Force-velocity relationship and maximal power on a cycle ergometer. Correlation with the height of a vertical jump

The force-velocity relationship on a Monark ergometer and the vertical jump height have been studied in 152 subjects practicing different athletic activities (sprint and endurance running, cycling on track and/or road, soccer, rugby, tennis and hockey) at an average or an elite level. There was an approximately linear relationship between braking force and peak velocity for velocities between 100 and 200 rev.min-1. The highest indices of force P0, velocity V0 and maximal anaerobic power (Wmax) were observed in the power athletes. There was a significant relationship between vertical jump height and Wmax related to body mass.     

Control of total peripheral resistance during hyperthermia in rats

To elucidate the effect of blood volume on the circulatory adjustment to heat stress, we studied alpha-chloralose-anesthetized rats at three levels of blood volume: normovolemia (NBV), hypervolemia (HBV; +32% plasma volume by isotonic albumin solution infusion), and hypovolemia (LBV; -16% plasma volume by furosemide administration). Body surface heating was performed with an infrared lamp to raise arterial blood temperature (Tb) at the rate of approximately 0.1 degree C/min. Before heating, central venous pressure (CVP) was significantly higher in HBV (0.41 +/- 0.25 mmHg) and lower in LBV (-1.44 +/- 0.22 mmHg) than in NBV (-0.41 +/- 0.10 mmHg). The Tb at which CVP started to decrease was approximately 40 degrees C in HBV, approximately 41 degrees C in NBV, and approximately 42 degrees C in LBV, and it decreased by 1.53 +/- 0.14, 1.92 +/- 0.24, and 0.62 +/- 0.14 mmHg from 37 to 43 degrees C of Tb in HBV, NBV, and LBV, respectively. Stroke volume was closely correlated with CVP, and this relationship was not affected by Tb. Heart rate responses to the raised Tb were similar among the three groups. Mean arterial pressure (MAP) was not affected by blood volume modification or CVP and was maintained at preheating (Tb 37 degrees C) level until Tb rose to 40 degrees C. Above this Tb, MAP increased until Tb reached 43 degrees C (+30-40 mmHg) for all three groups. Total peripheral resistance (TPR) was inversely correlated with CVP, and the slope of the linear relationship between TPR and CVP in LBV was three- to fourfold steeper than in NBV or HBV.(ABSTRACT TRUNCATED AT 250 WORDS)     

Thermal stress and Ca-independent contractile activation in mammalian skeletal muscle fibers at high temperatures.

Temperature dependence of the isometric tension was examined in chemically skinned, glycerinated, rabbit Psoas, muscle fibers immersed in relaxing solution (pH approximately 7.1 at 20 degrees C, pCa approximately 8, ionic strength 200 mM); the average rate of heating/cooling was 0.5-1 degree C/s. The resting tension increased reversibly with temperature (5-42 degrees C); the tension increase was slight in warming to approximately 25 degrees C (a linear thermal contraction, -alpha, of approximately 0.1%/degree C) but became more pronounced above approximately 30 degrees C (similar behavior was seen in intact rat muscle fibers). The extra tension rise at the high temperatures was depressed in acidic pH and in the presence of 10 mM inorganic phosphate; it was absent in rigor fibers in which the tension decreased with heating (a linear thermal expansion, alpha, of approximately 4 x 10(-5)/degree C). Below approximately 20 degrees C, the tension response after a approximately 1% length increase (complete < 0.5 ms) consisted of a fast decay (approximately 150.s-1 at 20 degrees C) and a slow decay (approximately 10.s-1) of tension. The rate of fast decay increased with temperature (Q10 approximately 2.4); at 35-40 degrees C, it was approximately 800.s-1, and it was followed by a delayed tension rise (stretch-activation) at 30-40.s-1. The linear rise of passive tension in warming to approximately 25 degrees C may be due to increase of thermal stress in titin (connectin)-myosin composite filament, whereas the extra tension above approximately 30 degrees C may arise from cycling cross-bridges; based on previous findings from regulated actomyosin in solution (Fuchs, 1975), it is suggested that heating reversibly inactivates the troponin-tropomyosin control mechanism and leads to Ca-independent thin filament activation at high temperatures. Additionally, we propose that the heating-induced increase of endo-sarcomeric stress within titin-myosin composite filament makes the cross-bridge mechanism stretch-sensitive at high temperatures.     

The effect of body temperature on the locomotory energetics of lizards

Oxygen consumption (VO2), carbon dioxide production (VCO2), and stamina were measured in the lizard Tupinambis nigropunctatus running at sustainable and non-sustainable velocities (v) on a motor-driven treadmill. Three experimental groups were measured: field-fresh animals at body temperature (Tb) = 35 degrees C and laboratory-maintained animals at Tb = 35 and 25 degrees C. Mean preferred Tb was determined to be 35.2 degrees C. At 35 degrees C, field-fresh animals had a greater maximal oxygen consumption (VO2max corr) (4.22 vs 3.60 ml O2 g-0.76h-1) and a greater endurance. The net cost of transport (slope of VO2 on v) did not differ between the groups (= 2.60 ml O2 g-0.76)km-1). Velocity at which VO2max is attained (MAS) is 0.84 km h-1. The respiratory exchange ratio (R) exceeded 1.0 at v above MAS, indicating supplementary anaerobic metabolism. At 25 degrees C, VO2max corr was lower (2.34 ml O2 g-0.76h-1) as was endurance, MAS occurring at 0.5 km h-1. Net cost of transport was not significantly different than at 35 degrees C. The effect of Tb on locomotory costs was analyzed for this lizard and other species. It was concluded that the net cost of transport is temperature independent in all species examined and the total cost of locomotion (VO2 v-1) is temperature dependent in Tupinambis (Q10 = 1.4-2.0) and all other species examined except one. The energetic cost of locomotion [(VO2active-VO2rest)v-1], previously reported to be temperature independent in lizards, is temperature dependent in Tupinambis (Q10 = 1.3-1.6) and in two other species.2r     

Oxygen consumption and body temperature of active and resting honeybees

We measured the energy turnover (oxygen consumption) of honeybees (Apis mellifera carnica), which were free to move within Warburg vessels. Oxygen consumption of active bees varied widely depending on ambient temperature and level of activity, but did not differ between foragers (>18 d) and middle-aged hive bees (7-10 d). In highly active bees, which were in an endothermic state ready for flight, it decreased almost linearly, from a maximum of 131.4 microl O(2) min(-1) at 15 degrees C ambient temperature to 81.1 microl min(-1) at 25 degrees C, and reached a minimum of 29.9 microl min(-1) at 40 degrees C. In bees with low activity, it decreased from 89.3 microl O(2) min(-1) at 15 degrees C to 47.9 microl min(-1) at 25 degrees C and 14.7 microl min(-1) at 40 degrees C. Thermographic measurements of body temperature showed that with increasing activity, the bees invested more energy to regulate the thorax temperature at increasingly higher levels (38.8-41.2 degrees C in highly active bees) and were more accurate. Resting metabolism was determined in young bees of 1-7 h age, which are not yet capable of endothermic heat production with their flight muscles. Their energy turnover increased from 0.21 microl O(2) min(-1) at 10 degrees C to 0.38 microl min(-1) at 15 degrees C, 1.12 microl min(-1) at 25 degrees C, and 3.03 microl min(-1) at 40 degrees C. At 15, 25 and 40 degrees C, this was 343, 73 and 10 times below the values of the highly active bees, respectively. The Q(10) value of the resting bees, however, was not constant but varied in a U-shaped manner with ambient temperature. It decreased from 4.24 in the temperature range 11-21 degrees C to 1.35 in the range 21-31 degrees C, and increased again to 2.49 in the range 30-40 degrees C. We conclude that attempts to describe the temperature dependence of the resting metabolism of honeybees by Q(10) values can lead to considerable errors if the measurements are performed at only two temperatures. An acceptable approximation can be derived by calculation of an interpolated Q(10) according to the exponential function (V(O(2))=0.151 x 1.0784(T(a))) (interpolated Q(10)=2.12).     

Microcalorimetric study of native cells of the microalgae Spirulina platensis in the temperature range (2-55)C

Thermal effects occurring upon heating a culture of blue-green microalgae Spirulina platensis in the temperature range 5-55 degrees C were studied. Under these conditions, an intensive heat evolution was observed. The heat evolution-versus-temperature curve has a peak with a maximum at approximately 45 degrees C and two distinct shoulders at approximately 25 and 40 degrees C. It was found that heat evolution Q at heating rates below 0.083 degree C/min does not change and is (381 +/- 20) J/g of dry biomass. It was concluded that this value is optimal for maintaining the cell viability (in particular, respiration) under anaerobic conditions, in the dark and in the stationary regime.     

Temperature dependence of ADP/ATP translocation in mitochondria

The temperature dependence of the adenine nucleotide exchange in mitochondria has been determined by employing a rapid mixing, quenching and sampling apparatus and the inhibitor quench-back exchange method. Thus the exchange is resolved down to 0.1 s. Rates are evaluated from accumulating the time-dependent progress at about 10 points. The exchange rate in liver mitochondria was determined from -10 degrees C to + 10 degrees C in the presence of 20% glycol, from 0 degrees C to 25 degrees C, and from 20 degrees C to 40 degrees C under partial inhibition by carboxyatractylate. The total range between -10 degrees C to + 40 degrees C has only one temperature break at 13 degrees C. From the Arrhenius plot between -10 degrees C to + 13 degrees C, EA approximately equal to 140 kJ and above 13 degrees C, EA approximately equal to 56 kJ is evaluated, corresponding to a Q10 of 8 and 2 respectively. In beef heart mitochondria the exchange rate was measured between 0 degrees C and 20 degrees C, and between 15 degrees C and 30 degrees C under partial inhibition with carboxyatractylate. There is a temperature break around 14 degrees C with EA approximately equal to 143 kJ between 0 degrees C and 14 degrees C and EA approximately equal to 60 kJ from 15 degrees C to 30 degrees C. The extrapolated translocation rates at 37 degrees C are 500 and 1800 mumol min-1 (g protein)-1 for rat liver and for beef heart mitochondria respectively. The temperature break is suggested to reflect a conformation change since there is no reversed break at low temperature, the temperature break changes in sonic particles and no lipid phase transition at 14 degrees C in mitochondria has been reported.     

Behavioral thermoregulation by hypoxic rats.

To address whether a shift in hypothalamic thermal setpoint might be a significant factor in induction of hypoxic hypothermia, behavioral thermoregulation was examined in 7 female Sprague-Dawley rats implanted with radiotelethermometers for deep body temperature (Tb) measurement in a thermocline during normoxia (PO2 = 125 torr) and hypoxia (PO2 = 60 torr). Normoxic rats (TNox) selected a mean ambient temperature of 19.7 +/- 1.4 (SE) degrees C and maintained Tb at 37.0 +/- 0.2 degrees C. Hypoxic rats selected a significantly higher ambient temperature (THox = 28.6 +/- 2.2 degrees C) but maintained Tb significantly lower at 35.5 +/- 0.3 degrees C. Without a thermal gradient (ambient temperature = 25 degrees C), Tb during hypoxia was 35.4 +/- 0.4 degrees C. The maintenance of a lower body temperature during hypoxia through behavioral thermoregulation despite having warmer temperatures available supports the hypothesis that the thermoregulatory setpoint of hypoxic rats is shifted to promote thermoregulation at a lower Tb, effectively reducing oxygen demand when oxygen supply is limited.     

Interaction of local and reflex thermal effects in control of forearm blood flow

We measured forearm blood flow (ABF) bilaterally on six subjects during 15-min periods of leg exercise and the first 10 min of recovery. One forearm (control) was kept at about 33 degrees C skin temperature in all experiments. In experiments at ambient temperature (Ta) of 15 degrees C, the other arm (experimental) was kept at about 26, 33, and 40 degrees C, respectively, during three successive cycles of exercise and recovery. ABF in the 26 degrees C forearm was linearly related to and averaged 42% of control. The relation of ABF in the 40 degrees C forearm to control ABF showed a bend at control ABF of 4-5 ml X 100 ml-1 X min-1. Below the bend, experimental ABF average 213% of control. Above the bend, experimental ABF averaged 5.09 ml X 100 ml-1 X min-1 above control. In four subjects, after heating the experimental forearm to 40 degrees C, we measured ABF for 25-30 min at rest in Ta of both 15 and 25 degrees C. At 25 degrees C Ta, ABF in the heated forearms rose gradually, but control ABF showed little change. At 15 degrees C Ta, the effect on ABF of local heating to 40 degrees C was much reduced, apparently due to reflex vasoconstrictor signals.     

Differential effects of temperature on E. coli and synthetic polyhydroxybutyrate/polyphosphate channels

Complexes of poly-(R)-3-hydroxybutyrate and inorganic polyphosphate (PHB/polyP), isolated from the plasma membranes of Escherichia coli or prepared synthetically (HB(128)/polyP(65)), form Ca(2+)-selective ion channels in planar lipid bilayers that exhibit indistinguishable gating and conductance characteristics at 22 degrees C. Here we examine the gating and conductance of E. coli and synthetic PHB/polyP complexes in planar lipid bilayers as a function of temperature from 15 to 45 degrees C. E. coli PHB/polyP channels remained effectively open throughout this range, with brief closures that became more rare at higher temperatures. Conversely, as temperatures were gradually increased, the open probability of HB(128)/polyP(65) channels progressively decreased. The effect was fully reversible. Channel conductance exhibited three distinct phases. Below 25 degrees C, as PHB approached its glass temperature (ca. 10 degrees C), the conductance of both E. coli and synthetic channels remained at about the same level (95-105 pS). Between 25 degrees C and ca. 40 degrees C, the conductance of E. coli and synthetic channels increased gradually with temperature coefficients (Q(10)) of 1.45 and 1.42, respectively. Above 40 degrees C, E. coli channel conductance increased sharply, whereas the conductance of HB(128)/polyP(65) channels leveled off. The discontinuities in the temperature curves for E. coli channels coincide with discontinuities in thermotropic fluorescence spectra and specific growth rates of E. coli cells. It is postulated that E. coli PHB/polyP complexes are associated with membrane components that inhibit their closure at elevated temperatures.     

Voltage-dependent calcium channels in ventricular cells of rainbow trout: effect of temperature changes in vitro

Voltage-dependent calcium channels (VDCC) in ventricular myocytes from rainbow trout (Oncorhynchus mykiss) were investigated in vitro using the perforated patch-clamp technique, which maintains the integrity of the intracellular milieu. First, we characterized the current using barium as the charge carrier and established the doses of various pharmacological agents to use these agents in additional studies. Second, we examined the current at several physiological temperatures to determine temperature dependency. The calcium currents at 10 degrees C (acclimation temperature) were identified as L-type calcium currents based on their kinetic behavior and response to various calcium channel agonists and antagonists. Myocytes were chilled (4 degrees C) and warmed (18 and 22 degrees C), and the response of VDCC to varying temperatures was observed. There was no significant dependency of the current amplitude and kinetics on temperature. Amplitude decreased 25-36% at 4 degrees C (Q(10) approximately 1.89) and increased 18% at 18 degrees C (Q(10) approximately 1.23) in control, Bay K8644 (Bay K)-, and forskolin-enhanced currents. The inactivation rates (tau(i)) did not demonstrate a temperature sensitivity for the VDCC (Q(10) 1.23-1. 92); Bay K treatment, however, increased temperature sensitivity of tau(i) between 10 and 18 degrees C (Q(10) 3.98). The low Q(10) values for VDCC are consistent with a minimal temperature sensitivity of trout myocytes between 4 and 22 degrees C. This low-temperature dependency may provide an important role for sarcolemmal calcium channels in adaptation to varying environmental temperatures in trout.     

The temperature dependence of some kinetic and conductance properties of acetylcholine receptor channels

We examined the temperature dependence of single-channel properties of the nicotinic acetylcholine receptor channel from clonal BC3H-1 cells over a range of 10-40 degrees C. We found temperature sensitivities (Q10 values) of 2-4 for the mean channel open time. The Q10 did not depend strongly on voltage and the voltage dependence of the mean open time was temperature-independent. The Q10 of closing rate of the long-lived open state was 3-4 but the Q10 of closing rate of the brief open state was independent of temperature. The duration of brief closures could be measured only between 10 and 25 degrees C. Since this approached the limit of the experimental time resolution, an accurate determination of the Q10 could not be made. The current decay due to desensitization after rapid application of high concentrations of agonist varied with a Q10 of about 2. The conductance of single channels (the inverse of the ion translocation rate) had a Q10 of 1.3-1.5. We found no obvious nonlinearities in the Arrhenius curves for any of the measured properties.     

Deformability and stability of erythrocytes in high-frequency electric fields down to subzero temperatures.

High-frequency electric fields can be used to induce deformation of red blood cells. In the temperature domain T = 0 degrees to -15 degrees C (supercooled suspension) and for 25 degrees C this paper examines for human erythrocytes (discocytes, young cell population suspended in a low ionic strength solution with conductivity sigma(25 degrees) = 154 microS/cm) in a sinusoidal electric field (nu = 1 MHz, E0 = 0-18 kV/cm) the following properties and effects as a function of field strength and temperature: 1) viscoelastic response, 2) (shear) deformation (steady-state value obtained from the viscoelastic response time), 3) stability (by experimentally observed breakdown of cell polarization and hemolysis), 4) electrical membrane breakdown and field-induced hemolysis (theoretical calculations for ellipsoidal particles), and 5) mechanical hemolysis. The items 2-4 were also examined for the frequency nu = 100 kHz and for a nonionic solution of very low conductivity (sigma(25 degrees) = 10 microS/cm) to support our interpretations of the results for 1 MHz. Below 0 degrees C with decreasing temperature the viscoelastic response time tau(res)(T) for the cells to reach steady-state deformation values d(infinity,E) increases and the deformation d(infinity,E)(T) decreases strongly. Both effects are especially high for low field strengths. The longest response time of approximately 30 s was obtained for -15 degrees C and small deformations. For 1 MHz the cells can be highly elongated up to 2.3 times their initial diameter a0 for 25 degrees and 0 degrees C, 2.1a0 for -10 degrees C and still 1.95a0 for -15 degrees C. For T > or = 0 degrees C the deformation is limited by hemolysis of the cells, which sets in for E0(lysis)(25 degrees) approximately 8 kV/cm and E0(lysis)(0 degrees) approximately 14 kV/cm. These values are approximately three times higher than the corresponding calculated critical field strengths for electrically induced pore formation. Nevertheless, the observed depolarization and hemolysis of the cells is provoked by electrical membrane breakdown rather than by mechanical forces due to the high deformation. For the nonionic solution, where no electrical breakdown is expected in the whole range for E0, the cells can indeed be deformed to even higher values with a low hemolytic rate. Below 0 degrees C we observe no hemolysis at all, not even for the frequency 100 kHz, where the cells hemolyze at 25 degrees C for the much lower field strength E0(lysis) approximately 2.5 kV/cm. Obviously, pore formation and growth are weak for subzero temperatures.     

The influence of thermoregulation on behavioural recovery from exercise in a lizard

1. Past work on the thermal preferences of Dipsosaurus dorsalis (Biard & Giard) has indicated that intense, exhaustive exercise causes these lizards to select a body temperature (33·5 °C) which is cooler than their preferred activity temperature of 40°C during the first 1–2 h of exercise recovery.
2. In order to test the hypothesis that the thermal regime selected by exhausted D. dorsalis is beneficial to the process of exercise recovery, lizards were forced to undergo both exhaustive and sprinting exercise at their preferred body temperature of 40°C. The peak speeds attained and the total distances travelled by these animals during these two different exercise protocols were measured and the animals were then forced to undergo a second bout of either sprinting or exhaustive exercise, following a 30–330 min recovery at either 20°C, 40°C or under a variable thermal regime which duplicated that selected by animals following exercise.
3. Animals recovering at a constant 40°C regained their ability to repeat exhaustive activity in less than 85 min, while animals recovering under the other two thermal regimes required between 85 and 100 min of recovery to be able to repeat this activity. Animals recovering at both 40°C and under the variable thermal regime regained their ability to repeat sprint behaviour within 60 min of recovery, while animals recovering at 20°C required more than 100 min of recovery to be able to repeat sprint behaviour.
4. These results formed the basis of the conclusion that the post-exercise behaviour selected by D. dorsalis retards the rate at which the animals recover their ability to repeat exhaustive exercise when compared with recovery at a constant 40°C but does not retard their ability to repeat sprint exercise.     

Thermodynamics of heat activation of single capsaicin ion channels VR1

Temperature affects functions of all ion channels, but few of them can be gated directly. The vanilloid receptor VR1 provides one exception. As a pain receptor, it is activated by heat >42 degrees C in addition to other noxious stimuli, e.g. acids and vanilloids. Although it is understood how ligand- and voltage-gated channels might detect their stimuli, little is known on how heat could be sensed and activate a channel. In this study, we characterized the heat-induced single-channel activity of VR1, in an attempt to localize the temperature-dependent components involved in the activation of the channel. At <42 degrees C, openings were few and brief. Raising the ambient temperature rapidly increased the frequency of openings. Despite the large temperature coefficient of the apparent activity (Q(10) approximately 27), the unitary current, the open dwell-times, and the intraburst closures were all only weakly temperature dependent (Q(10) < 2). Instead, heat had a localized effect on the reduction of long closures between bursts (Q(10) approximately 7) and the elongation of burst durations (Q(10) approximately 32). Both membrane lipids and solution ionic strength affected the temperature threshold of the activation, but neither diminished the response. The thermodynamic basis of heat activation is discussed, to elucidate what makes a thermal-sensitive channel unique.     

Behavioral thermoregulation of the toad, Bufo marinus: effects of air humidity

Two experiments were performed. The first tested the hypothesis that the toad, Bufo marinus, will select a lower ambient temperature under dry environmental conditions. This behavioral response would reduce evaporative water loss and facilitate survival on land. The second experiment measured the effects of temperature on evaporative water loss. In the first experiment, toads were placed in a thermal gradient (11-40 degrees C) for 3 days. On days 1 and 3, water-filled dishes were placed along the temperature gradient and humid air was circulated through the chamber. On day 2, water dishes were removed, and dry air was circulated through the chamber. Body temperature (Tb) was recorded with a cloacal thermistor. Selected Tb was approximately 8.6 degrees C lower during the dry conditions than during the humid conditions. The behavioral hypothermia took about 6 h to develop. In the second experiment, a reduction in Tb from 17.7 to 12 degrees C reduced evaporative water loss by 42%. Consequently, behavioral hypothermia of the toad is an important adaptation to dry environmental conditions.     

The dependence of the rate of heart contractions in the long-tailed Yakutsk suslik Citellus undulatus on environmental temperature]

Decrease of ambient temperature (Ta) leads to the increase of the heart rate (HR) in active ground squirrels C. undulatus by 5.3/min/1 degree C in summer and by 3.8/min/1 degree C in winter. In a hibernation state, the dependence of the HR on Ta was in a good agreement with equation HT = 2.53.exp.(0.1.Ta). On entering into hibernation and on arousal, the HR change outruns the corresponding body temperature (Tb) change by 1.5-2 hours. A maximum HR level (up to 400/min and more) was registered on arousal when Tb reached 17-20 degrees C. A minimal HR level (4-5/min) was observed during hibernation at Ta 2-5 degrees C. The maximum Ta, at witch C. undulatus was hibernating, reached 23-24 degrees C, the HR being 23-25/min.     

An infrared thermographic study of surface temperature in relation to external thermal stress in the Mongolian gerbil, Meriones unguiculatus

1. Temperatures of different body surface regions and deep body temperature (Tb) of unrestrained adult Mongolia gerbils exposed to ambient temperatures (Ta) of -10-35 degrees C were measured using infrared (i.r.) thermography and a thermocouple. 2. A strong positive linear relationship between the surface temperature and Ta was found. For Ta range -4-35 degrees C, the slope was lowest for the areas around the eyes and dorsal head, and steepest for the body extremities. At -10 degrees C, surface temperatures of the areas around the eyes and dorsal head were significantly lower then predicted. 3. Tb was lowest at Ta of 25 and 30 degrees C, increased at all temperatures above and up to Ta of -4 degrees C below this range, and began decline at -10 degrees C. 4. The thermoneutral zone (TNZ) is probably between 28 and 32 degrees C, and the absolute lower critical temperature (Tabsl) is probably -4 degrees C. 5. The Mongolian gerbil shows little control of surface temperature and in contrast to larger mammals it has not developed any special thermoregulatory surface areas to regulate heat exchange with its environment. At temperatures below -4 degrees C, this species is unable to maintain the surface temperature of body extremities above the freezing point. 6. It is suggested that the Mongolian gerbil uses mainly behavioral and ecological adaptive strategies to attenuate the stressful effects of its habitat.     

The cryopreservation of liposomes. 2. Effect of particle size on crystallization behavior and marker retention.

Liposome dispersions (bilayer composition Phospholipon 100H/dicetylphosphate (molar ratio 10:1) dispersed in 10 mM Tris buffer) are frozen in a differential scanning calorimeter. In the cooling curves of the dispersions a heat-flow below -40 degrees C is observed. This heat-flow is due to the crystallization of maximally supercooled water. Evidence is provided that at this temperature, defined as the homogeneous nucleation temperature, part or all encapsulated water in the liposomes crystallizes. At a cooling rate of 10 degrees C/min only for small liposomes with particle sizes below approximately 0.2 micron the internal volume crystallizes at the homogeneous nucleation temperature. After a freezing/thawing cycle of the liposomal dispersions retention of the water-soluble marker carboxyfluorescein (CF) was significantly better if crystallization of the encapsulated volume occurred at the homogeneous nucleation temperature. Up to 55% retention of CF in dispersions with mean vesicle sizes below 0.2 micron was found after storage for 45 min at -50 or -75 degrees C. Only relatively small particle size alterations were found in comparison with the original mean particle sizes after a freezing/thawing cycle with storage for 45 min at -50 or -75 degrees C. Independent of particle size, dispersions stored for 45 min at -25 degrees C showed low CF retention (less than 10%) after thawing. For most of the liposome dispersions stored at -25 degrees C, large particle size alterations compared to the original particle sizes were observed after a freezing/thawing cycle.