Effects of acute cardioselective and non-selective beta-adrenergic blockade on left-ventricular volumes and vascular resistance at rest and during exercise

The effects of beta1-adrenergic blockade on cardiovascular oxygen flow in normoxic and hypoxic humans at exercise

At exercise steady state, the lower the arterial oxygen saturation (SaO(2)), the lower the O(2) return (QvO(2)). A linear relationship between these variables was demonstrated. Our conjecture is that this relationship describes a condition of predominant sympathetic activation, from which it is hypothesized that selective beta1-adrenergic blockade (BB) would reduce O(2) delivery (QaO(2)) and QvO(2). To test this hypothesis, we studied the effects of BB on QaO(2) and QvO(2) in exercising humans in normoxia and hypoxia. O(2) consumption VO(2), cardiac output Q, CO(2) rebreathing), heart rate, SaO(2) and haemoglobin concentration were measured on six subjects (age 25.5 +/- 2.4 years, mass 78.1 +/- 9.0 kg) in normoxia and hypoxia (inspired O(2) fraction of 0.11) at rest and steady-state exercises of 50, 100, and 150 W without (C) and with BB with metoprolol. Arterial O(2) concentration (CaO(2)), QaO(2) and QvO(2) were then computed. Heart rate, higher in hypoxia than in normoxia, decreased with BB. At each VO(2), Q was higher in hypoxia than in normoxia. With BB, it decreased during intense exercise in normoxia, at rest, and during light exercise in hypoxia. SaO(2) and CaO(2) were unaffected by BB. The QaO(2) changes under BB were parallel to those in Q.QvO(2) was unaffected by exercise in normoxia. In hypoxia the slope of the relationship between QaO(2) and VO(2) was lower than 1, indicating a reduction of QvO(2) with increasing workload. QvO(2) was a linear function of SaO(2) both in C and in BB. The line for BB was flatter than and below that for C. The resting QvO(2) in normoxia, lower than the corresponding exercise values, lied on the BB line. These results agree with the tested hypothesis. The two observed relationships between QvO(2) and SaO(2) apply to conditions of predominant sympathetic or vagal activation, respectively. Moving from one line to the other implies resetting of the cardiovascular regulation.

Direct measurement of cardiac sympathetic efferent nerve activity during dynamic exercise

The assumption that tachycardia during light to moderate exercise was predominantly controlled by withdrawal of cardiac parasympathetic nerve activity but not by augmentation of cardiac sympathetic nerve activity (CSNA) was challenged by measuring CSNA during treadmill exercise (speed, 10-60 m/min) for 1 min in five conscious cats. As soon as exercise started, CSNA and heart rate (HR) increased and mean arterial pressure (MAP) decreased; their time courses at the initial 12-s period of exercise were irrespective of the running speed. CSNA increased 168-297% at 7.1 +/- 0.4 s from the exercise onset, and MAP decreased 8-13 mmHg at 6.0 +/- 0.3 s, preceding the increase of 40-53 beats/min in HR at 10.5 +/- 0.4 s. CSNA remained elevated during the later period of exercise, whereas HR and MAP gradually increased until the end of exercise. After the cessation of exercise, CSNA returned quickly to the control, whereas HR was slowly restored. In conclusion, cardiac sympathetic outflow augments at the onset of and during dynamic exercise even though the exercise intensity is low to moderate, which may contribute to acceleration of cardiac pacemaker rhythm.

Effects of acute beta-adrenergic receptor blockade on age-associated changes in cardiovascular performance during dynamic exercise

The cardiovascular response to beta-adrenergic stimulation is markedly blunted with advancing age, and this blunting may underlie some of the prominent age-associated changes in the hemodynamic profile during dynamic exercise. To examine this hypothesis, we administered the nonselective beta-adrenergic receptor blocker propranolol (0.15 mg/kg IV) to 25 healthy normotensive men ages 28 to 72 years from the Baltimore Longitudinal Study of Aging (BLSA) immediately before maximal upright cycle ergometry with 99mTc gated cardiac blood pool scintigraphy. Their hemodynamic responses to exercise were compared with those of 70 age-matched healthy unmedicated male BLSA control subjects. The maximal cycle work rate achieved was similar in propranolol-treated men (158 +/- 32 W) and control subjects (148 +/- 32 W) and declined similarly with age in both groups. Hemodynamics at seated rest were not age-related in either group; however, propranolol-treated men had lower heart rates (HR), systolic blood pressure (SBP), ejection fraction, and cardiac index than control subjects but higher end-diastolic volume index (EDVI) and end-systolic volume index (ESVI) by covariance analysis. At maximal effort, several striking age-drug interactions were evident: Propranolol caused a greater reduction in HR and greater increases in EDVI and stroke volume index (SVI) in younger than in older men. Hence, at maximal work rate, HR declined less with age in the propranolol group (0.46 versus 1.09 beats per minute per year, P < .05 by covariance analysis); EDVI and SVI decreased with age (0.27 and 0.48 mL/m2 per year, respectively) after propranolol compared with increases of 0.47 and 0.16 mL/m2 per year in control subjects, respectively, each P < or = .05 by covariance analysis. The left ventricular contractility index, SBP/ESVI, at exhaustion was reduced by propranolol to a greater extent in younger than older men. Thus, acute beta-adrenergic blockade reverses the age-associated ventricular dilation at end diastole and end systole observed during upright cycle exercise and blunts the decline in maximal HR and myocardial contractility. These data suggest that the age-associated declines in maximal HR and left ventricular contractility during vigorous exercise are manifestations of reduced beta-adrenergic responsivity with advancing age which is partially offset by exercise-induced ventricular dilation.