Is adenosine a modulator of peripheral vasoconstrictor responses?

Background: Local vasoconstrictor reflexes, the vascular myogenic response (VMR) and the veno-arterial reflex (VAR) are necessary for the maintenance of regional blood flow and systemic arterial blood pressure during orthostatic stress. Their molecular mechanism is unknown. We postulated that adenosine is involved in the activation of these local reflexes.

Methods: This hypothesis was tested in 10 healthy male volunteers (age 29 ± 3 years, BMI 24 ± 1 kg/m(2)). We used veno-occlusive plethysmography method for the assessment of forearm arterial blood flow at baseline and upon causing local venous congestion by inflating a second cuff to 40 mmHg for 4 min (VAR) and during placement of the forearm 40 cm below cardiac level for 4 min (VMR). These measurements were repeated after local infusion of either saline or aminophylline, non-selective adenosine blockers, using the Bier block method.

Results: Rest baseline forearm blood flow was comparable in both arms. Saline did not affect the baseline forearm blood flow. However, aminophylline causes a significant increase in baseline forearm blood flow of 34 ± 6 % (p = 0.002). VAR demonstrated a decrease in forearm blood flow of 49 ± 4.5 % and after saline infusion it remained unchanged, 49 ± 5 % (p = 0.92). However, aminophylline causes significant decrease in the VAR by 35 ± 3 % (p = 0.02). But, both, saline and aminophylline did not affect the VMR.

Conclusion: Arterial vasoconstriction triggered by venous congestion, which is the veno-arterial reflexis seems to be modulated by adenosine, at least partially. This "sensory" reflex requires further pharmacologic physiologic investigation.