The influence of temperature on power production during swimming. I. In vivo length change and stimulation pattern

Ectothermal animals are able to locomote effectively over a wide range of temperatures despite low temperature reducing the power output of their muscles. It has been suggested that animals recruit more muscle fibres and faster fibre types to compensate for the reduced power output at low temperature, but it is not known how much low temperature actually reduces power output in vivo. ‘Optimized’ work-loop measurements, which are thought to approximate muscle function in vivo, give a Q(10) of approximately 2.3 for power output of scup (Stenotomus chrysops) red muscle between 10 degrees C and 20 degrees C. However, because of the slower muscle relaxation rate at low temperatures, ‘optimizing’ work loops requires stimulation duration to be reduced and oscillation frequency to be decreased to obtain maximal power output. Previous fish swimming experiments suggest that similar optimization may not occur in vivo, and this may have substantial consequences in terms of muscle power generation and swimming at low temperatures. To assess more precisely the effects of temperature on muscle performance and swimming, in the present study, we measured the length change, stimulation duration and stimulus phase of red muscle at various positions along scup swimming at several speeds at 10 degrees C and 20 degrees C. In a companion study, we determined the effects of temperature on in vivo power generation by driving muscle fibre bundles through these in vivo length changes and stimulation conditions, and measuring the resulting power output. The most significant finding from the present study is that, despite large differences in the in vivo parameters along the length of the fish (a decrease in stimulus duration, an increase in strain and a negative shift in phase) moving posteriorly, these parameters do not change with temperature. Thus, although the nervous system of fish could, in theory, compensate for slow muscle relaxation by greatly reducing muscle stimulation duration at low temperatures, it does not. This lack of compensation to low temperatures might reflect a potential limitation in neural control.</jats:p

The influence of temperature on power production during swimming. II. Mechanics of red muscle fibres in vivo

We found previously that scup (Stenotomus chrysops) reduce neither their stimulation duration nor their tail-beat frequency to compensate for the slow relaxation rates of their muscles at low swimming temperatures. To assess the impact of this ‘lack of compensation’ on power generation during swimming, we drove red muscle bundles under their in vivo conditions and measured the resulting power output. Although these in vivo conditions were near the optimal conditions for much of the muscle at 20 degrees C, they were far from optimal at 10 degrees C. Accordingly, in vivo power output was extremely low at 10 degrees C. Although at 30 cm s(−)(1), muscles from all regions of the fish generated positive work, at 40 and 50 cm s(−)(1), only the POST region (70 % total length) generated positive work, and that level was low. This led to a Q(10) of 4–14 in the POST region (depending on swimming speed), and extremely high or indeterminate Q(10) values (if power at 10 degrees C is zero or negative, Q(10) is indeterminate) for the other regions while swimming at 40 or 50 cm s(−)(1). To assess whether errors in measurement of the in vivo conditions could cause artificially reduced power measurements at 10 degrees C, we drove muscle bundles through a series of conditions in which the stimulation duration was shortened and other parameters were made closer to optimal. This sensitivity analysis revealed that the low power output could not be explained by realistic levels of systematic or random error. By integrating the muscle power output over the fish's mass and comparing it with power requirements for swimming, we conclude that, although the fish could swim at 30 cm s(−)(1) with the red muscle alone, it is very unlikely that it could do so at 40 and 50 cm s(−)(1), thus raising the question of how the fish powers swimming at these speeds. By integrating in vivo pink muscle power output along the length of the fish, we obtained the surprising finding that, at 50 cm s(−)(1), the pink muscle (despite having one-third the mass) contributes six times more power to swimming than does the red muscle. Thus, in scup, pink muscle is crucial for powering swimming at low temperatures. This overall analysis shows that Q(10) values determined in experiments on isolated tissue under arbitrarily selected conditions can be very different from Q(10) values in vivo, and therefore that predicting whole-animal performance from these isolated tissue experiments may lead to qualitatively incorrect conclusions. To make a meaningful assessment of the effects of temperature on muscle and locomotory performance, muscle performance must be studied under the conditions at which the muscle operates in vivo.</jats:p

RXTE and Swift Discovery of the Intermittent Source XTE J1704-445

We report the detection of an intermittent source XTE J1704-445 (hereafter J1704). This source was first detected in RXTE PCA scans of the galactic bulge and ridge regions over approximately a month. Because J1704 was near another bright source 4U 1705-440, whose variability can sometimes mask other nearby positions, it took some time to recognize J1704 as "real.