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Free and phosphorylated thiamine were determined in the liver and 9 regions of rat nervous system, after i.p. injection of labeled thiamine. A compartmental model was used to evaluate the rates of the phosphorylation of thiamine and the complete dephosphorylation of its phosphates. Remarkable differences between the liver and the nervous tissue as well as among different nervous regions were shown: high rates were found in hypothalamus, midbrain and cerebellum, low rates in cerebral cortex.

A mathematical model of contracting skeletal muscle is studied. The model is composed by an elastic element (SE) in series with a contractile element (CE) that describes the cross bridge kinetics with a formulation derived by that proposed by Eisenberg and Hill (1978). An analytical study of the system of nonlinear partial differential equations of the model allows the existence and the uniqueness of the solution to be proved. A suitable approach to the numerical solution is defined and a series of numerical tests are performed. These tests lead to select an appropriate set of parameters and allow to compare model predictions and experimental observations on frog skeletal muscle.

The content of total thiamine radioactivity in some nervous structures and liver of the rat was determined in a steady state condition, using [thiazole-2-14C]thiamine as a tracer. The contents were analyzed by a mamillary type compartmental model which enabled us to calculate the influx and efflux fractional rate constants, turnover times, turnover rates and relative accuracy. Total thiamine turnover rates of the central nervous system regions were found to be ordered in the following sequence: cerebellum (0.55 microgram/g.h) greater than medullar and pons greater than spinal cord and hypothalamus greater than midbrain (plus thalamic area) and corpus striatum greater than cerebral cortex (0.16 microgram/g.h). Sciatic nerve turnover rate was 0.58 microgram/g.h. The turnover times were mainly between 5 and 10 h (range 2.4--16.4 h). The influx rate constants could be ordered as follows: cerebellum greater than hypothalamus, pons and medulla greater than corpus striatum, spinal cord, midbrain (plus thalamic area) and sciatic nerve greater than cerebral cortex. The results show in general a good agreement between turnover rate values and brain regional sensitivity to thiamine deficiency, the most vulnerable areas to thiamine depletion being those with the highest turnover rates.

The relaxation phase and its load dependence were studied in papillary muscles isolated from the left ventricle of rats of the following ages: 20 days, 2, 8, 18, and 24 months. The myofibrillar ATPase activity and the force-velocity relation were determined in each age group in order to characterize the kinetic properties of the contractile material. Both shortening velocity and myofibrillar ATPase activity showed a progressive reduction with maturation and aging. This observation suggested an age-dependent decrease in cross bridge formation rate. The relaxation phase was characterized by its duration and the maximum rate of tension decline in isometric conditions, and by the speed of relengthening in isotonic conditions. Relaxation became faster and of shorter duration with maturation from 20 days to 2 months and then became slower and of longer duration with further maturation and aging. The sensitivity of relaxation to changes in length or load was evaluated by measuring how much earlier tension declined in the presence of a given length change. An increase in load sensitivity of relaxation was observed during maturation from 20 days to 8 months. This increase was followed by a reduction during aging from 8 to 24 months. Such a biphasic trend of the age-related changes in load sensitivity of relaxation could result from the interplay between the progressive decrease in cross bridge formation rate and a reduction in activation decay rate. The latter was suggested by the prolongation of the relaxation phase and by the maintenance of developed tension during aging.

The effect of active shortening on tension development has been studied in rat papillary muscle preparations at 20 degrees C. Total tension at the peak of a contraction with a shortening phase is reduced below the value predicted by the isometric length-tension relationship. At the same instantaneous length, total tension is less in contractions with an isotonic phase than in isometric responses and the amount of the tension deficit increases with the extent of shortening. The influence of the time at which shortening takes place, on the amount of tension deficit, has been explored. The increase in [Ca2+]o does not affect the tension deficit. Conversely an increase in [Ca2+]o associated to addition of caffeine to the bathing solution almost abolished the effect of active shortening on tension development. Several possible explanations of the reduction of tension development by shortening are discussed.

Shortening deactivation is studied in isolated preparations of rat cardiac muscle by comparing tension redevelopment in partially fused isometric contractions with quick releases of different extent ("control" and "test" displacements). The active shortening takes place during the ascending phase of the 1st contraction of the couple and the comparison of redeveloped tension is carried out at the peaks of both 1st and 2nd contractions. The depressant effect of active shortening can be demonstrated at the 1st peak and also at the 2nd one, after the restimulation of the muscle. This effect slowly decreases with time and disappears within approximately one second after active shortening took place. By keeping constant the control shortening and changing the test shortening, a clear dependence of depressant effect on the amount of displacement can be shown in the 1st and in the 2nd contraction.

The choice of a suitable muscle model consistent with the mechanical behaviour of Rat papillary muscle at rest or during isotonic or isometric contraction has been considered. Three different preload levels within the ascending limb of the Frank-Starling curve have been used in five papillary muscles. Series elastic (SE) and parallel elastic (PE) length-tension relationships have been evaluated according to five mechanical muscle models using data from systolic and diastolic quick-release manoeuvres. For each preload level and muscle model, the time course of the force-velocity relationship of the contractile element (CE) has been calculated using a PDP 8/L digital computer. Several mechanical and biological characteristics are considered which exclude four of the five selected muscle models. The Maxwell model with a damper seems to be the most suitable mechanical model for Rat papillary muscle. According to the selected muscle model it seems that VCEmax is dependent on muscle length variation; the reliability of this parameter as an index of myocardial contractility is discussed.

To calculate the kinetic parameters of thiamine monophosphate transport across the rat blood-brain barrier in vivo, different doses of a [35S]thiamine monophosphate preparation with a specific activity of 14.8 mCi.mmol-1 were injected in the femoral vein and the radioactivity was measured in arterial femoral blood and in the cerebellum, cerebral cortex, pons, and medulla 20 s after the injection. This short experimental time was used to prevent thiamine monophosphate hydrolysis. Thiamine monophosphate was transported into the nervous tissue by a saturable mechanism. The maximal transport rate (Jmax) and the half-saturation concentration (Km) equaled 27-39 pmol.g-1.min-1 and 2.6-4.8 microM, respectively. When compared with that of thiamine, thiamine monophosphate transport seemed to be characterized by a lower affinity and a lower maximal influx rate. At physiological plasma concentrations, thiamine monophosphate transport rate ranged from 2.06 to 4.90 pmol.g-1.min-1, thus representing a significant component of thiamine supply to nervous tissue.

Different steps of the metabolism of thiamine (T), thiamine mono- (TMP), pyro- (TPP) and triphosphate (TTP) in the cerebellum, brainstem, cerebral cortex and the sciatic nerve were evaluated in the rat in vivo. The radioactivity of T and its phosphoesters was determined at fixed time intervals (0.5-240 h) after an intraperitoneal injection of [14C]T (30 micrograms:1.25 muCi), under steady state conditions. The dynamics of thiamine compounds was evaluated using a compartmental mathematical model that allowed the fractional rate constants (FRC), turnover rates (TR) and turnover times to be calculated. The phosphorylation of T to TPP and the dephosphorylations of TPP to TMP and TMP to T could be estimated in all the structures investigated. Their turnover rates were found to be ordered in the sequence: cerebellum greater than brainstem greater than cerebral cortex greater than sciatic nerve. The transphosphorylation of TPP to TTP was so small that it could not be determined in a reliable way. Regional differences were found both the rate and in the composition of T and TMP mixture released from nervous structures. The shortest turnover time of TPP was found in the cerebellum, while the sciatic nerve exhibited the fastest renewal of T and TMP. In all the structures investigated TPP had a rather short turnover time, suggesting that its function might be associated to a rapid conversion into chemically different forms. The possible relationships between the rates of turnover of T compounds are the sensitivity of the nervous structures to T deficiency are discussed.

The load sensitivity of the relaxation phase was studied in rat papillary muscle, with isotonic afterloaded contractions and stretches applied after the peak of isometric twitches. The tension decay occurred earlier in isotonic than in isometric contractions. When a central region of the preparation was marked with small stainless steel pins, a lengthening of this region could be shown during relaxation of isometric (fixed end) contractions. This lengthening was earlier and faster in isotonic afterloaded contractions. Therefore the sensitivity of relaxation to load or length changes could be described in the context of the general mechanism of relaxation which takes into account the non uniform behaviour of the muscle and the internal movement during contractions. Interventions which decelerate the activation decay rate had different effects on the load dependence of relaxation. Caffeine addition and substitution of strontium for calcium abolished the load sensitivity while a temperature reduction had no influence on it.

The elastic and mechanical behaviour of isolated papillar muscle was studied in aged and in young rats. The spontaneous fall in tension (stress relaxation phenomenon) and the duration of the active state of the contractile element during the isometric and isotonic contraction were greater in aged than in young rats. The elastic properties of the elements in parallel and in series and the intensity of the active state in the contractile elements, on the contrary, did not exhibit any significant difference between aged and young rats.

The effect of stretch on the time-course of relaxation has been investigated in rat papillary muscle. In control conditions, a quick stretch carried out at or after the peak of an isometric twitch promotes an earlier and faster relaxation. Following inotropic interventions which decelerate the activation decay rate (caffeine addition and Ca2+ replacement with Sr2+) the onset and time-course of relaxation are unaffected by the stretch.

The transport of thiamine (T) and thiamine monophosphate (TMP) across the blood-brain barrier was measured in vivo in the rat. Different doses of [14C]T (15-550 nmol) and [14C]TMP (11-110 nmol) were injected into the femoral vein. The content of T and its phosphoesters in blood and brain tissue (cerebellum, pons, medulla and cerebral cortex) 20 s after the injection was determined radiometrically after electrophoretic separation. Blood flow and blood volume in the same regions of the brain was also determined. Both T and TMP entered rapidly the cerebral tissue, where they were found chemically unmodified. The cerebral tissue extracted less than 7% of plasma T. At physiological plasma T concentrations, the rate of transport ranged from 0.43 to 0.65 nmol X g-1 X h-1 with only minor differences among the various regions. T was transported into the nervous tissue by two separate mechanisms: one saturable, that at physiological plasma T levels accounted for 95% (cerebellum) to 91% (cerebral cortex) of the total T taken up, and one non-saturable, that was most efficient in the cerebral cortex. The Km (half-saturation constant) of the former transport mechanism ranged from 1.95 to 2.75 nmol X ml-1 in the 4 areas investigated. Vmax (maximal transport rate) values ranged from 6 to 9 nmol X g-1 X h-1, the highest value being found in the cerebellum. The overall transport rate of TMP was on average 5-10 times as low as that of T and also showed a saturable and a non-saturable component. Both components were slower than those observed for T.

When caffeine plus calcium is added to the perfusing medium, isometric relaxation of rat myocardium is no longer affected by length changes occurring during the twitch. The dependence of isometric relaxation on the initial muscle length is still present and more pronounced after caffeine addition.

In order to estimate wave propagation "in vitro", aortic compliance has been studied by means of rings and stripes from 6 adult male Wistar rats. Lengthening and shortening maneuvres have been performed; the pulse transmission rate was evaluated by calculating the elastic modulus during the maneuvres. A Norepinephrine infusion has been done. Results obtained show that transmission rate is higher in shortening than in lengthening phases. Norepinephrine did not significantly affect the values. The rate values were not greatly different whether "in vitro" or "in vivo". By increasing specimen stress, the rate too was increased, but the amount was lower than "in vivo" values. Hence it was assumable that rate variations "in vivo", should be partially influenced by elastic modulus, and a vegetative intervention should be implied.

In order to investigate internal shortening effects upon time course of active state, a study on rat papillary muscle has been carried out. Internal (or CE) shortening was evaluated according to a mechanical muscle model in which parallel elastic (PE), series elastic (SE), contractile (CE) elements are represented. CE force-velocity-shortening-time relationships have been used to describe time course of muscle active state in isometric, completely isotonic, afterloaded, and stopped isotonic contractions. The results obtained suggest that, regardless of type of contraction utilized, the parameters of muscle active state are strongly influenced by the amount of CE shortening.

Length changes of consecutive, 0.5-0.8 mm long segments of frog single muscle fibres were studied by photoelectric recording of opaque markers placed on the fibre surface. There was a marked redistribution of segment length during an ordinary isometric contraction (fixed fibre ends) at both 2.15 and 2.6-2.8 microns sarcomere length. This length redistribution can explain the tension 'creep' that occurs during standard isometric contractions on the descending limb of the length-tension relation. Length clamp of individual segments eliminated tension creep completely. Active force of length-clamped segments was investigated within the range 2.20-3.65 microns sarcomere length. The descending limb of the length-tension relation (determined in segments where no tension creep occurred) was not strictly linear but had a slightly sigmoid shape. Active force was reduced to zero at a sarcomere length close to 3.65 microns. While isometric force varied only moderately between different segments, the velocity of unloaded shortening (V0) was found to vary greatly (by 22-50%) along the length of a fibre. V0 did not correlate with the passive resistance to a length change, the isometric force or the cross-sectional area of the individual segments. Local differences of the internal milieu and/or coexistence of myosins of different kinetic properties within a single fibre may account for the observed differences in V0.