Central modulation of cardiac baroreflex moment‐to‐moment sensitivity during treadmill exercise in conscious cats

Figures

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  FIGURE 1. Time courses of HR and AP during treadmill exercise at three levels of speed. Cats performed treadmill exercises for 1 min (orange areas) each at speeds of 20–30, 40, and 50–60 m/min. In the lower panels, black, dark gray, and light gray indicate systolic, mean, and diastolic AP, respectively. Values are means ± standard deviations. AP, arterial blood pressure; HR, heart rate.

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  FIGURE 2. Averaged HR and AP responses around the onset (1–5 s) and during the late period (51–60 s) of treadmill exercise at three levels of speed. Cardiovascular responses, except systolic AP, were independent of treadmill speed around the onset of exercise, and they were augmented in proportion to the speed during the late period of exercise. The number of trials was 25 at low speed (20–30 m/min), 8 at moderate speed (40 m/min), and 9 at high speed (50–60 m/min). Open circles indicate individual values. Filled circles with error bars indicate means ± standard deviations. Each p value for post hoc tests is described when p < 0.1. p < 0.05 are in bold. AP, arterial blood pressure; DAP, diastolic arterial blood pressure; HR, heart rate; MAP, mean arterial blood pressure; SAP, systolic arterial blood pressure.

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  FIGURE 3. Examples of cardiac baroreflex responses to brief occlusion of the abdominal aorta at rest and during treadmill exercise at three levels of speed in a cat. (a) Baroreflex bradycardia caused by aortic occlusion appeared to be blunted at the onset of treadmill exercise, irrespective of speed. Thereafter, it recovered to the resting level during treadmill exercise at 30 and 40 m/min, while it was blunted again during the latter half of treadmill exercise at 50 m/min. (b) The inhibition of baroreflex bradycardia (two‐headed red arrows) at the exercise onset is shown at a higher chart speed. The data were taken from (a) as indicated by dotted lines. The gray line of the left side (at rest) indicates preocclusion HR, while the gray line of the right side (during the transition from rest to exercise) indicates control HR response to treadmill exercise without occlusion. Orange areas indicate periods of treadmill exercise. Light blue areas indicate periods of occlusion. AP, arterial blood pressure; EMG, electromyogram; HR, heart rate.

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  FIGURE 4. Averaged baroreflex curves between changes in SAP and HR in response to aortic occlusion (a) and the relative baroreflex slope and ratio (b) at rest and during treadmill exercise. The baroreflex ratio was calculated by dividing the net baroreflex‐induced decrease in HR by the peak increase in SAP due to aortic occlusion (detailed information was described in “Methods”). In each cat, relative baroreflex slope and ratio were calculated against the individual mean values at rest, which were taken as “1.” The baroreflex curve and slope were altered at the onset (red) of treadmill exercise, irrespective of speed. As exercise proceeded (light gray → gray → dark gray), the baroreflex curve and slope were restored to the resting level (white) in the cases of low (20–30 m/min) and moderate speed (40 m/min), whereas the inhibition of baroreflex slope occurred again at 45 s of exercise at high speed (50–60 m/min). A similar blunting effect on the relative baroreflex ratio was confirmed. The cardiac baroreflex curves are shown as means without standard deviations for data visibility, while relative baroreflex slopes and ratios are shown as means ± standard deviations. Each P value for post…

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  FIGURE 5. Averaged relative baroreflex slope and ratio during treadmill exercise for the three cats. In each cat, relative baroreflex slope and ratio were calculated against the individual mean values at rest, which were taken as “1.” The averaged relative baroreflex slope and ratio for the three cats were examined, and the values at rest and at the onset of exercise were compared by using a paired t test (as indicated by green areas). The relative baroreflex slope was significantly blunted at the onset of treadmill exercise, irrespective of the speed. The inhibitory effect on relative baroreflex ratio at the onset of exercise was significant only at 40 m/min due to the small number of cats (20–30 m/min, p = 0.369; 40 m/min, p = 0.0423; 50–60 m/min, p = 0.179). Each p value is described. p < 0.05 are in bold.

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  FIGURE 6. Moment‐to‐moment modulation of cardiac baroreflex during treadmill exercise with convex changes in speed. (a) An example of cardiac baroreflex responses to brief aortic occlusion during treadmill exercise with convex changes in speed (30 → 60 → 30 m/min) in a cat. In this case, arrhythmia occurred around 120 s of exercise. (b) Averaged baroreflex slope during the stepwise treadmill exercise summarized in nine trials (upper panel) and three cats (lower panel). In each cat, the relative baroreflex slope was calculated against the individual mean value at rest, which was taken as “1.” Values indicated by the green circle were statistically compared against the resting value to test our hypothesis. The relative baroreflex slope was blunted (p < 0.001) during the speed transition from low to high. On the other hand, such inhibition of baroreflex was restored immediately during the speed transition from high to low (p = 0.277 in trials; p = 0.135 in cats), and then the restored slope was maintained until the end of the exercise. Each p value for post hoc tests is described when p < 0.1. p value for the paired t test is described irrespective of p value. p < 0.05 are in bold. AP,…

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  FIGURE 7. Preocclusion values of HR and AP during treadmill exercise with convex changes in speed. HR and AP values changed depending on the treadmill speed. It is noteworthy that the values of cardiovascular variables were similar (p ≥ 0.779) at 45 s of exercise at high speed and during the second speed transition (from high to low). Each p value for post hoc tests is described. AP, arterial blood pressure; DAP, diastolic arterial blood pressure; HR, heart rate; MAP, mean arterial blood pressure; SAP, systolic arterial blood pressure.