INTRODUCTION

Top Abstract Introduction Methods Results Discussion References

WHEN OXYGEN delivery to active skeletal muscle is insufficient for ongoing metabolic demands, metabolites accumulate and stimulate group III and IV afferent neurons within the active muscle, eliciting a reflex increase in efferent sympathetic nerve activity and systemic arterial pressure (SAP) termed the muscle metaboreflex (1, 5, 7-9, 11, 16). Data from Wyss et al. (18) indicate that when this reflex is activated in conscious dogs via progressive reductions in hindlimb perfusion during treadmill exercise, a considerable fraction of the metaboreflex pressor response is due to an increase in cardiac output (CO) in addition to peripheral vasoconstriction. Other studies utilizing anesthetized animal models have shown that static muscle contraction elicits increases in the first derivative of left ventricular pressure (LV dP/dt) (14), indicating that reflexes arising from skeletal muscle afferents can induce significant increases in ventricular contractility, although with static muscle contraction it can be difficult to differentiate the relative roles of mechanosensitive vs. metabosensitive afferents in mediating these responses.

Previously, O'Leary (7) observed that in conscious dogs during dynamic exercise, the majority of the metaboreflex-induced increase in heart rate (HR) occurs via an increase in sympathetic activity to the heart. In that study, -adrenergic blockade markedly decreased the tachycardic response to hindlimb ischemia. In addition, after -adrenergic blockade the magnitude of the metaboreflex-induced pressor response was reduced, leading the author to conclude that the reflex tachycardia contributes importantly to the increase in arterial blood pressure. However, CO was not measured in that study. Data from Wyss et al. (18) indicate that muscle metaboreflex-induced increases in CO occur via increases in HR with little change in stroke volume (SV). Inasmuch as White et al. (17) demonstrated using conscious dogs that changes in HR over a wide range (from ~100 to 180 beats/min) per se do not elicit changes in CO due to the inverse relationship between HR and SV, muscle metaboreflex-induced tachycardia by itself likely would not elicit substantial increases in CO. Thus the data from Wyss et al. (18) indirectly allude to a reflex increase in ventricular performance; that is, muscle metaboreflex-induced increases in ventricular contractility could offset the reduction in ventricular filling time (concomitant with the increases in HR), maintaining SV essentially constant despite the tachycardia and thereby increasing CO. Alternatively, activation of the muscle metaboreflex could elicit increases in filling pressure that could maintain SV via a Frank-Starling mechanism. Thus the present study was designed to determine whether the muscle metaboreflex elicits substantial increases in ventricular performance.

	METHODS

Top Abstract Introduction Methods Results Discussion References

All experiments were performed using five conscious dogs of either gender (22-28 kg) trained to run on a motor-driven treadmill. All procedures were reviewed and approved by the Institutional Animal Care Committee and conformed to National Institutes of Health guidelines.

Surgical preparation. The animals were prepared in a series of surgical sessions with at least 1 wk of recovery between surgeries and between the last surgery and the first experiment. For all procedures anesthesia was induced with Pentothal Sodium and maintained with isoflurane. Immediately before and at the completion of each surgery the animals were treated with cefazolin (500 mg iv) and then with cephalexin (30 mg/kg by mouth, 2 times/day) for at least 1 wk postoperatively. Postoperative discomfort was controlled using acepromazine (0.1 mg/kg im) and buprenorphine (0.015 mg/kg iv) whenever deemed necessary.

Experimental procedures. All experiments were performed after the animals had fully recovered from surgery and were active, afebrile, and of good appetite. The animal was brought to the laboratory and allowed to roam freely for 15-30 min. The animal was then directed to the treadmill, and the blood flow transducers were connected to a flowmeter (Transonic Instruments). The FAP, SAP, and CVP catheters were connected to pressure transducers (Spectromed P-10 EZ). HR was monitored via a cardiotachometer triggered by the CO signal. SV was calculated as CO/HR. All data were sampled by a laboratory computer at 1,000 Hz, and mean values for each cardiac cycle were saved on hard disk for subsequent analysis.

Statistical analysis. Each animal served as its own control. One-minute averages of all variables were taken during steady-state exercise and at each level of partial vascular occlusion. The data were analyzed as described by Wyss et al. (18). At this workload, initial reductions in hindlimb perfusion do not elicit metaboreflex responses. Once TAQ is reduced below a threshold level, further reductions in hindlimb perfusion elicit substantial increases in SAP, CO, and HR. Thus the relationship between hindlimb perfusion and the efferent responses (e.g., SAP, CO, HR) is "dogleg" in shape at this workload. Therefore, the data were approximated to two linear regressions, an initial response line wherein no substantial change in SAP, CO, and HR occurred with the initial reductions in TAQ and a pressor response line when further reductions in TAQ elicited substantial increases in the efferent responses. The intersection of these two lines is taken as the threshold for the reflex. In control experiments no change in SV occurred, and therefore these data could not be fitted by two regression lines, and the values during free-flow exercise were compared with those at the lowest level of hindlimb perfusion. The data were analyzed using repeated-measures ANOVA, and individual means were compared using modified Bonferroni tests.

	RESULTS

Top Abstract Introduction Methods Results Discussion References

Figure 1 shows a typical control experiment. As described previously (7-9, 11, 18), initial reductions in hindlimb perfusion during mild exercise do not elicit metaboreflex responses. However, once hindlimb perfusion is reduced below a threshold level, substantial increases in SAP, HR, and CO occur. Note that little change in SV occurred.

View larger version (24K): [in this window] [in a new window]   Fig. 1.   Responses in systemic arterial pressure (SAP), heart rate (HR), cardiac output (CO) and stroke volume (SV) to progressive reductions in femoral arterial pressure (FAP) and terminal aortic blood flow (TAQ) via graded, partial vascular occlusion of the terminal aorta from 1 animal during a control experiment. Note that with metaboreflex activation marked increases in SAP, HR, and CO occurred with little change in SV. KPH, km/h; bpm, beats/min.

Figure 2 shows average values of SAP, HR, CO, SV, and CVP as a function of TAQ during free-flow exercise, at metaboreflex threshold, and at the maximal level attained with the largest reductions in hindlimb perfusion during control experiments. Metaboreflex activation caused substantial and highly significant (P  0.002) increases in SAP, HR, and CO. No significant change in SV or CVP occurred with metaboreflex activation (P = 0.74 and 0.67, respectively).

View larger version (12K): [in this window] [in a new window]   Fig. 2.   Average levels of SAP, HR, CO, SV, and central venous pressure (CVP) during control experiments plotted as a function of TAQ during free-flow exercise (circles at right), at metaboreflex threshold (circles at middle), and at lowest imposed level of TAQ (circles at left). Once TAQ was reduced below threshold, substantial increases in SAP, HR, and CO occurred. No significant changes in either SV or CVP occurred. * P < 0.05 vs. free-flow exercise; NS, no significant change from free-flow exercise.

Figure 3 shows the average values of SAP, HR, CO, SV, and CVP as a function of TAQ during free-flow exercise, at metaboreflex threshold, and at the maximal level attained with the largest reductions in hindlimb perfusion when HR was maintained constant at 225 beats/min. In the absence of any change in HR, substantial increases in SAP and CO still occurred with metaboreflex activation because of the large, significant increases in SV.

View larger version (11K): [in this window] [in a new window]   Fig. 3.   Average levels of SAP, HR, CO, SV, and CVP during constant ventricular pacing plotted as a function of TAQ during free-flow exercise (circles at right), at metaboreflex threshold (circles in middle), and at lowest imposed level of TAQ (circles at left). Note that at constant HR, metaboreflex activation elicited a substantial increase in SV such that the increases in SAP and CO were similar to control experiments. * P < 0.05 vs. free-flow exercise; NS, no significant change from free flow exercise.

Figure 4 shows the maximal changes in SAP, CO, SV, HR, and CVP during control experiments and during constant HR. Ventricular pacing did not significantly affect the maximal levels of SAP or CO. During control experiments, SV remained essentially constant with metaboreflex activation, whereas, at constant HR, SV increased markedly during hindlimb ischemia such that the changes in CO and SAP were not different between the control experiments vs. at constant HR. No significant change in CVP occurred in either experiment.

View larger version (11K): [in this window] [in a new window]   Fig. 4.   Average maximal changes in SAP, CO, SV, HR, and CVP in response to metaboreflex activation during control experiments and during constant HR. Note that maximal increases in SAP and CO were not significantly different between control experiments and at constant HR. * P < 0.05, change significantly different from constant HR; NS, no significant difference.

	DISCUSSION

Top Abstract Introduction Methods Results Discussion References

The major new finding in this study is that activation of the muscle metaboreflex in conscious dogs during dynamic exercise elicits marked increases in ventricular performance. Normally, with metaboreflex activation CO increases substantially via an increase in HR with constant SV. However, we observed that when HR was maintained constant, SV increased significantly with metaboreflex activation such that the increases in CO and SAP were indistinguishable from control experiments. Thus, with activation of the muscle metaboreflex in the normal animal, despite decreases in ventricular filling time due to the reflex tachycardia, SV remains unchanged due to the increase in ventricular performance, thereby allowing CO to increase substantially and contribute importantly to the pressor response.

Previously, O'Leary (7) observed that after -adrenergic blockade both the reflex tachycardia and pressor responses to metaboreflex activation in conscious dogs were significantly attenuated, leading to the conclusion that the HR component of the reflex contributed importantly to the pressor response. However, CO was not measured in that study. Because -adrenergic blockade will affect both chronotropic and inotropic function, we utilized ventricular pacing to investigate whether metaboreflex activation elicits increases in ventricular performance. The present investigation indicates that increases in ventricular contractility, rather than HR per se, are likely more important in eliciting substantial increases in CO. With metaboreflex activation in control experiments, SV remained constant despite the reduction in ventricular filling time due to the reflex tachycardia. White et al. (17) demonstrated using conscious dogs that changes in ventricular rate within the range observed with metaboreflex activation in this model do not elicit any significant change in CO due to the reduction in SV. As HR increases, SV decreases proportionately, presumably due to the reduction in ventricular filling time, thereby causing the product of SV and HR (e.g., CO) to remain unchanged. Thus our results from the control experiments indirectly indicate a significant role for increases in ventricular contractility with metaboreflex activation. In the control experiments SV remained unchanged despite significant increases in HR (and thus reductions in ventricular filling time), thereby allowing CO to increase markedly. Furthermore, when HR was maintained constant, SV increased significantly during metaboreflex activation such that the increases in CO (and SAP) were virtually the same as in control experiments (Fig. 4). Because no significant change in CVP occurred in either control experiments or during constant HR, the rise in SV during metaboreflex activation at constant HR is not likely to be a consequence of the Frank-Starling effect but likely reflects the effects of increases in sympathetic activity to the ventricular myocardium eliciting increases in ventricular contractility. Thus, in the previous study by O'Leary (7) in which -adrenergic blockade attenuated both the tachycardia and pressor responses to muscle metaboreflex activation, it is likely that the reflex increases in ventricular contractility were attenuated (or abolished) by the -adrenergic blockade, and thus the reduced pressor responses were a consequence of attenuated increases in CO.

In support of the concept that activation of muscle afferents can elicit changes in ventricular contractility, Stebbins et al. (12, 14) demonstrated using anesthetized cats that electrically induced static muscle contraction causes significant increases in LV dP/dt. In addition, recently Nobrega et al. (6) observed in humans with atrioventricular block that SV increased significantly during static knee extension when HR was maintained constant at the peak level observed in control experiments when HR was allowed to increase (via a dual-chamber sensing and pacing pacemaker). In that experiment, at constant high HR, the increase in SV during static contraction was accomplished via a decrease in end-systolic volume with no change in end-diastolic volume, indicating that the rise in SV and CO was mediated via an increase in ventricular contractility. However, because static muscle contraction activates both mechano- and metabosensitive afferents (2, 3) and that activation of mechanosensitive afferents also elicits increases in ventricular contractility (13), to what extent the ventricular contractility responses observed with static muscle contraction are attributable to the muscle metaboreflex vs. mechanoreflex is unclear.

In summary, activation of the muscle metaboreflex during dynamic exercise elicits increases in ventricular performance such that SV remains unchanged despite large increases in HR. The combination of constant SV with the reflex tachycardia results in marked increases in CO that, combined with peripheral vasoconstriction, cause substantial increases in arterial blood pressure.