION OF THE REM SLEEP CYCLE AS A RHYTHM

The goal of this study was to develop an objective quantitative method for representing the temporal organization of sleep in terms of the period and rhythmicity of REM sleep occurrences. Data on normative human sleep, already scored for stage REM and not stage REM, were subjected to a “binary autocorrelation.” The mean period over 92 nights of sleep for 10 Ss was 101.5 min and quite stable. Data is also presented on variability of the rhythm in terms of an “index of rhythmicity.” Measures of temporal organization may prove to be as significant for sleep research as amount of the various sleep stages.     

A method for assessing muscle fatigue during sprint exercise in humans using a friction-loaded cycle ergometer

This study investigated the mechanical changes induced by muscle fatigue caused by repeated sprints and determined whether a friction-loaded cycle ergometer has any advantages for assessing muscle fatigue. Nine subjects performed 15 sprints, each of 5 s with a 25-s rest, on a friction-loaded cycle ergometer. The averaged force, power and velocity of each push-off were calculated. Maximal power decreased by 17.9%, with a concomittent slowing of muscle contraction, but without any change in the maximal force. These results demonstrated that repeated sprints slow down muscle contraction, leading to a fall in maximal power without any loss of force. This would suggest that fast twitch fibres are selectively fatigued by repeated sprints. However, the ergometer used in the present study made it difficult to evaluate the relative influences of contraction velocity and sprinting time. This was certainly the most important limitation. On the other hand, it showed the advantage of measuring instantaneous power and total work dissipated in the environment simultaneously. It also permitted a force-velocity relationship to be obtained from a single sprint and this relationship is known to be closely related to the muscle fibre composition. Accepted: 5 March 1998     

Superior cycle control with a contraceptive vaginal ring compared with an oral contraceptive containing 30 microg ethinylestradiol and 150 microg levonorgestrel: a randomized trial

BACKGROUND: This trial was conducted to compare cycle control with vaginal ring a combined contraceptive vaginal ring, and a combined oral contraceptive (COC) delivering 30 mug ethinylestradiol (EE) and 150 mug levonorgestrel. METHODS: This open-label, randomized, multi-centre, Phase III study involved adult women from 11 countries. Subjects were treated with either vaginal ring or a COC for 13 cycles (12 months). RESULTS: A total of 1030 subjects (vaginal ring, n=512; COC, n=518) comprised the intention-to-treat (ITT) population. The percentage of women in the ITT population who completed the trial was 70.9% for vaginal ring and 71.2% for the COC group. The incidence of breakthrough bleeding and spotting over cycles 2-13, the primary efficacy parameter, was lower with vaginal ring (range 2.0-6.4%) than the COC (range 3.5-12.6%), and for cycles 2 and 9 the lower incidence with vaginal ring was confirmed as statistically significant (P=0.003 and P=0.002 respectively). The incidence of intended bleeding was significantly higher over all cycles with vaginal ring (58.8-72.8%) than with the COC (43.4-57.9%). CONCLUSIONS: Cycle control with vaginal ring was excellent and superior to that of a COC containing 30 mug EE.     

<Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> Ats1p interacts with Nap1p,a cytoplasmic protein that controls bud morphogenesis

Saccharomyces cerevisiae ATS1 (-tubulin suppressor 1) was originally identified as a high-copy suppressor of class two -tubulin mutations and was proposed to have a regulatory role in coordinating the microtubule state with the cell cycle. Here, we show that Ats1p interacts with Nap1p, a cytoplasmic protein that regulates the activity of the Cdc28p/Clb2p complex. Loss of Nap1p results in a delayed switch from polar to isotropic bud growth. The delayed switch results in elongated buds. Nap1p and Ats1p interact in two-hybrid and co-immunoprecipitation assays. Both nap1 and ats1 cells have a Clb2p-dependent elongated bud morphology. Deletion of ATS1 partially suppresses the elongated bud morphology and benomyl resistance of nap1 mutants. Our results suggest Ats1p might regulate coordination of the microtubule state with the cell cycle through an interaction with Nap1p.Communicated by S. Hohmann     

Single-equation models for the tear film in a blink cycle: realistic lid motion.

We consider model problems for the tear film over multiple blink cycles that utilize a single equation for the tear film; the single non-linear partial differential equation that governs the film thickness arises from lubrication theory. The two models that we consider arise from considering the absence of naturally occurring surfactant and the case when the surfactant is strongly affecting the surface tension. The film is considered on a time-varying domain length with specified film thickness and volume flux at each end; only one end of the domain is moving, which is analogous to the upper eyelid moving with each blink. Realistic lid motion from observed blinks is included in the model with end fluxes specified to more closely match the blink cycle than those previously reported. Numerical computations show quantitative agreement with in vivo tear film thickness measurements under partial blink conditions. A transition between periodic and non-periodic solutions has been estimated as a function of closure fraction and this may be a criterion for what is effectively a full blink according to fluid dynamics.