Letter to H.B. Stenzel from L.W. Stephenson on 1949-10-25

Jackson School of Geoscience

Muscle metaboreflex attenuates spontaneous heart rate baroreflex sensitivity during dynamic exercise

Hypoperfusion of active skeletal muscle elicits a reflex pressor response termed the muscle metabore-flex. Dynamic exercise attenuates spontaneous baroreflex sensitivity (SBRS) in the control of heart rate (HR) during rapid, spontaneous changes in blood pressure (BP). Our objective was to determine whether muscle metaboreflex activation (MRA) further diminishes SBRS. Conscious dogs were chronically instrumented for measurement of HR. cardiac Output, mean arterial pressure. and left ventricular systolic pressure (LVSP) at rest and during mild (3.2 km/h) or moderate (6.4 km/h at 10% grade) dynamic exercise before and after MRA (via partial reduction of hindlimb blood flow). SBRS was evaluated as the slopes of the linear relations (LRs) between HR and LVSP during spontaneous sequences of at least three consecutive beats when HR changed inversely vs. pressure (expressed as beats (.) min(-1) (.) mmHg(-1)). During mild exercise, these LRs shifted upward. with a significant decrease in SBRS (-3.0 +/- 0.4 vs. -5.2 +/- 0.4. P < 0.05 vs, rest). MRA shifted LRS upward and rightward and decreased SBRS (-2.1 +/- 0.1, P < 0.05 vs. mild exercise). Moderate exercise shifted LRS upward and rightward and significantly decreased SBRS (-1.2 +/- 0. 1. P < 0.05 vs. rest). MRA elicited further Upward and rightward shifts of the LRs and reductions in SBRS (-0.9 +/- 0.1. P < 0.05 vs. moderate exercise). We Conclude that dynamic exercise resets the arterial baroreflex to higher BP and HR as exercise intensity increases. In addition, increases in exercise intensity. as well as MRA. attenuate SBRS

Spontaneous baroreflex control of heart rate during exercise and muscle metaboreflex activation in heart failure

In heart failure (HF), there is a reduced baroreflex sensitivity at rest, and during dynamic exercise there is enhanced muscle metaboreflex activation (MRA). However, how the arterial baroreflex modulates HR during exercise is unknown. We tested the hypothesis that spontaneous baroreflex sensitivity (SBRS) is attenuated during exercise in HF and that MRA further depresses SBRS. In seven conscious dogs we measured heart rate (HR), cardiac output, and left ventricular systolic pressure at rest and during mild and moderate dynamic exercise, before and during MRA (via imposed reductions of hindlimb blood flow), and before and after induction of HF (by rapid ventricular pacing). SBRS was assessed by the sequences method. In control, SBRS was reduced from rest with a progressive resetting of the baroreflex stimulus-response relationship in proportion to exercise intensity and magnitude of MRA. In HF, SBRS was significantly depressed in all settings; however, the changes with exercise and MRA occurred with a pattern similar to the control state. As in control, the baroreflex stimulus-response relationship showed an intensity- and muscle metaboreflex (MMR)-dependent rightward and upward shift. The results of this study indicate that HF induces an impairment in baroreflex control of HR at rest and during exercise, although the effects of exercise and MRA on SBRS occur with a similar pattern as in control, indicating the persistence of some vagal activity