INTRODUCTION

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THE EXERCISE PRESSOR REFLEX is characterized by an increase in discharge of groups III and IV muscle afferents within contracting muscle. This reflex evokes an increase in heart rate (HR) and peripheral sympathetic discharge. These afferents may be stimulated by metabolites produced by muscle contraction. Potassium has been thought to play a role in the induction of this response (20). In a series of studies in dogs, Rybicki and colleagues (12, 13) demonstrated that potassium may play a role in the activation of this afferent limb. These investigators examined the discharge responses of single groups III and IV muscle afferents to increasing concentrations of interstitial potassium ([K⁺]_I) (13) and demonstrated a potassium concentration-dependent activation of these fibers. Of note, afferent stimulation was maximal early (6 ± 1 s) and returned to control firing rates within 30 s, leading to the conclusion that [K⁺]_I played a role in the initiation, but not the maintenance, of the pressor response.

To assess the relationship between potassium and the exercise pressor reflex in humans, Fallentin et al. (2) measured forearm venous plasma potassium concentration ([K⁺]_V) and mean arterial pressure (MAP) as normal subjects performed bouts of static handgrip (SHG) exercise. SHG exercise was followed by circulatory arrest to "trap" metabolic products and selectively engage the metabolite-sensitive component of this reflex. Low-intensity [15% maximal voluntary contraction (MVC)] and high-intensity (30% MVC) paradigms were used as venous blood was sampled from the exercising forearm. The investigators demonstrated a correlation between [K⁺]_V and MAP during and after static exercise, further strengthening the relationship between potassium and the exercise pressor reflex. Fallentin and colleagues (2) did not measure muscle sympathetic nerve activity (MSNA).

Recently, Green et al. (3) investigated the response of [K⁺]_I to exercise in human skeletal muscle. In dynamic contraction of calf muscle, [K⁺]_I increased as a function of exercise intensity, but it did not appear responsible for evoking muscle pain during exercise. Muscle pain, like engagement of the exercise pressor reflex, is thought to be transmitted by thin fiber muscle afferents. Therefore, this study suggests that [K⁺]_I may not be responsible for evoking the exercise pressor reflex.

Given these conflicting results, we sought to investigate the relationship between K⁺ and muscle afferent stimulation in humans. Subjects performed dynamic and SHG exercise as MSNA and [K⁺]_V were measured. We tested the hypothesis that MSNA would correlate with [K⁺]_V during handgrip exercise and posthandgrip circulatory arrest (PHG-CA).

	METHODS

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Subjects. Eight healthy volunteers (28.8 ± 3.7 yr; 6 males, 2 females) participated in the study. Body mass index (BMI) was 24.8 ± 0.9 kg/m², with a range of 20.4 to 28.7. The subjects were normotensive nonsmokers and were not taking any medications. Each subject gave written informed consent. All procedures used in the study had prior approval of the Institutional Review Board of the Milton S. Hershey Medical Center of the Pennsylvania State University.

Protocol. Subjects reported to the General Clinical Research Center, provided a prestudy history, and received a physical examination. All handgrip exercise was conducted using the nondominant arm (2). Subjects performed rhythmic handgrip (RHG) exercise at 40% MVC for 20 min or until fatigue. After 45 min of rest, subjects then performed SHG exercise for 2 min at 40% of MVC. During the static exercise, before release of handgrip, limb circulation was arrested by inflating an upper arm blood pressure (BP) cuff to suprasystolic pressure (~300 mmHg). PHG-CA was maintained for 2 min. During RHG, blood samples (1 ml/sample) were drawn twice during baseline and at 1-min intervals during the exercise paradigms. During SHG, blood samples were drawn twice during baseline, at 30 s of exercise, peak exercise (2 min), and 1-min increments during circulatory arrest. MSNA was recorded continuously in the peroneal nerve.

Microneurography. The microneurographic technique provides direct recordings of sympathetic nerve traffic directed to blood vessels in skeletal muscle. Sympathetic neural discharge causes an increase in vascular smooth muscle tone and vasoconstriction (5, 15, 17, 19).

Blood samples. Venous blood was obtained via a 20-gauge intravenous catheter placed in a retrograde fashion in the deep antecubital vein of the exercising forearm. Plasma was obtained immediately by centrifuge of the specimen for 30 s at 3,000 rpm (Eppendorf Centrifuge model 5415C, Hamburg, Germany). Analysis of the samples for metabolites was conducted with an ABL System 510 (Radiometer) in the Core Laboratory of the Milton S. Hershey Medical Center General Clinical Research Center.

HR and BP. HR was monitored via three-lead electrocardiography. Systemic BP was measured continuously by using photoplethysmography (Finapres; Ohmeda, Madison, WI) on the dominant hand. Resting BP was obtained with an automated sphygmomanometer (Dinamap; Critikon, Tampa, FL).

Statistics. All values are expressed as means ± SE. Statistical significance was accepted at the 0.05 probability level and was determined with repeated-measures ANOVA. Pairwise comparisons relative to baseline were performed by Dunnett's procedure. Pearson's partial correlation, adjusted for paradigm, was performed for analysis of MSNA vs. [K⁺]_V and MSNA vs. MAP.

	RESULTS

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We examined MAP, MSNA (expressed as bursts/min), and [K⁺]_V during RHG and SHG and PHG-CA following static exercise. Tension was 40% MVC. The results are expressed in Figs. 1-4.

View larger version (11K): [in this window] [in a new window]   Fig. 1.   Left: muscle sympathetic nerve activity (MSNA) burst frequency before and during rhythmic handgrip (RHG) at 40% maximal voluntary contraction (MVC). Mean values ± SE, n = 8. Middle: venous plasma potassium concentration ([K+]V) before and during RHG at 40% MVC. Mean values ± SE, n = 8. Right: mean arterial blood pressure (MAP) before and during RHG at 40% MVC. Mean values ± SE, n = 6. Base, baseline; Ex2, grip at 2 min; End, end grip. *P < 0.05 vs. baseline.

View larger version (13K): [in this window] [in a new window]   Fig. 2.   Left: MSNA burst frequency before and during static handgrip (SHG) at 40% MVC and during 2 min of posthandgrip circulatory arrest (PHG-CA). Mean values ± SE, n = 8. Middle: [K+]V before and during SHG at 40% MVC and during 2 min of PHG-CA. Mean values ± SE, n = 8. Right: MAP before and during SHG at 40% MVC and during 2 min of PHG-CA. Mean values ± SE, n = 8. Ex, grip at 30 s; CA1, circulatory arrest at 1 min; CA2, circulatory arrest at 2 min. *P < 0.05 vs. baseline.

View larger version (15K): [in this window] [in a new window]   Fig. 3.   MSNA (burst frequency) vs. [K+]V (meq/l) during RHG (A), SHG (B), and PHG-CA (C), adjusted for paradigm.

View larger version (14K): [in this window] [in a new window]   Fig. 4.   MAP (mmHg) vs. [K+]V (meq/l) during RHG (A), SHG (B), and PHG-CA (C), adjusted for paradigm.

RHG resulted in statistically significant elevation of MAP and [K⁺]_V but not MSNA (Fig. 1). Both MAP and [K⁺]_V were elevated above control at 2 min of exercise; however, MSNA was not. The dissociation of [K⁺]_V from MSNA reduces the likelihood of a causal relationship.

SHG elicited a robust early rise in MAP, which was maximal at peak exercise. This elevation in MAP was sustained above baseline during the period of PHG-CA (Fig. 2). Sympathetic nerve activity rose to maximum levels at peak exercise and was also sustained during the PHG-CA (Fig. 2). Although [K⁺]_V rose quickly during SHG, there was a gradual return to control levels during the PHG-CA period. We speculate that this was due to decreased K⁺ loss from muscle at the termination of exercise coupled with sustained Na-K pump activity induced by exercise and postexercise ischemia (16). By the end of the PHG-CA, there was no significant difference between [K⁺]_V and baseline (Fig. 2). Furthermore, [K⁺]_V was dissociated from MAP and MSNA.

Although [K⁺]_V and MSNA seemed to follow a similar temporal course during SHG, they did not correlate with one another during the active exercise period or PHG-CA (Fig. 3). Similarly, [K⁺]_V and MAP did not demonstrate a strong association during RHG, SHG, or PHG-CA (Fig. 4).

	DISCUSSION

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We found that [K⁺]_V did not correlate with MSNA during dynamic or SHG exercise. Furthermore, [K⁺]_V did not correlate with MSNA during PHG-CA, a manipulation thought to isolate metaboreceptor stimulation of MSNA from any mechanoreceptor or central command component. Although these data do not entirely exclude the possibility that K⁺ is involved with the initiation of the exercise reflex, they do support the concept that K⁺ is not directly involved in the maintenance of the exercise pressor reflex.

It is important to note that dynamic and static exercise paradigms evoke different metabolic and reflex responses. During dynamic (or nonischemic) exercise, the muscle is freely perfused. As such, the accumulation rate of potassium and other metabolites in the interstitium and plasma compartments may be less than that seen in static (or ischemic) exercise (16). Thus metabolite and reflex effects are often less impressive than during static exercise (1). Under these conditions, we demonstrated dissociation of [K⁺]_V from MSNA during dynamic exercise. Thus, in freely perfused exercising muscle, potassium does not appear to play a role in the maintenance of the muscle reflex.

During static exercise, subjects demonstrated a robust exercise pressor response that was sustained during circulatory arrest (Fig. 2). [K⁺]_V rose significantly within 30 s of exercise, peaked before termination of muscle tension, and gradually declined during circulatory arrest, returning to control levels by the end of PHG-CA. Similar to the dynamic exercise paradigm, however, [K⁺]_V did not correlate with MSNA during SHG (Fig. 3). Furthermore, during PHG-CA, the MSNA remained significantly elevated vs. control. However, [K⁺]_V returned to control levels. The lack of correlation, and clear dissociation during PHG-CA, again suggests that potassium is not associated with maintenance of the exercise reflex.

Our findings support those of Rybicki et al. (12, 13). Rybicki and colleagues (13) demonstrated that potassium only transiently increased the discharge of groups III and IV thin fiber afferents. Conceptually, our work is also consistent with the recent findings of Green and colleagues (3). These investigators examined whether potassium stimulates thin fiber muscle afferents that mediate the muscle-pain response. They showed no association between [K⁺]_I and muscle pain (3). Thin fiber muscle afferents are thought to be responsible for muscle pain and the exercise pressor reflex. Thus the current report in conjunction with these studies suggests that potassium is not responsible for maintaining the exercise pressor reflex.

Our data are contrary to those of Fallentin et al. (2). Although Fallentin and colleagues (2) reported an association between [K⁺]_V and MAP, we do not. To account for repeated measures, we performed a partial correlation, adjusted for paradigm, whereas Fallentin et al. assumed independence. Thus we believe that [K⁺]_V and MAP are not strongly associated. In addition, the lack of convincing association between [K⁺]_V and MSNA further strengthens the argument that K⁺ is not involved with maintenance of the pressor reflex.

To determine if K⁺ evoked the muscle reflex, venous potassium was used as a surrogate for [K⁺]_I. It is clear that venous K⁺ will underestimate [K⁺]_I, and the magnitude of this underestimation will be dependent on K⁺ loss from the muscle (16). The amount of K⁺ lost from muscle is, in turn, related to the mass of muscle and the mode of exercise. The greater the muscle mass involved and the more dynamic the exercise paradigm, the greater the muscle loss (16). In our studies, a handgrip paradigm using intermittent contractions and sustained static contractions was employed. Under these circumstances, the loss of K⁺ from muscle will be minimized. Accordingly, under the experimental conditions used in this report, we believe that the relationship between venous and interstitial K⁺ is strong and constant, making venous potassium a reliable index of [K⁺]_I.

Parenthetically, we believe that the relationship between venous K⁺ and [K⁺]_I was likely to be very similar during static exercise at 40% MVC and during PHG-CA that followed. Static contractions at 40% MVC are likely to be predominantly ischemic, and the loss of K⁺ from muscle will be small.

Potential limitations. A potential limitation of the present study is that the volunteers in our report ranged in BMI from 20.4 to 28.7. BMI may influence sympathetic nerve activity responses (14). This factor could affect the slope of the MSNA/metabolite relationship such that this slope could be attenuated at higher BMI values. Nevertheless, we do not believe that BMI variability affected our results because MSNA findings in this report are similar to those reported previously (9, 10).

Perspectives

We previously provided data that adenosine (8), lactate, and H⁺ are not important afferent stimulants. Although controversy still surrounds each of these observations, we believe that the sum of the human and animal literature to date suggests that under physiological conditions, adenosine, lactate and H⁺ do not play important roles in evoking the muscle reflex. Rather, we believe that diprotonated phosphate is likely to play a very important role in evoking the muscle reflex (18; unpublished observations by MacLean et al.). Finally, it should also be mentioned that recent exciting findings by Kaufman and colleagues suggest that ATP (not adenosine) may play an important role in evoking the muscle reflex (4). Clearly, additional experiments to elucidate the causes of muscle afferent stimulation are necessary.

In summary, we showed that [K⁺]_V does not correlate with MSNA during dynamic or SHG exercise in humans. These findings suggest that potassium may not play an important role in sustaining the exercise pressor response in intact exercising humans. It is important to note, however, that absence of correlation in our study does not exclude the possibility that potassium may play a transient role in engaging this reflex, as suggested by Rybicki et al. (13). Whether other factors, such as afferent sensitization, changes in tissue volume, and interactions between K⁺ and other metabolites, contribute to this reflex is unknown (6, 11).