Baroreflex modulation of muscle sympathetic nerve activity during posthandgrip muscle ischemia in humans

To identify whether muscle metaboreceptor stimulation alters baroreflex con trol of muscle sympathetic nerve activity (MSNA), MSNA, beat-by-beat arteri al blood pressure (Finapres), and electrocardiogram were recorded in 11 hea lthy subjects in the supine position. Subjects performed 2 min of isometric handgrip exercise at 40% of maximal voluntary contraction followed by 2.5 min of posthandgrip muscle ischemia. During muscle ischemia, blood pressure was lowered and then raised by intravenous bolus infusions of sodium nitro prusside and phenylephrine HCl, respectively. The slope of the relationship between MSNA and diastolic blood pressure was more negative (P < 0.001) du ring posthandgrip muscle ischemia (-201.9 +/- 20.4 units.beat(-1).mmHg(-1)) when compared with control conditions (- 142.7 +/- 17.3 units.beat(-1).mmH g(-1)). No significant change in the slope of the relationship between hear t rate and systolic blood pressure was observed. However, both curves shift ed during postexercise ischemia to accommodate the elevation in blood press ure and MSNA that occurs with this condition. These data suggest that the s ensitivity of baroreflex modulation of MSNA is elevated by muscle metaborec eptor stimulation, whereas the sensitivity of baroreflex of modulate heart rate is unchanged during posthandgrip muscle ischemia.