Inhibition of alpha-adrenergic vasoconstriction in exercising human thigh muscles

Abstract

The mechanisms underlying metabolic inhibition of sympathetic responses within exercising skeletal muscle remain incompletely understood. The aim of the present study was to test whether alpha(2)-adrenoreceptor-mediated vasoconstriction was more sensitive to metabolic inhibition than alpha(1)-vasoconstriction during dynamic knee-extensor exercise. We studied healthy volunteers using two protocols: (1) wide dose ranges of the alpha-adrenoreceptor agonists phenylephrine (PE, alpha(1) selective) and BHT-933 (BHT, alpha(2) selective) were administered intra-arterially at rest and during 27 W knee-extensor exercise (n= 13); (2) flow-adjusted doses of PE (0.3 microg kg(-1) l(-1)) and BHT (15 microg kg(-1) l(-1)) were administered at rest and during ramped exercise (7 W to 37 W; n= 10). Ultrasound Doppler and thermodilution techniques provided direct measurements of femoral blood flow (FBF). PE (0.8 microg kg(-1)) and BHT (40 microg kg(-1)) produced comparable maximal reductions in FBF at rest (-58 +/- 6 versus-64 +/- 4%). Despite increasing the doses, PE (1.6 microg kg(-1) min(-1)) and BHT (80 microg kg(-1) min(-1)) caused significantly smaller changes in FBF during 27 W exercise (-13 +/- 4 versus-3 +/- 5%). During ramped exercise, significant vasoconstriction at lower intensities (7 and 17 W) was seen following PE (-16 +/- 5 and -16 +/- 4%), but not BHT (-2 +/- 4 and -4 +/- 5%). At the highest intensity (37 W), FBF was not significantly changed by either drug. Collectively, these data demonstrate metabolic inhibition of alpha-adrenergic vasoconstriction in large postural muscles of healthy humans. Both alpha(1)- and alpha(2)-adrenoreceptor agonists produce comparable vasoconstriction in the resting leg, and dynamic thigh exercise attenuates alpha(1)- and alpha(2)-mediated vasoconstriction similarly. However, alpha(2)-mediated vasoconstriction appears more sensitive to metabolic inhibition, because alpha(2) is completely inhibited even at low workloads, whereas alpha(1) becomes progressively inhibited with increasing workloads.

Figure 1. Experimental protocols 1–4 represented graphically

Time courses are given in minutes on the top axis of each panel. BL, baseline; BHT, BHT-933; PE, phenylephrine; Prop, propranolol; Yoh, yohimbine. Protocol 1: the β-receptor agonist isoproterenol (1, 2, 4 and 8 ng kg⁻¹ min⁻¹, 2-min infusions) was administered before and after 150 min of β-blockade with propranalol (12.5 μg kg⁻¹ bolus and 0.125 μg kg⁻¹ min⁻¹ maintenance). Protocol 2: the selective α₂-adrenoreceptor agonist BHT-933 (BHT, 20 and 40 μg kg⁻¹ min⁻¹, 2-min infusions) and the selective α₁-agonist phenylephrine (PE, 0.4 and 0.8 μg kg⁻¹ min⁻¹, 2-min infusions) were applied before and during the last 10 min of a 20 min infusion of the α₂-antagonist yohimbine (5 μg kg⁻¹ min⁻¹). Protocol 3: dose–response for BHT (2.5–80 μg kg⁻¹ min⁻¹, 2-min infusions) and PE (0.025–1.6 μg kg⁻¹ min⁻¹, 2-min infusions) at rest and during 27 W knee extensor exercise. Protocol 4: flow-adjusted administration of BHT (15 μg kg⁻¹ l⁻¹, 3-min infusions) and PE (0.3 μg kg⁻¹ l⁻¹, 3-min infusions) at rest and during ramped exercise.

Figure 2. Femoral blood flow (FBF) during infusion of the non-selective β-agonist isoproterenol (1–8 ng kg⁻¹ min⁻¹) before (black bars), and 150 min after initiation of non-selective β-blockade with propranolol (grey bars)

Propranolol provided virtually complete blockade of isoproterenol-mediated vasodilatation (n= 9). *P < 0.05 compared to baseline and compared to responses after propranolol.

Figure 3. Specificity of BHT for α₂-adrenoreceptors was validated by measurements of femoral blood flow before and after selective α₂-blockade with yohimbine

Top panel, administration of the selective α₂-agonist BHT (20 and 40 μg kg⁻¹ min⁻¹) produced significant vasoconstriction (grey bars), and this effect was inhibited following yohimbine (5 μg kg⁻¹ min⁻¹) (hatched bars). Bottom panel, the selective α₁-agonist phenylephrine (PE, 0.4 and 0.8 μg kg⁻¹ min⁻¹) produced significant vasoconstriction both before (black bars) and after administration of yohimbine (n= 7) (hatched bars). *P < 0.05 compared to baseline; §P < 0.05 compared to BHT responses after yohimbine.

Figure 4. Dose–response relationships for intra-arterial PE and BHT at rest

A, during incremental doses of PE, femoral artery diameter (FAD, top) decreased 35%, with concomitant reductions in femoral blood flow (FBF, middle) and femoral vascular conductance (FVC, bottom). B, during incremental doses of BHT, FAD decreased only slightly and only at the highest doses. However, FBF and FVC both decreased significantly, and of similar magnitudes to the responses seen during PE. *P < 0.05 compared to resting baseline, §P < 0.05 between comparable doses of PE and BHT. On the lower four panels the mean ±s.e. symbols are accurate for the absolute changes, whereas the alternative y axis for relative changes has been included for easier comparison to Fig. 5.

Figure 5. Dose–response relationships for intra-arterial PE and BHT during 27 W exercise

A, during exercise, PE reduced FAD only at the highest dose (top). The changes in FBF (middle) and FVC (bottom) during PE were blunted compared to rest. A significant change in FVC occurred at the highest doses of PE compared to baseline and compared to the highest dose of BHT, and FBF also fell slightly during high doses of PE. B, during exercise, the response to BHT was completely abolished, with no significant change in FAD, FBF, or FVC, even at the highest dose. *P < 0.05 compared to exercising baseline, §P < 0.05 between comparable doses of PE and BHT. On the lower four panels the mean ±s.e. symbols are accurate for the absolute changes, whereas the alternative y axis for relative changes has been included for easier comparison to Fig. 4.

Figure 6. Post-hoc flow-adjusted doses for PE and BHT at rest (triangles) and during 27 W exercise (circles)

A, PE-induced reduction in FAD (top) was significantly smaller during exercise compared to rest for all flow-adjusted doses. However, only the FBF response (middle) to the highest flow-adjusted dose of PE was statistically reduced during exercise, and FVC responses (bottom) at rest and during exercise were not statistically different during flow-adjusted doses of PE. B, BHT-induced reduction in FAD (top) was attenuated at the lower flow-adjusted dose, and the FBF (middle) and FVC (bottom) responses to both flow-adjusted doses of BHT were significantly reduced. §P < 0.05 compared to flow-adjusted dose–response at rest.

Figure 7. Absolute FBF and FVC during ramped exercise of 7–37 W with superimposed administration of PE (A) and BHT (B)

Both PE and BHT were flow-adjusted to the exercise-induced increase in FBF. A, at lower workloads (7 and 17 W), PE-induced decreases in FBF and FVC were seen (black bars), and this response was abolished at higher intensities. B, BHT administration caused no changes in FBF or FVC (grey bars) at any exercise intensity. *P < 0.05 compared to predrug value for each level of exercise.

Figure 8. Changes in FBF (top) and FVC (bottom) during ramped exercise of 7–37 W with superimposed administration of PE (black bars) and BHT (grey bars)

All responses to BHT and PE during exercise were significantly smaller than at rest. At lower exercise intensities, PE produced significant changes in FBF and FVC, and these responses were significantly larger than the response to BHT at 7 W §P < 0.05 between PE and BHT at same level of exercise.

Figure 9. Comparison of ultrasound Doppler and thermodilution techniques for determining femoral blood flow during knee-extensor exercise

The individual data shown by small filled circles are obtained within the same minute for the two methods in 10 subjects during increasing levels of exercise intensity from 7 to 37 W before administration of α-agonists. The mean values obtained by the two methods, shown by the larger filled circles, did not differ significantly at any intensity of exercise. The variation of the data obtained by the Doppler method was generally larger than the variation of the thermodilution data. Summary data are shown as means ± 95% confidence intervals.