Halliwill, John R.

We tested the hypothesis that ACh or nitric oxide (NO) might be involved in the vasodilation that accompanies a single contraction of the forearm. Eight adults (3 women and 5 men) completed single 1-s-duration contractions of the forearm to raise and lower a weight equivalent to ∼20% maximal voluntary contraction through a distance of 5 cm. In a second protocol, each subject had a cuff, placed completely about the forearm, inflated to 120 mmHg for a 1-s period, then released as a simulation of the mechanical effect of muscle contraction. Three conditions were studied, always in this order: 1) control, with intra-arterial infusion of saline; 2) after muscarinic blockade with atropine; and 3) after NO synthase inhibition with N G-monomethyl-l-arginine (l-NMMA) plus atropine. Forearm blood flow (FBF), measured by combined pulsed and echo Doppler ultrasound, was reduced at rest with l-NMMA-atropine compared with the other two conditions. After the single contraction, there were no effects of atropine, butl-NMMA reduced the peak FBF and the total postcontraction hyperemia. After the single cuff inflation, atropine had no effects, whereasl-NMMA caused changes similar to those seen after contraction, reducing the peak FBF and the total hyperemia. The observation thatl-NMMA reduced FBF in response to both cuff inflation and a brief contraction indicates that NO from the vascular endothelium might modulate the basal level of vascular tone and the mechanical component of the hyperemia with exercise. It is unlikely that ACh and NO from the endothelium are involved in the dilator response to a single muscle contraction.

The goal of this study was to merge the methods currently used to assess beat-by-beat changes in muscle sympathetic nerve activity with a signal-averaging approach and overcome the inherent subjectivity and time-consuming nature of manual analysis of baroreflex-mediated sympathetic responses in humans. This is a retrospective study using data obtained during two prior studies [J. R. Halliwill, J. A. Taylor, and D. L. Eckberg. J. Physiol. (Lond.) 495: 279–288, 1996; C. T. Minson, J. R. Halliwill, T. Young, and M. J. Joyner. FASEB J. 13: A1044, 1999]. Beat-by-beat arterial pressure (Finapres device) and muscle sympathetic nerve activity (microneurography) were recorded in seven healthy, nonsmoking, normotensive subjects (2 men, 5 women) between the ages of 23 and 32 yr during arterial pressure changes induced by bolus injections of nitroprusside and phenylephrine. The muscle sympathetic nerve activity-diastolic pressure relationship was analyzed by both the traditional manual detection method and a novel segregated signal-averaging method. The results show the two analysis approaches are highly correlated across subjects ( r = 0.914, P < 0.05) and are in close agreement [slope for manual detection −6.17 ± 0.91 (SE) vs. slope for segregated signal averaging −5.98 ± 0.83 total integrated activity ⋅ beat−1 ⋅ mmHg−1; P = 0.60]. However, a considerable time savings is seen with the new method (min vs. h). Segregated signal averaging as developed here provides a valid alternative to “by-hand” analysis of beat-by-beat changes in muscle sympathetic nerve activity that occur during dynamic baroreflex-mediated changes in sympathetic outflow. This approach provides an objective, rapid method to analyze nerve recordings.

Dietz, Niki M., John R. Halliwill, John M. Spielmann, Lori A. Lawler, Bettina G. Papouchado, Tamara J. Eickhoff, and Michael J. Joyner. Sympathetic withdrawal and forearm vasodilation during vasovagal syncope in humans. J. Appl. Physiol. 82(6): 1785–1793, 1997.—Our aim was to determine whether sympathetic withdrawal alone can account for the profound forearm vasodilation that occurs during syncope in humans. We also determined whether either vasodilating β2-adrenergic receptors or nitric oxide (NO) contributes to this dilation. Forearm blood flow was measured bilaterally in healthy volunteers ( n = 10) by using plethysmography during two bouts of graded lower body negative pressure (LBNP) to syncope. In one forearm, drugs were infused via a brachial artery catheter while the other forearm served as a control. In the control arm, forearm vascular resistance (FVR) increased from 77 ± 7 units at baseline to 191 ± 36 units with −40 mmHg of LBNP ( P < 0.05). Mean arterial pressure fell from 94 ± 2 to 47 ± 4 mmHg just before syncope, and all subjects demonstrated sudden bradycardia at the time of syncope. At the onset of syncope, there was sudden vasodilation and FVR fell to 26 ± 6 units ( P < 0.05 vs. baseline). When the experimental forearm was treated with bretylium, phentolamine, and propranolol, baseline FVR fell to 26 ± 2 units, the vasoconstriction during LBNP was absent, and FVR fell further to 16 ± 1 units at syncope ( P < 0.05 vs. baseline). During the second trial of LBNP, mean arterial pressure again fell to 47 ± 4 mmHg and bradycardia was again observed. Treatment of the experimental forearm with the NO synthase inhibitor NG-monomethyl-l-arginine in addition to bretylium, phentolamine, and propranolol significantly increased baseline FVR to 65 ± 5 units but did not prevent the marked forearm vasodilation during syncope (FVR = 24 ± 4 vs. 29 ± 8 units in the control forearm). These data suggest that the profound vasodilation observed in the human forearm during syncope is not mediated solely by sympathetic withdrawal and also suggest that neither β2-adrenergic-receptor-mediated vasodilation nor NO is essential to observe this response.

Engelke, Keith A., John R. Halliwill, David N. Proctor, Niki M. Dietz, and Michael J. Joyner. Contribution of nitric oxide and prostaglandins to reactive hyperemia in the human forearm. J. Appl. Physiol. 81(4): 1807–1814, 1996.—We investigated the separate and combined contributions of nitric oxide (NO) and vasodilating prostaglandins as mediators of reactive hyperemia in the human forearm. Forearm blood flow (FBF) was measured with venous occlusion plethysmography after 5 min of ischemia. In one protocol ( n = 12), measurements were made before and after intra-arterial administration of the NO synthase inhibitor NG-monomethyl-l-arginine (l-NMMA) to one forearm. In a separate protocol ( n = 7), measurements were made before and after systemic administration of the cyclooxygenase inhibitor ibuprofen and again afterl-NMMA.l-NMMA reduced baseline FBF at rest (2.7 ± 0.4 to 1.6 ± 0.2 ml ⋅ 100 ml−1⋅ min−1; P < 0.05) and had a modest effect on peak forearm vascular conductance and flow (forearm vascular conductance = 31.1 ± 3.1 vs. 25.7 ± 2.5 ml ⋅ min−1⋅ 100 ml forearm−1⋅ 100 mmHg of perfusion pressure−1⋅ min−1, P < 0.05; FBF = 26.6 ± 2.9 vs. 22.8 ± 2.6 ml ⋅ 100 ml−1⋅ min−1, P = 0.055). Total excess flow above baseline during reactive hyperemia was unaffected byl-NMMA (14.3 ± 3.0 vs. 13.1 ± 2.3 ml/100 ml; P < 0.05). Ibuprofen did not change FBF at rest, reduced peak FBF from 27.6 ± 1.9 to 20.3 ± 2.7 ml ⋅ 100 ml−1⋅ min−1( P < 0.05), but had no effect on total excess flow above baseline. Infusion ofl-NMMA after ibuprofen reduced FBF at rest by 40%, had no effect on peak flow, but reduced total excess flow above baseline from 12.0 ± 2.5 to 7.6 ± 1.3 ml/100 ml ( P < 0.05). These data demonstrate that NO synthase inhibition has a modest effect on peak vasodilation during reactive hyperemia but plays a minimal role later. Prostaglandins appear to be important determinants of peak flow. The effects of NO synthase inhibition during reactive hyperemia may also be potentiated by concurrent cyclooxygenase inhibition.

We evaluated a method of baroreflex testing involving sequential intravenous bolus injections of nitroprusside followed by phenylephrine and phenylephrine followed by nitroprusside in 18 healthy men and women, and we drew inferences regarding human sympathetic and vagal baroreflex mechanisms. We recorded the electrocardiogram, photoplethysmographic finger arterial pressure, and peroneal nerve muscle sympathetic activity. We then contrasted least squares linear regression slopes derived from the depressor (nitroprusside) and pressor (phenylephrine) phases with 1) slopes derived from spontaneous fluctuations of systolic arterial pressures and R-R intervals, and 2) baroreflex gain derived from cross-spectral analyses of systolic pressures and R-R intervals. We calculated sympathetic baroreflex gain from integrated muscle sympathetic nerve activity and diastolic pressures. We found that vagal baroreflex slopes are less when arterial pressures are falling than when they are rising and that this hysteresis exists over pressure ranges both below and above baseline levels. Although pharmacological and spontaneous vagal baroreflex responses correlate closely, pharmacological baroreflex slopes tend to be lower than those derived from spontaneous fluctuations. Sympathetic baroreflex slopes are similar when arterial pressure is falling and rising; however, small pressure elevations above baseline silence sympathetic motoneurons. Vagal, but not sympathetic baroreflex gains vary inversely with subjects’ ages and their baseline arterial pressures. There is no correlation between sympathetic and vagal baroreflex gains. We recommend repeated sequential nitroprusside followed by phenylephrine doses as a simple, efficientmeans to provoke and characterize human vagal and sympathetic baroreflex responses.

Estimates of calf blood flow with venous occlusion plethysmography vary widely between studies, perhaps due to the use of different plethysmographs. Consequently, we compared calf blood flow estimates at rest and during reactive hyperemia in eight healthy subjects (four men and four women) with two commonly used plethysmographs: the mercury-in-silastic (Whitney) strain gauge and Dohn air-filled cuff. To minimize technical variability, flow estimates were compared with a Whitney gauge and a Dohn cuff on opposite calves before and after 10 min of bilateral femoral arterial occlusion. To account for any differences between limbs, a second trial was conducted in which the plethysmographs were switched. Resting flows did not differ between the plethysmographs (P = 0.096), but a trend toward lower values with the Whitney was apparent. Peak flows averaged 37% lower with the Whitney (27.8 +/- 2.8 ml.dl-1.min-1) than with the Dohn plethysmograph (44.4 +/- 2.8 ml.dl-1.min-1; P < 0.05). Peak flow expressed as a multiple above baseline was also lower with the Whitney (10-fold) than with the Dohn plethysmograph (14.5-fold; P = 0.02). Across all flows at rest and during reactive hyperemia, estimates were highly correlated between the plethysmographs in all subjects (r2 = 0.96-0.99). However, the mean slope for the Whitney-Dohn relationship was only 60 +/- 2%, indicating that over a wide range of flows the Whitney gauge estimate was 40% lower than that for the Dohn cuff. These results demonstrate that the same qualitative results can be obtained with either plethysmograph but that absolute flow values will generally be lower with Whitney gauges.

The contributions of acetylcholine and/or nitric oxide (NO) to the rapid changes in human forearm blood flow (FBF) at the onset and recovery from mild exercise were studied in eight subjects. Rhythmic handgrip contractions were performed during brachial artery infusions of saline (2 ml/min; control), atropine (0.2 mg over 3 min), to block acetylcholine binding to muscarinic receptors, or atropine + N G-monomethyl-l-arginine (l-NMMA; 4 mg/min for 4 min), to additionally inhibit NO synthase. Brachial artery mean blood velocity (MBV; pulsed Doppler ultrasound) and diameter (echo Doppler) were measured continuously, and FBF was calculated. Atropine reduced acetylcholine-induced increases in FBF by ∼71% ( P < 0.05). FBF at rest was reduced by atropine and further reduced with atropine +l-NMMA. Both drug conditions reduced FBF during exercise by ∼10% compared with control, with no difference between drug treatments. Brachial artery diameter was unchanged from rest by exercise, recovery, and drug treatments. Neither drug treatment altered the rate or magnitude of the increase in FBF above rest. Peak FBF after exercise was reduced by atropine and atropine + l-NMMA. Total FBF during 5 min of recovery was reduced with atropine +l-NMMA compared with control and atropine. The results suggest that 1) acetylcholine and NO mechanisms additively contribute to FBF levels at rest, 2) a cholinergic mechanism adjusts the absolute FBF levels during exercise, 3) neither acetylcholine nor NO is essential to observe the normal time course or magnitude of the exercise response, and 4) NO contributes to the FBF response during recovery from exercise.

We tested the hypothesis that acute hypoxia would alter the sensitivity of arterial baroreflex control of both heart rate and sympathetic vasoconstrictor outflow. In 16 healthy, nonsmoking, normotensive subjects (8 women, 8 men, age 20–33 yr), we assessed baroreflex control of heart rate and muscle sympathetic nerve activity by using the modified Oxford technique during both normoxia and hypoxia (12% O2). Compared with normoxia, hypoxia reduced arterial O2 saturation levels from 96.8 ± 0.3 to 80.7 ± 1.4% ( P < 0.001), increased heart rate from 59.8 ± 2.4 to 79.4 ± 2.9 beats/min ( P < 0.001), increased mean arterial pressure from 96.7 ± 2.5 to 105.0 ± 3.3 mmHg ( P = 0.002), and increased sympathetic activity 126 ± 58% ( P < 0.05). The sensitivity for baroreflex control of both heart rate and sympathetic activity was not altered by hypoxia (heart rate: −1.02 ± 0.09 vs. −1.02 ± 0.11 beats · min−1 · mmHg−1; nerve activity: −5.6 ± 0.9 vs. −6.2 ± 0.9 integrated activity · beat−1 · mmHg−1; both P > 0.05). Acute exposure to hypoxia reset baroreflex control of both heart rate and sympathetic activity to higher pressures without changes in baroreflex sensitivity.

In sedentary individuals, postexercise hypotension after a single bout of aerobic exercise is due to a peripheral vasodilation. Endurance exercise training has the potential to modify this response and perhaps reduce the degree of postexercise hypotension. We tested the hypothesis that endurance exercise-trained men and women would have blunted postexercise hypotension compared with sedentary subjects but that the mechanism of hypotension would be similar (i.e., vasodilation). We studied 16 endurance-trained and 16 sedentary men and women. Arterial pressure, cardiac output, and total peripheral resistance were determined before and after a single 60-min bout of exercise at 60% peak oxygen consumption. All groups exhibited a similar degree of postexercise hypotension (∼4–5 mmHg; P < 0.05 vs. preexercise). In sedentary men and women, hypotension was the result of vasodilation (Δresistance: −8.9 ± 2.2%). In endurance-trained women, hypotension was also the result of vasodilation (−8.1 ± 4.1%). However, in endurance-trained men, hypotension was the result of a reduced cardiac output (−5.2 ± 2.4%; P < 0.05 vs. all others) and vasodilation was absent (−0.7 ± 3.3%; P < 0.05 vs. all others). Thus we conclude the magnitude of postexercise hypotension is similar in sedentary and endurance-trained men and women but that endurance-trained men and women achieve this fall in pressure via different mechanisms.

Background —We tested sympathetic and cardiovagal baroreflex sensitivity during the placebo or “low-hormone” phase (LH) and 2 to 3 weeks later during the “high-hormone” phase (HH) of oral contraceptive (OC) use in 9 women. Methods and Results —Sympathetic baroreflex sensitivity was assessed by intravenous doses of sodium nitroprusside and phenylephrine and defined as the slope relating muscle sympathetic nerve activity (by microneurography) and diastolic blood pressure. Cardiovagal baroreflex sensitivity was defined as the slope relating R-R interval and systolic blood pressure. No difference was observed for resting muscle sympathetic nerve activity or plasma norepinephrine levels. However, sympathetic baroreflex sensitivity was greater and mean arterial pressure was higher during the LH than in the HH phase. Similarly, cardiovagal baroreflex sensitivity was greater in the LH than in the HH phase. Conclusions —Sympathetic and cardiovagal baroreflex sensitivities change during the 28-day course of OC use. Furthermore, changes in baroreflex sensitivity with OC differ from changes in baroreflex sensitivity during the normal menstrual cycle.