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Articles in PresS, published online ahead of print April 11, 2002
Am J Physiol Heart Circ Physiol, 10.1152/ajpheart.00152.2002
Submitted on February 25, 2002
Accepted on April 9, 2002
1 Physiology, Wayne State University School of Medicine, Detroit, MI, USA
2 Surgery, Wayne State University School of Medicine, Detroit, MI, USA
3 Medicine, John D. Dingell Veterans Administration Medical Center, Detroit, MI, USA
* To whom correspondence should be addressed. E-mail: doleary{at}med.wayne.edu.
We investigated the effect of muscle metaboreflex activation on left circumflex coronary blood flow (CBF) and vascular conductance (CVC) in conscious, chronically instrumented dogs during treadmill exercise ranging from mild to severe workloads. Metaboreflex responses were also observed during mild exercise with constant heart rate (HR; 225 beats/min) and B1 adrenergic receptor blockade in order to attenuate the substantial reflex increases in cardiac work. The muscle metaboreflex was activated via graded partial occlusion of hindlimb blood flow. During mild exercise, with muscle metaboreflex activation, hindlimb ischemia elicited significant increases in MAP, HR, and CO (+39.0 [plusmn]] 5.2 mmHg, +29.9 [plusmn]] 7.7 beats/min, and +2.0 [plusmn]] 0.4 l/min, respectively; all changes P < 0.05). Coronary blood flow increased from 51.9 [plusmn]] 4.3 to 88.5 [plusmn]] 6.6 ml/min, (P< 0.05), whereas no significant change in CVC occurred (0.56 [plusmn]] 0.06 vs. 0.59 [plusmn]] 0.05 ml/min/mmHg; P> 0.05). Similar responses were observed during moderate exercise. In contrast, with metaboreflex activation during severe exercise, no further increases in CO or HR occurred, the increases in MAP and CBF were attenuated and a significant reduction in CVC was observed (1.00 [plusmn]] 0.12 vs. 0.90 [plusmn]] 0.13 ml/min/mmHg; P< 0 .05). Similarly, when the metaboreflex was activated during mild exercise with the rise in cardiac work lessened (via constant HR and B1 blockade), no increase in CO occurred, the MAP and CBF responses were attenuated (+15.6 [plusmn]] 4.5 mmHg, +8.3 [plusmn]] 2 ml/min) and CVC significantly decreased from 0.63 [plusmn]] 0.11 to 0.53 [plusmn]] 0.10 ml/min/mmHg. We conclude that the muscle metaboreflex induced increases in sympathetic nerve activity to the heart functionally vasoconstricts the coronary vasculature.
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