Dynamic carotid baroreflex control of the peripheral circulation during exercise in humans

Abstract

We sought to determine the dynamic relationship between carotid baroreflex (CBR)-mediated control and local control of the skeletal muscle vasculature during dynamic exercise. In 12 subjects (18-35 years old), oscillatory neck pressure (NP, +40 mmHg) was applied at 0.1 Hz (i.e. 5 s on, 5 s off) for 5 min to determine the degree of CBR control over heart rate (HR), arterial blood pressure (ABP), muscle sympathetic nerve activity (MSNA), femoral blood velocity and skeletal muscle tissue oxygenation at rest and during 7 W dynamic knee-extension exercise. Skeletal muscle tissue oxygenation measurements of both the exercising and nonexercising leg were evaluated. Fast Fourier transformation was performed on 5 min segments to calculate spectral power of the R-R interval (RRI), ABP, MSNA, femoral blood velocity and tissue oxygenation time series, and the low-frequency (LF, 0.085-0.115 Hz) power spectra were compared to evaluate the degree of CBR-mediated entrainment for each variable. At rest, sinusoidal NP significantly increased LF spectral power of RRI, ABP, MSNA and femoral blood velocity. During exercise, sinusoidal NP provoked a similar increase in spectral power for RRI and MSNA, while CBR-mediated changes in ABP and femoral blood velocity were attenuated compared to rest. Changes in spectral power of skeletal muscle tissue oxygenation during sinusoidal NP were similar between the exercising and nonexercising leg at rest. However, during exercise the changes in skeletal muscle tissue oxygenation power were significantly less in the exercising leg, while changes in the nonexercising leg were similar to rest. We have demonstrated simultaneous entrainment of all CBR end-organ measurements, ranging from cardiac chronotropic effects to alterations at the level of the skeletal muscle microcirculation. Moreover, we have identified a significant and specific attenuation of end-organ responsiveness to CBR-mediated sympathoexcitation in the vasculature of the exercising muscle. However, despite a shift towards more predominant local control over the exercising muscle vasculature, systemic arterial blood pressure was well preserved.

Figure 1. Individual end-organ responses

Sample tracing for R–R interval (RRI), muscle sympathetic nerve activity (MSNA), arterial blood pressure (ABP), femoral blood velocity (FBV) and tissue oxygenation (TO_m) during sinusoidal NP (0.1 Hz) at rest (left panels) and during 7 W exercise (middle panels). Right panels indicate the corresponding spectra at rest (black shading) and during exercise (grey shading). Note that spectral power of RRI and MSNA are similar between rest and exercise and are thus superimposed at the LF (0.085–0.115 Hz) frequency range.

Figure 2. Resting LF (0.085–0.115 Hz) power of all end-organ measurements during baseline (black bars) and 0.1 Hz sinusoidal NP (grey bars) after ln data transformation

Note that values for femoral blood velocity and TO_m become negative following ln transformation since starting value is <1. *Significantly different from baseline, P < 0.05.

Figure 3. ln LF (0.085–0.115 Hz) power of all end-organ measurements during exercise baseline (black bars) and 0.1 Hz sinusoidal NP (grey bars) during 7 W knee extension exercise

*Significantly different from baseline, P < 0.05.

Figure 4. Changes (from baseline) in ln power of all measurements in response to sinusoidal NP at rest (black bars) and during exercise (grey)

*Significantly different from resting sinusoidal NP, P < 0.05.

Figure 5. Changes (from baseline) in ln power of tissue oxygenation (TO_m) in the exercising leg and nonexercising leg at rest and during unilateral knee-extension exercise

*Significantly different from rest, P < 0.05.