Search PubMed⌕ Search

Biomedical subjects

V Lombardi

Publications and source records attributed to V Lombardi.

At least 73 records · Page 4Linked to original sources
PubMed1989

The recovery of tension in transients during steady lengthening of frog muscle fibres.

The tension transients following step length changes imposed on tetanized muscle fibres during the steady phase of force response to lengthening were determined at different velocities. At low velocities the early partial recovery after a step was smaller and slower than under isometric conditions, while the speed of the final total recovery was faster. The degree of depression of the early recovery and the speed of the final recovery increased with the lengthening velocity. At a given lengthening velocity the speed of the total recovery depended on size and direction of the steps, increasing from the region of the larger releases to that of the larger stretches. The changes in the early partial recovery are explained qualitatively by the theory of Huxley and Simmons (1971), while the changes in the speed of the final recovery are explained by assuming that detachment of cross-bridges is negligible until a certain range of cross-bridge strain is reached, and then increases rapidly. It is also necessary to assume that cross-bridges detached in this way re-attach much more rapidly than when they detach on completion of their cycle during shortening.

Animals↗
PubMed1988

The mechanisms of force enhancement during constant velocity lengthening in tetanized single fibres of frog muscle.

The present paper deals with the mechanism responsible for the force enhancement in response to constant velocity lengthenings imposed at the plateau of isometric tetanic contractions. Experiments were performed at 3.5-5.6 degrees C on single fibres isolated from the tibialis anterior muscle of the frog. The resting sarcomere length was about 2.15 microns. The large variation observed in the characteristics of the force responses to lengthening and therefore in the shape of the force-lengthening velocity relations is mainly determined by the degree of longitudinal dishomogenities of muscle fibres. "Homogeneous" fibres and individual "tendon-free segments" selected along them exhibited superimposable force-lengthening velocity relations with negligible shape variation. Stiffness and force increased with lengthening, reaching, approximately steady values, respectively 10-15% and 60-70% greater than at the isometric tetanus plateau, at velocities of about 0.1 micron/s per half-sarcomere (hs). At higher lengthening velocities stiffness remained practically unvaried, whereas the steady component of the force responses continued to increase, reaching asymptotically a maximum value, about 100% greater than at the isometric tetanus plateau, at velocities between about 0.5 and 1 micron/s per hs. The amount of lengthening required to attain the peak value was smaller for stiffness (10 nm per hs) than for force (12-14 nm per hs). In terms of 1957 model of A. F. Huxley, the results indicate that lengthening (i) potentiates the mechanical output of muscle by increasing the number and the degree of extension of attached cross-bridges, (ii) enhances the speed of cross-bridge attachment and detachment.

Animals↗
PubMed1987

Stiffness of frog muscle fibres during rise of tension and relaxation in fixed-end or length-clamped tetani.

Stiffness measurements were performed during the rise, the plateau and the relaxation of tetanic contractions both in whole single muscle fibres and in tendon-free fibre segments under either fixed-end or length-clamp conditions. Fibres were isolated from the tibialis anterior muscle of the frog. Experiments were performed at 2-6 degrees C. Changes in length of tendon-free fibre segments were monitored by means of a "striation follower", an opto-electronic device which, during contraction, measured sarcomere displacement at the level of two selected regions of a fibre. Fast length perturbations imposed at one tendon end of a fibre during the plateau of tetanic contractions distribute uniformly along its length. During the tetanus rise stiffness led isometric tension in whole fibres under fixed conditions as well as in tendon-free fibre segments under length-clamp conditions. It was confirmed that a significant part of the unlinearity of T1 relations is determined by tendon compliance. During the isometric phase of relaxation in fixed-end tetani, the decline of tension led that of stiffness both in whole fibres and in tendon-free fibre segments. It is concluded that the shift observed between stiffness and tension during tetanus rise and relaxation represents a true specific event in the contractile process.

Animals↗
PubMed1986

A velocity-dependent shortening depression in the development of the force-velocity relation in frog muscle fibres.

During the onset of activation in isolated frog muscle fibres the development of the force-velocity (T-V) relation was determined by imposing single and double ramp releases. The experiments were performed at 3.5-6 degrees C or 19-22 degrees C and at a starting sarcomere length of about 2.25 micron. A velocity- and time-dependent shortening deactivation was shown to exist during the development of contraction. It was found that, early during the tetanus rise, at submaximal levels of activation, the values of T (the steady force exerted by the muscle fibres at any velocity of shortening V lower than V0) were significantly affected by previous conditioning shortening. Conditioning shortening at lower speeds led to potentiation of T and, at higher speeds, to depression. Both these effects were independent of the amount of shortening and, in addition, were not present at the tetanus plateau. At each given time or isometric tension throughout the tetanus rise the values of T. normalized for those determined at the same velocities at the tetanus plateau, were found to be inversely correlated with the actual velocities of shortening. The slope of this relation (a measure of the velocity-dependent shortening deactivation) decreased exponentially with time, attaining, in six fibres at low temperature, 10% of its initial value within 26-73 ms. The results may be explained in terms of a cross-bridge model of contraction by assuming that the rate of development of activation is controlled by the rate of release of the Ca2+ as well as by the velocity at which the muscle fibres are allowed to shorten and in turn by the actual number of attached cross-bridges.

Animals↗
PubMed1984

Pressure-induced metabolic activation of platelets demonstrated in vitro by determination of malondialdehyde formation.

Platelet rich plasma (PRP) exposed in vitro to 200 mm Hg above atmospheric pressure showed a significant increase in malondialdehyde (MDA) formation compared to PRP at atmospheric pressure. This difference is also evident when platelets are incubated with arachidonic acid. The increase of MDA demonstrates that the increased beta-thromboglobulin and platelet factor 4 in plasma and the shape changes of platelets after pressure stimulation in vitro that were described in a previous paper result from the release reaction. Pressure-induced effects in vivo are discussed.

Blood Platelets↗
PubMed1984

Force-velocity relation and stiffness in frog single muscle fibres during the rise of tension in an isometric tetanus.

The force-velocity (T-V) relation and the force-extension (T1) relation are determined at present times and at increasing isometric tensions during a tetanic contraction in frog single muscle fibres in which the passive compliance in series with the sarcomeres was made very small. The slope of the instantaneous T1 relation, the fibre stiffness, increases roughly in proportion to the level of the rising isometric tension at which the measurements were made. The value of V0 (the velocity of shortening under zero load) is time-independent, whereas the force T exerted during shortening at any velocity V lower than V0 increases gradually with time after the beginning of the tetanus volley and attains its steady state level before the isometric tension has attained the tetanus plateau and the fibre stiffness its final value. It is concluded that the delay of the development of the isometric tension and of the fibre stiffness with respect to the development of the T-V relation is determined by a specific factor of the contractile process. It is interesting to note that in a cross-bridge model of contraction, in which the value of the rate constant for cross-bridge formation is moderate, the recruitment of actin sites which is measured by the characteristics of the instantaneous T-V relation, is expected to lead significantly the actual cross-bridge formation, which is measured both by the instantaneous isometric tension and by the instantaneous stiffness.

Animals↗
PubMed1984

Development of force-velocity relation and rise of isometric tetanic tension measure the time course of different processes.

The time course of the contractile process was investigated in the presence of AR-L 115BS, a twitch potentiator which is thought to increase the rate of the Ca2+ binding by troponin and to improve the Ca2+ mobilization from the sarcoplasmic reticulum. AR-L 115BS increased markedly the rate of development of the force-velocity (T-V) relation and of the isometric tetanic tension. The effect on the rate of development of the T-V relation was however substantially more intense than that on the speed of rise of tetanic tension, thus reducing considerably the isometric tension level at which the T-V relation attained its final characteristics. The velocity of shortening under zero load and the degree of curvature of the T-V relation were not affected by AR-L 115BS. These findings support the view that the development of the T-V relation and the rise of the isometric tetanic tension measure the time course of two different processes.

Animals↗
PubMed1984

The maximum velocity of shortening during the early phases of the contraction in frog single muscle fibres.

The maximum velocity of shortening (Vmax) was determined at preset times during the development and the plateau of isometric tetani in single fibres isolated from the tibialis anterior muscle of the frog. Experiments were performed at low temperature (3.6-6 degrees C) and at about 2.25 micron sarcomere length. The controlled velocity release method was used. Vmax was measured by determining the lowest velocity of release required to keep the tension at zero. Extreme care was taken in dissection and mounting of the fibres in order to make the passive series compliance very small. The value of Vmax at the end of the latent period for the development of isometric tension (at 4.5 degrees C about 10 ms after the beginning of the stimulus volley) was already the same as later during either the tension rise or at the plateau of isometric tetani. These results show that the value of Vmax of intact fibres is independent of time and activation subsequent to the latent period, and suggest that the cycling rate of the crossbridges may thus attain its steady-state value just at the end of the isometric latent period.

Animals↗
PubMed1983

Development of force-velocity relation, stiffness and isometric tension in frog single muscle fibres.

The force-velocity (T-V) relation and the force-extension (T1) relation from single fibres isolated from the muscle tibialis anterior of the frog were determined at present times during the rise of tension and the plateau of an isometric tetanus. During the rise of an isometric tetanus the value of V0 (the velocity of shortening at zero load) remained constant, whereas both the force T exerted during shortening at a given velocity lower than V0 and the slope of the instantaneous T1 curve increased with time after the beginning of the stimulus volley. However, while the value of T (a measure of the level of activation) attained its final value much before the isometric tension attained the plateau, the slope of the T1 curve (the fibre stiffness, a measure of the number of attached crossbridges) increased throughout the course of tension development. In six muscle fibres, at 2.25 microns sarcomere length and at about 4 degrees C, at a time during the tetanus rise when T had attained 81% of its final value, the fibre stiffness and the isometric tension had risen, respectively, to 50% and to 38% of the values attained at the tetanus plateau. Later, when T had already attained 99% of its final value, the fibre stiffness and the isometric tension had risen, respectively, to 80% and to 76% of their plateau values. In the same muscle fibres, the average value for the amount of step release required to drop the plateau tetanic tension to zero was only 8.62 nm (observed) or 6.16 nm (extrapolated from the linear part of the T1 curves) per half sarcomere. It is concluded that the large delay in the development of the isometric tension with respect to the development of T, and therefore with respect to the development of the whole T-V relation, cannot be attributed to the passive series compliance of the muscle fibres. It is likely that this delay is due to a specific event in the contractile process.

Animals↗
PubMed1982

Acute collapse of thoracic vertebral bodies after long-term steroid treatment in leprosy.

Prolonged steroid therapy may produce sufficient osteoporosis to permit collapse of a vertebral body. Decompressive laminectomy and aggressive postoperative therapy can give gratifying results. One can postulate whether these results are related in any way to the protective effect of the steroid being taken during the time of the collapse of the vertebral bodies.

Adult↗
PubMed1981

Force-velocity relation in deuterium oxide-treated frog single muscle fibres during the rise of tension in an isometric tetanus.

1. The force-velocity (P-V) relation from a single fibres isolated from the semitendinosus muscle of the frog was determined at pre-set times during the rise of tension and the plateau of isometric tetani. The controlled-velocity release method was used. Experiments were performed at about 2.25 micrometers sarcomere length and at 3-4 degrees C or at 19-21 degrees C. 2. Replacing H2O with D2O resulted in a rapid large reduction of the peak twitch tension and of the speed of development of twitch and tetanic tensions. The tetanic tension (P0) was usually reduced, in certain fibres to as low as 5% of the value in H2O-Ringer solution. 3. The depression of twitch and tetanus characteristics was followed by a recovery, the duration of which varied greatly in different fibres. During the recovery period previous conditioning activity potentiated the tetanus characteristics. 4. After the end of the recovery period in D2O-Ringer solution both the peak twitch tension and the speed of development of tetanic tension was still greatly depressed, whereas the value of P0 was slightly greater than in H2O-Ringer. The speed of rise of isometric tension after a quick release imposed at the tetanus plateau was reduced in D2O-Ringer, usually to about 50% of the value in H2O-Ringer. 5. D2O increased the development time of the P-V relation and produced a conspicuous increase in the degree of its curvature. The value of V0 (the velocity of shortening at zero load) was not significantly depressed by D2O and it was the same independent both of the time after the beginning of stimulation and of the isometric tension at which the measurement was made. The P-V relation attained its final characteristics before the isometric tension reached the plateau. During the recovery period in D2O-Ringer, at the plateau of isometric tetani of different size, the relative force exerted at a given velocity of shortening was constant. 6. In D2O-treated fibres, NO3- and caffeine (i) potentiated the peak twitch tension and the speed of development of tetanic tension without affecting significantly the speed of the redevelopment of tension after a quick release imposed at the tetanus plateau and (ii) reduced the development time of the P-V relation, but did not affect either the degree of its curvature or the value of V0 and P0. 7. The results are discussed by assuming that the release of Ca2+ from the sarcoplasmic reticulum is a rate-limiting process for the development of activation and in turn for the development of isometric tension. In terms of the cross-bridge model of Huxley (1957), the time or Ca2+-dependent factor of activation appears to be the recruitment of actin sites for cross-bridge formation, whereas the value of the rate constants regulating the cross-bridge kinetics appears to be time and Ca2+-independent.

Animals↗
PubMed1979

The variation of characteristics of twitch and tetanic contractions with sarcomere length in isolated muscle fibres of the frog.

The relation between sarcomere length, tension and time course of tension development in twitch and tetanic contractions at 20 degrees C was determined for isolated fibres from the semitendinosus muscle of the frog (Rana esculenta). In twenty fibres at about 2.15 micron sarcomere length, the peak twitch tension, the maximum tetanic tension and the twitch/tetanus ratio ranged, respectively, from 0.22 to 1.6 kg/cm2, from 2.13 o 3.96 kg/cm2 an from 0.07 to 0.53. The peak twitch tension was found to be: i) directly correlated with the twitch/tetanus ratio and the time to the peak of the first derivative of the twitch tension, ii) inversely correlated with the time to the peak of the first derivative of tetanic tension. No significant correlation was found between the maximal tetanic tension and the peak twitch tension or the twitch/tetanus ratio. Peak twitch tension and twitch/tetanus ratio were not correlated with the fibre cross-sectional area which ranged from 1.052 to 6,283 micron2. Sarcomere length-tension curves for twitch and tetanic isometric contractions at 20 degrees C were determined in twelve fibres. Increases in sarcomere length from about 2.15 to 2.85 micron produced, depending on the peak twitch tension or the twitch/tetanus ratio at about 2.15 micron, either decrease and no change or increase in peak twitch tension, but constantly enhanced the twitch/tetanus ratio and the degree of this potentiation was inversely correlated with the twitch/tetanus ratio at 2.15 micron. Increase in sarcomere length above 2.15 micron did not alter the course of the early development of twitch and tetanic tensions, reduced considerably the variation in peak twitch tension and twitch/tetanus ratio, without altering that of tetanic tension and swamped the correlation between the peak twitch tension and the time to peak of the differentiated twitch tension. However, the peak twitch tension at about 2.85 micron resulted to be directly correlated with the peak twitch tension at about 2.15 micron and in addition the relative length-dependent change in the time of the peak of the first derivative of the twitch tension resulted to be directly correlated with the relative length-dependent change in the peak twitch tension. It is concluded that both the duration of the active state and the rate factors of activation contribute to the determining of the large variation in peak twitch tension at about 2.15 micron, whereas the length-dependent increase in twitch/tetanus ratio appears to be mainly determined by prolongation of the active state duration.

Animals↗
PubMed1979

Development of activation and rise of tension in an isometric tetanus.

The force-velocity relation was determined at preset times after the beginning of the stimulus volley, either at low isometric tensions, during the raising phase of tetanus and the recovery after a quick release imposed at the plateau, or at the steady state. It is concluded that the characteristics of the force-velocity relation may be used to express the actual level of activation at any time during the contractile process and that the mechanical output of frog twitch muscle fibres implies two distinct steps: (i) activation of the contractile machinery; (ii) production of force.

Animals↗