Autonomic mechanisms of muscle metaboreflex control of heart rate

Ischemia in active skeletal muscle induces reflex increases in systemic arterial pressure (SAP) and heart rate (HR), termed the muscle metaboreflex. When metaboreflex activation is maintained during postexercise muscle ischemia, SAP remains elevated; however, HR decreases. Why the HR responses differ with metaboreflex activation during exercise vs. during postexercise ischemia while the SAP responses are similar in each setting remains unclear. Two hypotheses were tested: 1) the increase in HR with muscle ischemia occurs predominantly via an increase in sympathetic activity, and 2) sympathetic activity to the heart remains elevated during post-exercise ischemia; however, HR decreases because of an increase in parasympathetic outflow. The muscle metaboreflex was activated in conscious dogs during treadmill exercise (3.2 kph, 0% grade) by progressively decreasing perfusion to the hindlimbs. Experiments were performed before and after muscarinic (atropine) or beta- (atenolol or propranolol) receptor blockade. In control experiments, once beyond the threshold for the reflex, the HR sensitivity of the muscle metaboreflex averaged -2.4 +/- 0.3 beats.min-1.mmHg-1 and the reflex open-loop gain averaged -3.2 +/- 0.3 (calculated as the ratio of the increase in HR or SAP to the decrease in hindlimb perfusion pressure beyond threshold). Atropine had no effect on either HR sensitivity (-2.7 +/- 0.4 beats.min-1.mmHg-1) or open-loop gain (-3.3 +/- 0.5, both P > 0.05 vs. control). However, pretreatment with beta-receptor antagonist significantly decreased both HR sensitivity (-0.7 +/- 0.1 beats.min-1.mmHg-1, P < 0.001) and open-loop gain (-1.9 +/- 0.3, P < 0.01). During postexercise ischemia, HR decreased while SAP remained elevated.(ABSTRACT TRUNCATED AT 250 WORDS)