The effect of temperature on transmission at the neuromuscular junction of the sartorius muscle of Rana pipiens

Feeling the heat: source–sink mismatch as a mechanism underlying the failure of thermal tolerance

A mechanistic explanation for the tolerance limits of animals at high temperatures is still missing, but one potential target for thermal failure is the electrical signaling off cells and tissues. With this in mind, here I review the effects of high temperature on the electrical excitability of heart, muscle and nerves, and refine a hypothesis regarding high temperature-induced failure of electrical excitation and signal transfer [the temperature-dependent deterioration of electrical excitability (TDEE) hypothesis]. A central tenet of the hypothesis is temperature-dependent mismatch between the depolarizing ion current (i.e. source) of the signaling cell and the repolarizing ion current (i.e. sink) of the receiving cell, which prevents the generation of action potentials (APs) in the latter. A source–sink mismatch can develop in heart, muscles and nerves at high temperatures owing to opposite effects of temperature on source and sink currents. AP propagation is more likely to fail at the sites of structural discontinuities, including electrically coupled cells, synapses and branching points of nerves and muscle, which impose an increased demand of inward current. At these sites, temperature-induced source–sink mismatch can reduce AP frequency, resulting in low-pass filtering or a complete block of signal transmission. In principle, this hypothesis can explain a number of heat-induced effects, including reduced heart rate, reduced synaptic transmission between neurons and reduced impulse transfer from neurons to muscles. The hypothesis is equally valid for ectothermic and endothermic animals, and for both aquatic and terrestrial species. Importantly, the hypothesis is strictly mechanistic and lends itself to experimental falsification.

Activation of respiratory muscles does not occur during cold-submergence in bullfrogs, Lithobates catesbeianus

Semiaquatic frogs may not breathe air for several months because they overwinter in ice-covered ponds. In contrast to many vertebrates that experience decreased motor performance after inactivity, respiratory motor function in bullfrogs, Lithobates catesbeianus, remains functional following cold-submergence. Unlike mammalian hibernators with unloaded limb muscles and inactive locomotor systems, respiratory mechanics of frogs counterintuitively allow for ventilatory maneuvers when submerged. Thus we hypothesized that bullfrogs generate respiratory motor patterns during cold-submergence to avoid disuse and preserve motor performance. Accordingly, we measured activity of respiratory muscles (buccal floor compressor and glottal dilator) via electromyography in freely behaving bullfrogs at 20°C and 2°C. Although we confirm that ventilation cycles occur underwater at 20°C, bullfrogs did not activate either respiratory muscle when submerged acutely or chronically at 2°C. We conclude that cold-submerged bullfrogs endure respiratory motor inactivity, implying that other mechanisms, excluding underwater muscle activation, maintain a functional respiratory motor system throughout overwintering.

Why do insects enter and recover from chill coma? Low temperature and high extracellular potassium compromise muscle function in Locusta migratoria

When exposed to low temperatures, many insect species enter a reversible comatose state (chill coma), which is driven by a failure of neuromuscular function. Chill coma and chill coma recovery have been associated with a loss and recovery of ion homeostasis (particularly extracellular [K+], [K+]o) and accordingly onset of chill coma has been hypothesized to result from depolarization of membrane potential caused by loss of ion homeostasis. Here, we examined whether onset of chill coma is associated with a disturbance in ion balance by examining the correlation between disruption of ion homeostasis and onset of chill coma in locusts exposed to cold at varying rates of cooling. Chill coma onset temperature changed maximally 1°C under different cooling rates and marked disturbances of ion homeostasis were not observed at any of the cooling rates. In a second set of experiments, we used isolated tibial muscle to determine how temperature and [K+]o, independently and together, affect tetanic force production. Tetanic force decreased by 80% when temperature was reduced from 23°C to 0.5°C, while an increase in [K+]o from 10 mmol l−1 to 30 mmol l−1 at 23°C caused a 40% reduction in force. Combining these two stressors almost abolished force production. Thus, low temperature alone may be responsible for chill coma entry, rather than a disruption of extracellular K+ homeostasis. As [K+] also has a large effect on tetanic force production, it is hypothesized that recovery of [K+]o following chill coma could be important for the time to recovery of normal neuromuscular function.

Enhancement of twitch force by stretch in a nerve-skeletal muscle preparation of the frog Rana porosa brevipoda and the effects of temperature on it

We investigated the mechanism of the enhancement of twitch force by stretch and the effects of temperature on it in nerve-skeletal muscle preparations of whole iliofibularis muscles isolated from the frog Rana brevipoda. When a preparation was stimulated indirectly and stretched, the twitch force after the stretch was enhanced remarkably in comparison to that observed before a stretch at low temperature. The enhanced force obtained by a stretch of 20% resting muscle length (l0) at low temperature was as high as the force obtained by direct stimulation. The phenomenon was not dependent on the velocity but on the amplitude of stretch. The enhanced force obeyed the length-force relationship when a stretch was long enough. The above results were observed when the frogs were kept at room temperature (20-22 degrees C). Measurements were also taken at low temperature (4 degrees C); when frogs were kept at low temperature for more than 2 months, twitch force obtained without stretch was considerably higher at l0. The amplitude of the action potential recorded extracellularly from the muscle surface increased remarkably after a stretch, but was same before and after a stretch when recorded from the nerve innervating muscle. The effects of temperature on twitch and tetanic force by direct or indirect stimulation without stretch were also studied as basic data of the stretch experiment. The results from this study suggest that stretch-induced force enhancement in a nerve-muscle preparation is caused by an increase in the transmission rate between nerve and muscle, and the amplitude of the enhanced force is determined by the length-force relationship of the muscle. The phenomenon is also strongly affected by the temperature at which the frogs are kept.

Body temperature and locomotor capacity in a heterothermic rodent

We quantify the locomotor capacity of the round-tailed ground squirrel (Spermophilus tereticaudus), a mammal that can lower energetic costs by relaxing thermoregulatory limits without becoming inactive. We measured maximum sprint speed, maximum limb cycling frequency and maximum force production in animals at body temperatures ranging from 31 degrees C to 41 degrees C. We found no thermal dependence in any of these parameters of locomotion. Results (means +/- S.E.M.) across this range of body temperatures were: sprint speed = 4.73+/-0.04 m s(-1), limb cycling frequency = 19.4+/-0.1 Hz and maximum force production = 0.012+/-0.0003 N g(-1). The neuro-muscular system of this species may thus be less thermally dependent at these temperatures than that of other mammals, allowing for the maintenance of whole-animal performance across a broader range of body temperatures. The absence of any significant loss of locomotor capabilities associated with either a decrease of 7-8 degrees C or a rise of 3-4 degrees C in body temperature from typical mammalian values raises significant questions regarding our understanding of the evolution and physiology of the mammalian mode of thermoregulation.

The influence of thermal acclimation on power production during swimming. I. In vivo stimulation and length change pattern of scup red muscle

Ectothermal animals are able to locomote in a kinematically similar manner over a wide range of temperatures. It has long been recognized that there can be a significant reduction in the power output of muscle during swimming at low temperatures because of the reduced steady-state (i.e. constant activation and shortening velocity) power-generating capabilities of muscle. However, an additional reduction in power involves the interplay between the non-steady-state contractile properties of the muscles (i.e. the rates of activation and relaxation) and the in vivo stimulation and length change pattern the muscle undergoes during locomotion. In particular, it has been found that isolated scup (Stenotomus chrysops) red muscle working under in vivo stimulus and length change conditions (measured in warm-acclimated scup swimming at low temperatures) generates very little power for swimming. Even though the relaxation of the muscle has slowed greatly, warm-acclimated fish swim with the same tail-beat frequencies and the same stimulus duty cycles at cold temperatures, thereby not affording the slow-relaxing muscle any extra time to relax. We hypothesize that considerable improvement in the power output of the red muscle at low temperatures could be achieved if cold acclimation resulted in either a faster muscle relaxation rate or in the muscle being given more time to relax (e.g. by shortening the stimulus duration or reducing the tail-beat frequency). We test these hypotheses in this paper and the accompanying paper. Scup were acclimated to 10 degrees C (cold-acclimated) and 20 degrees C (warm-acclimated) for at least 6 weeks. Electromyograms (EMGs) and high-speed cine films were taken of fish swimming steadily at 10 degrees C and 20 degrees C. At 10 degrees C, we found that, although there were no differences in tail-beat frequency, muscle strain or stimulation phase between acclimation groups, cold-acclimated scup had EMG duty cycles approximately 20 % shorter than warm-acclimated scup. In contrast at 20 degrees C, there was no difference between acclimation groups in EMG duty cycle, nor in any other muscle length change or stimulation parameter. Thus, in response to cold acclimation, there appears to be a reduction in EMG duty cycle at low swimming temperatures that is probably due to an alteration in the operation of the pattern generator. This novel acclimation probably improves muscle power output at low temperatures compared with that of warm-acclimated fish, an expectation we test in the accompanying paper using the work-loop technique.

Effects of thermal acclimation on nervous conduction and muscle contraction in the frog Rana temporaria

The effects of season and acclimation temperature on the latency of the leg withdrawal reflex and three of its components have been studied: conduction velocity in the sciatic nerve, spinal conduction time, and contraction time of gastrocnemius muscle. The latency of the leg withdrawal reflex was markedly shortened by cold acclimation: the reaction times were at 6 degrees C 1.54 s in 4 degrees C acclimated and 3.97 s in 24 degrees C acclimated winter frogs. Also, the temperature dependence of the reflex latency was reduced by cold acclimation. Thus, frogs acclimated to cold responded to external stimuli in cold more rapidly than warm-acclimated ones. This cold adaptation of the reflex could not be explained by changes in its studied components. These made up only one-tenth of the reflex response time, and either did not show significant cold acclimation (muscle contraction and spinal conduction times in summer) or showed inverse acclimation, especially when measured at high temperatures (i.e. conduction velocities were reduced by acclimation to cold). Thus, the cold acclimation of the reflex response probably resides in the sensory component of the response. The inverse temperature adaptation response of conduction velocities may reflect a reduced ion permeability across cellular membranes in cold which decreases metabolic energy expenditure during inactive periods.

Does behavioural hypothermia promote post-exercise recovery in cold-submerged frogs?

At the low temperatures of the overwintering environment of the frog Rana temporaria, small changes in ambient temperature have large effects on metabolism and behaviour, especially since Q10 values are often greatly elevated in the cold. How the overwintering aquatic frog copes with variable thermal environments in terms of its overall activity metabolism and recovery from pursuit by predators is poorly understood, as is the role of behavioural thermoregulation in furthering recovery from intense activity. Exhaustive exercise was chosen as the method of evaluating activity capacity (defined by time to exhaustion, total distance swum and number of leg contractions before exhaustion) and was determined at 1.5 and 7 degreesC. Other cohorts of frogs were examined at both temperatures to determine the metabolic (acid-base, lactate, glucose, ATP and creatine phosphate) and respiratory responses to exercise in cold-submerged frogs. Finally, temperature preference before and after exercise was determined in a thermal gradient to define the importance of behavioural thermoregulation on the recovery rates of relevant metabolic and respiratory processes. Activity capacity was significantly reduced in frogs exercised at 1.5 versus 7 degreesC, although similar levels of tissue acid-base metabolites and lactate were reached. Blood pH, plasma PCO2 and lactate levels recovered more rapidly at 1.5 degreesC than at 7 degreesC; however, intracellular pH and the recovery of tissue metabolite levels were independent of temperature. Resting aerobic metabolic rates were strongly affected by temperature (Q10=3.82); however, rates determined immediately after exercise showed a reduced temperature sensitivity (Q10=1.67) and, therefore, a reduced factorial aerobic scope. Excess oxygen consumption recovered to resting values after 5–6.25 h, and 67 % recovery times tended to be slightly faster at the lower temperatures. Exercise in the cold, therefore, provided an immediately higher factorial scope, which could be involved in the faster rate of recovery of blood lactate levels in the colder frogs. In addition, exercise significantly lowered the preferred temperature of the frogs from 6.7 to 3.6 degreesC for nearly 7 h, after which they returned to their normal, unstressed preferred temperatures. Thus, a transient behavioural hypothermia in the skin-breathing, overwintering frog may be an important strategy for minimising post-exercise stress and maintaining aerobic metabolism during recovery from intense activity.

The effect of reduced temperature on the inhibitory action of adenosine and magnesium ion at frog motor nerve terminals

1. A study was made to exclude the notion that adenosine receptor agonists exert a direct physical blockade of the depolarization-secretion process. Reduced temperature was employed as a tool for distinguishing between physico-chemical processes (such as those which mediate evoked transmitter release) and biochemical mechanisms (such as those which involve second messenger substances) in the action of adenosine. Adenosine and 2-chloroadenosine were used as agonists in this electrophysiological study of the release of acetylcholine (ACh) from frog motor nerve terminals. 2. The ability of these two adenosine receptor activators to reduce neurally-evoked ACh release was prevented or greatly attenuated by maintaining the preparation at temperatures between 5 and 10 degrees C. Such low temperatures inhibit the activation of receptors coupled to second messengers via guanine nucleotide binding proteins (e.g. adenylate cyclase). Low temperature alone did not substantially alter evoked ACh secretion under the conditions of these experiments. 3. Inhibition of evoked ACh release by the extracellular Ca antagonist Mg, which acts directly to block Ca channels, was not affected by low temperature. 4. The results are consistent with the hypothesis that a temperature-sensitive second messenger system controls the intracellular events linked to extracellular adenosine receptor activation.

Studies on the mode of action of botulinum toxin type A at the frog neuromuscular junction

In frogs poisoned with botulinum toxin type A the quantal content of endplate potentials is greatly reduced. Lowering the temperature of the preparation increases quantum content; between 14 and 4 degrees C the mean Q10 for this effect is 6.3. Facilitation of synaptic transmission is marked with pairs of stimuli and cooling further enhances facilitation. The time constant of decay of facilitation is 34 ms at 20 degrees C and 116 ms at 4 degrees C. The increase in facilitation and in its time constant of decay at low temperature are presumably not a result of a prolongation of the duration of the nerve terminal action potential since such changes are not seen in the presence of K+-channel blockade by 3,4-diaminopyridine. Electrotonic depolarization of nerve terminals in the presence of tetrodotoxin and 3,4-diaminopyridine induces all-or-none endplate currents. Such endplate currents, at a holding potential of -50 mV, show that the amount of charge entry is about 1/3 of that in unpoisoned junctions but still corresponds to 5-10 X 10(3) transmitter quanta. Transmitter release at this level is maintained during repetitive stimulation even in the presence of 82 mM Ca2+ in the extracellular solution. We speculate that the blockade of transmitter release in BoTx -poisoned muscles results from a stimulatory effect of the toxin on metabolic systems of Ca2+ disposal in the nerve terminal.

Electron-microscope cytochemistry of acetylcholine-like cation by means of low-temperature "ionic fixation"

A fresh preparation of frog neuromuscle was fixed at low temperatures (0 degree-4 degrees C) by means of an "ionic-fixation" procedure which is based on the precipitation of quaternary ammonium cations, such as choline and acetylcholine, with molybdic or tungstic heteropolyanions. A low temperature was used to slow down drastically the biological processus of vesicular exocytosis so that ionic fixation, the speed of which is only slightly influenced by temperature variation, could be performed efficiently. In addition to the conventional point-like precipitate in the synaptic vesicle which is considered to be vesicular acetylcholine, numerous spot-like precipitates were observed in the synaptic cleft. Most of these were contiguous to the active zone, and some were in a paired form and corresponded to the double rows of the synaptic vesicles in contact with active zones. It is concluded that these spot-like precipitates were acetylcholine-like cations of the synaptic vesicles which had been discharged into the synaptic cleft by exocytosis and captured by the ionic fixation procedure. The results are discussed in relation to the vesicular and non-vesicular hypothesis of acetylcholine release.

The effects of temperature and thermal acclimation upon the osmotic properties and nonelectrolyte permeability of liver and gill mitochondria from rainbow trout (Salmo gairdneri)

Thermal acclimation of rainbow trout (Salmo gairdneri) taken from 20 degrees C to 7 degrees C resulted in adaptation of mitochondrial function, as evidenced by increases in the specific activities of NADH- and succinate-cytochrome c reductase of 1.93- and 2.7-fold respectively. Mitochondria from both gill and liver obeyed the Boyle-van't Hoff relationship in the range from 400 to 60 mosM. Thermal acclimation had no effect on the osmotic properties of liver mitochondria, whereas gill mitochondria from cold-acclimated trout were more sensitive to osmotic swelling than mitochondria from warm-acclimated individuals. The non-electrolyte permeability of liver mitochondria was assessed by optically monitoring mitochondrial swelling rates in isosmotic solutions of urea, glycerol, mannitol and glucose. Two parameters of mitochondrial swelling were determined: (a) initial swelling rates, d(1/A)dt, and (b) swelling constants, ks, derived from the time required to swell a fixed volume. Regardless of the assay temperature or the permeant employed, liver mitochondria from cold-acclimated trout exhibited greater initial swelling rates than mitochondria from warm-acclimated trout, indicating properties of temperature-compensated permeability. The apparent ranking of nonelectrolyte permeabilities was urea greater than glycerol greater than mannitol greater than glucose. ks values for urea and glycerol from cold-acclimated trout were greater than values typical of warm-acclimated populations; however ks values for glucose and mannitol were not influenced by thermal acclimation. Regardless of the permeant considered, activation energies for ks values were 3- to 5-fold greater than those for initial swelling rates. The time course of mitochondrial swelling consists of two components, an initial rapid swelling phase characterized by a half-life of 3-12 seconds, and a slower swelling phase characterized by a half life of 1-6 minutes. Initial swelling rates, which approximate the rapid swelling component, are considered to be the least ambiguous index of permeability, whereas ks values are more complex and strongly influenced by the slower swelling component.