Direct effects of alpha 2-adrenergic receptor stimulation on intravascular systemic capacity in the dog

Alpha adrenergic receptor blockade increases capillarization and fractional O2 extraction and lowers blood flow in contracting human skeletal muscle

AIM

To assess the effect of elevated basal shear stress on angiogenesis in humans and the role of enhanced skeletal muscle capillarization on blood flow and O₂ extraction.

METHODS

Limb haemodynamics and O₂ extraction were measured at rest and during one-leg knee-extensor exercise (12 and 24 W) in 10 healthy untrained young men before and after 4-week treatment with an α₁ receptor-antagonist (Terazosin, 1-2 mg day^-1 ). Corresponding biopsies were taken from the m. vastus lateralis.

RESULTS

Resting leg blood flow was increased by 57% 6 h following Terazosin treatment (P < 0.05), while basal capillary-to-fibre ratio was 1.69 ± 0.08 and increased to 1.90 ± 0.08 after treatment (P < 0.05). Leg O₂ extraction during knee-extensor exercise was higher (4-5%; P < 0.05), leg blood flow and venous lactate levels lower (6-7%; P < 0.05), while leg VO₂ was not different after Terazosin treatment.

CONCLUSIONS

These results demonstrate that daily treatment with an α-adrenergic receptor blocker induces capillary growth in human skeletal muscle, likely due to increased shear stress. The increase in capillarization resulted in an increased fractional O₂ extraction, a lower blood flow and venous lactate levels in the exercising leg. The increase in capillarization, and concomitant functional readouts in the exercising leg, may provide a basis for novel angiotherapy.

Vascular capacitance and cardiac output in pacing-induced canine models of acute and chronic heart failure

The relationship between stressed and total blood volume, total vascular capacitance, central blood volume, cardiac output (CO), and pulmonary capillary wedge pressure (Ppcw) was investigated in pacing-induced acute and chronic heart failure. Acute heart failure was induced in anesthetized splenectomized dogs by a volume load (20 mL/kg over 10 min) during rapid right ventricular pacing at 250 beats/min (RRVP) for 60 min. Chronic heart failure was induced by continuous RRVP for 2-6 weeks (average 24 +/- 2 days). Total vascular compliance and capacitance were calculated from the mean circulatory filling pressure (Pmcf) during transient circulatory arrest after acetylcholine at three different circulating volumes. Stressed blood volume was calculated as a product of compliance and Pmcf, with the total blood volume measured by a dye dilution. Central blood volume (CBV) and CO were measured by thermodilution. Central (heart and lung) vascular capacitance was estimated from the plot of Ppcw against CBV. Acute volume loading without RRVP increased capacitance and CO, whereas after volume loading with RRVP, capacitance and CO were unaltered from baseline. Chronic RRVP reduced capacitance and CO. All interventions, volume +/- RRVP or chronic RRVP, increased stressed and central blood volumes and Ppcw. Acute or chronic RRVP reduced central vascular capacitance. Cardiac output was increased when stressed and unstressed blood volumes increased proportionately as during volume loading alone. When CO was reduced and Ppcw increased, as during chronic RRVP or acute RRVP plus a volume load, stressed blood volume was increased and unstressed blood volume was decreased. Thus, interventions that reduced CO and increased Ppcw also increased stressed and reduced unstressed blood volume and total vascular capacitance.

Effect of chronic rapid ventricular pacing on total vascular capacitance

BACKGROUND

Rapid right ventricular pacing (RRVP) at 250 bpm for 3-6 weeks produces chronic heart failure manifested by a reduction in cardiac output and increases in right atrial, pulmonary artery, and capillary wedge pressures.

METHODS AND RESULTS

One week after splenectomy and pacemaker placement, vascular capacitance, unstressed volume, and compliance were determined in 19 anesthetized dogs from pressure-volume curves using transient circulatory arrests induced by acetylcholine. Nine dogs were restudied 31 +/- 1 days later without RRVP, and 10 dogs underwent RRVP at 250 bpm and were restudied at 23 +/- 8 and 38 +/- 8 days in cardiac failure and after 1 and 2 weeks of postpacing recovery. Control animals had no changes in vascular capacitance or compliance. Dogs undergoing RRVP exhibited a marked increase in mean circulatory filling pressure (5.4 +/- 0.4 to 10.5 +/- 1.5 mm Hg) during the development of cardiac failure with a reduction in unstressed volume (81.9 +/- 5.7 to 43.9 +/- 8.1 ml.kg-1) without changing total vascular compliance. Total blood volume decreased (95.4 +/- 6.2 to 66.7 +/- 6.5 ml.kg-1) primarily due to a reduction in packed cell volume. The pressure gradient for venous return and overall venous resistance was unaltered. Central blood volume as a proportion of total blood volume increased (9.3 +/- 1.7% to 16.0 +/- 2.7%). Arterial compliance and capacity and pulmonary vascular compliance were reduced. In the 2-week postpacing period, except for a reduced cardiac response to a volume load, all of these parameters returned to baseline values.

CONCLUSIONS

Chronic RRVP induced cardiac failure with a marked reduction in total vascular capacitance due to a reduction in unstressed volume without altering compliance. The rise in mean circulatory filling pressure was limited by a reduction in total blood volume.

Effect of the alpha 2-adrenergic antagonist yohimbine on orthostatic tolerance

We studied the effect of yohimbine, a drug that inhibits presynaptic alpha 2-adrenergic receptors and increases the neuronal release of norepinephrine from the central and sympathetic nervous systems, on tolerance to cardiovascular stress in 10 untrained, healthy subjects. Using radioligand binding of tritiated yohimbine to platelets, these subjects were found to have a normal complement of alpha 2-adrenergic receptors (174 +/- 18 [+/- SEM] receptors/platelet) with normal Kd (1.93 +/- 0.17 nmol/l). Lower body negative pressure was used to test responses to cardiovascular stress in the subjects after they received either placebo or 20 mg yohimbine. Graded lower body negative pressure from 0 to -40 mm Hg significantly decreased systolic blood pressure from 116 +/- 3.7 to 106 +/- 5.8 mm Hg, increased heart rate from 54 +/- 3 to 68 +/- 7 beats/min, decreased forearm blood flow from 1.8 +/- 0.21 to 1.36 +/- 0.25 ml/100 ml/min, and increased forearm vascular resistance from 55.76 +/- 12.1 to 77.26 +/- 15.8 mm Hg/ml/min. Yohimbine increased the blood pressure at rest and during lower body negative pressure, but these changes were not significantly different from values recorded from the individuals when they were given placebo. Compared with placebo, however, yohimbine significantly increased forearm blood flow at rest (1.80 +/- 0.21 vs. 2.66 +/- 0.31 ml/100 ml/min, p less than 0.05) and during -40 mm Hg of lower body negative pressure (1.36 +/- 0.25 vs. 1.91 +/- 0.28 ml/100 ml/min, p less than 0.05). We also found that yohimbine significantly increased the plasma insulin concentration in these fasted subjects (9.4 +/- 2.4 vs. 14.5 +/- 1.4 ng/ml, p less than 0.05) without inducing hypoglycemia.(ABSTRACT TRUNCATED AT 250 WORDS)