Letters to the Editor

	LETTER

Effects of muscle temperature on oxygen uptake kinetics during exercise

To the Editor: I have read with great interest the paper entitled "Effect of increased muscle temperature on oxygen uptake kinetics during exercise," by Koga and co-workers (3), published in the October, 1997, issue of the Journal of Applied Physiology. In this paper, the temperature dependence of the oxygen uptake kinetics is analyzed during light and intense exercise, i.e., below and above the lactate threshold. The analysis of the slow component of the oxygen uptake kinetics at varying muscle temperature is certainly original and attractive to me. I am also glad to observe that the light exercise part of the study confirms previous results from our laboratory, published in Respiration Physiology (2). The differences between the two studies are as follows: 1) Koga et al. (3) increased muscle temperature, Ishii et al. (2) decreased it; 2) for this reason, the induced change in muscle temperature was 3.0°C in the former study, 7.5°C in the latter; and 3) muscle blood flow was measured in the latter study. The hypothesis put forward by Koga et al. (3) was the same as for our study (2) and was already rejected. It was proposed that during light exercise the recruitment of a greater muscle mass simultaneously contracting at each instant might have compensated for the expected temperature dependence of the velocity constant of the oxygen uptake kinetics. This hypothesis was tested during maximal exercise, a condition in which the proposed compensation may not operate, due to limitation in oxygen flow (1). Indeed, the fast component of the kinetics of oxygen uptake during exercise at the maximal power was found to be slower at cold than at warm muscle temperature. The statement that "... it is unknown whether the kinetics of adjustment of muscle and pulmonary oxygen uptake during the first few minutes of moderate- or heavy-intensity exercise would be altered by increased temperature," made in the introduction of the paper by Koga et al. (3), is therefore not correct.

When discussing the role of oxygen delivery, following the argument of the temperature effects on the oxygen equilibrium curve, Koga et al. (3) remark that they did not measure muscle blood flow and speculate, from the observation of an unaltered fast component of the oxygen uptake kinetics, that the factors affecting oxygen delivery must have remained unchanged. Indeed, Ishii et al. (2) showed no significant changes in muscle blood flow at cold vs. warm muscle temperature, although a tendency toward lower blood flow values and slower blood flow kinetics in the former case was evident.

	REFERENCES

1.   Ferretti, G., Binzoni T., Hulo N., Kayser B., Thomet J. M., and Cerretelli P. Kinetics of oxygen consumption during maximal exercise at different muscle temperatures. Respir. Physiol. 102: 261-268, 1995[Medline].

2.   Ishii, M., Ferretti G., and Cerretelli P. Effects of muscle temperature on the O₂ kinetics at the onset of exercise in man. Respir. Physiol. 88: 343-353, 1992[Medline].

3.   Koga, S., Shiojiri T., Kondo N., and Barstow T. J. Effect of increased muscle temperature on oxygen uptake kinetics during exercise. J. Appl. Physiol. 83: 1333-1338, 1997[Abstract/Free Full Text].

Guido Ferretti, Département de Physiologie Centre Médicale Universitaire Université de Genève CH-1211 Genève 4, Switzerland

	REPLY

To the Editor: We are pleased to respond to the letter by Ferretti regarding our recent paper (5). We are, of course, familiar with his previous work regarding the effects of lowered muscle temperature (T_m) on oxygen uptake (O₂) kinetics at the onset of exercise (2, 3). In fact, we referred to his papers in our previous work on this topic (6). However, we disagree with several statements made in the letter.

1) Our data do not "confirm," but rather "extend to states of elevated muscle temperatures," the "previous results from (his) laboratory." As correctly noted by Ferretti, this earlier work (3) dealt with the kinetics of response to reduced T_m.

2) The work of Ferretti et al. (2), which examined the kinetics of O₂ after maximal exercise, did so during conditions of reduced T_m. As stated above, these results do not a priori predict responses to either conditions where T_m is elevated or to submaximal exercise. Thus the conclusion that our introductory statement was not correct is itself not correct.

We believe that a key issue in this discussion is whether the integrated cardiorespiratory responses during exercise change linearly with changing T_m so that responses during elevated T_m can be predicted from responses to reduced T_m. Our studies collectively demonstrate that varying T_m induces nonlinear behavior in O₂ kinetics during exercise: raising T_m before exercise onset had no appreciable effect on O₂ kinetics (5), whereas moderate exercise with reduced T_m resulted in a significant slowing of the O₂ response compared with the control T_m condition (6). This latter observation disagrees with previous work by Ishii et al. (3). Potential physiological mechanisms for this nonlinear behavior of O₂ kinetics as a function of T_m include the following observations.

1) Mitochondria O₂, ADP:O ratio, states 3 and 4 of respiration, and the respiratory control ratio all exhibit nonlinear behavior as a function of increasing temperature above 37°C (1). Thus changes in metabolic rate as a function of decreasing T_m would not be expected to predict any changes when T_m is elevated.

2) It is currently unclear whether muscle blood flow, both kinetically and in the steady-state of exercise, responds linearly as T_m varies from subnormal to supranormal values. Since Ferretti et al. (2) and Ishii et al. (3) measured blood flow during exercise, when T_m was normal or reduced, and did not examine the muscle blood flow responses to elevated T_m, we do not know that blood flow will be greater when T_m is elevated. In fact, Johnson et al. (4) found that, despite a rise in T_m to 38.5°C (similar to our elevated preexercise T_m), forearm muscle blood flow did not increase at rest.

3) Finally, there could be some factors that modify the physiological responses to changes in temperature. First, concomitant changes in core temperature with T_m [as occurred in the studies of both Ishii et al. (3) and Ferretti et al. (2)] may result in alteration of the systemic responses to exercise. We designed our protocol to cause an isolated increase in T_m without concomitant changes in core temperature. Second, a shift to greater anaerobic metabolism has been shown in both elevated and reduced T_m conditions (for review, see Ref. 6), although the apparent mechanical efficiency of working muscles seems linear.

Based on the above arguments and the paucity of information describing muscle blood flow and O₂ kinetics during heavy exercise with elevated T_m, we believe that our work provides new insights into the effects of elevated T_m on the cardiorespiratory responses to moderate and heavy exercise, which could not be predicted from previous work with reduced T_m. We hope that further study on the effects of T_m on O₂ kinetics during exercise would be stimulated by the work of Ferretti and our work.

	REFERENCES

1.   Brooks, G. A., Hittelman K. J., Faulkner J. A., and Beyer R. E. Temperature, skeletal muscle mitochondrial functions, and oxygen debt. Am. J. Physiol. 220: 1053-1059, 1971.

2.   Ferretti, G., Binzoni T., Hulo N., Kayser B., Thomet J. M., and Cerretelli P. Kinetics of oxygen consumption during maximal exercise at different muscle temperatures. Respir. Physiol. 102: 261-268, 1995.

3.   Ishii, M., Ferretti G., and Cerretelli P. Effects of muscle temperature on the O₂ kinetics at the onset of exercise in man. Respir. Physiol. 88: 343-353, 1992.

4.   Johnson, J. M., Brengelmann G. L., and Rowell L. B. Interactions between local and reflex influences on human forearm skin blood flow. J. Appl. Physiol. 41: 826-831, 1976[Abstract/Free Full Text].

5.   Koga, S., Shiojiri T., Kondo N., and Barstow T. J. Effect of increased muscle temperature on oxygen uptake kinetics during exercise. J. Appl. Physiol. 83: 1333-1338, 1997.

6.   Shiojiri, T., Shibasaki M., Aoki K., Kondo N., and Koga S. Effects of reduced muscle temperature on the oxygen uptake kinetics at the start of exercise. Acta Physiol. Scand. 159: 327-333, 1997[Medline].

Shunsaku Koga, Applied Physiology Laboratory Kobe Design University Kobe 651-2196, Japan Tomoyuki Shiojiri Laboratory of Exercise and Sports Science Yokohama City University Yokohama 236-0014, Japan Narihiko Kondo Faculty of Human Development Kobe University Kobe 657-0011, Japan Thomas J. Barstow Department of Kinesiology Kansas State University Manhattan, KS 66506-0302