Morphine is an arteriolar vasodilator in man

Area of Science:

• Pharmacology
• Cardiovascular Physiology

Background:

• The arterial effects of morphine are not fully understood.
• Investigating morphine's impact on vascular responses is crucial for clinical applications.

Purpose of the Study:

• To determine the forearm vascular responses to intra-arterial morphine in healthy subjects.
• To elucidate the mediating mechanisms of morphine-induced vasodilatation.

Main Methods:

• Forearm blood flow (FBF) was measured using venous occlusion plethysmography.
• Morphine was infused intra-arterially in a dose-ranging study, with assessments for acute tolerance.
• Mechanistic studies involved antagonists like naloxone, histamine receptor blockade, and nitric oxide (NO) clamp.

Main Results:

• Morphine significantly increased FBF in a dose-dependent manner.
• No acute tolerance to morphine's vasodilatory effect was observed.
• Vasodilatation was mediated by histamine and nitric oxide pathways, as it was blocked by antihistamines and NO clamp, but not by naloxone.

Conclusions:

• Intra-arterial morphine induces forearm vasodilatation.
• The vasodilatation is mediated by local histamine release and subsequent nitric oxide production.
• Opioid receptor involvement in these arterial effects requires further investigation.