Background: Endothelin-3 (ET-3) and its inactive precursor, big endothelin-3 (big ET-3), are both found in human plasma. We investigated whether big ET-3 is converted to ET-3 in the human forearm resistance vessels and dorsal hand veins in vivo.
Methods: In a 4-phase study, 6 subjects received 90 minute intrabrachial artery infusions of big ET-3 (50 and 100 pmol x min(-1)) and ET-3 (5 and 10 pmol x min(-1)) in random order. Forearm blood flow was measured by venous occlusion plethysmography. In a second 3-phase study, 6 subjects received 90-minute dorsal hand vein infusions of saline solution, big ET-3 (50 pmol x min(-1)) and ET-3 (5 pmol x min(-1)) in random order. In a third 2-phase study, 6 subjects received 90-minute dorsal hand vein infusions of big ET-3 (100 pmol x min(-1)) and ET-3 (10 pmol x min(-1)). In the dorsal hand vein studies, vessel diameter was measured by the Aellig technique.
Results: Intra-arterial ET-3 caused local forearm vasoconstriction of 20%+/-9% (P = .009) at 5 pmol x min(-1) and 20%+/-10% (P = .001) at 10 pmol x min(-1) after 90 minutes, with no difference between doses (P = .69). Intra-arterial big ET-3 also caused local forearm vasoconstriction of 22%+/-6% at 50 pmol x min(-1) (P = .004) and 18%+/-3% at 100 pmol x min(-1) (P<.0001) after 90 minutes, with no difference between doses (P = .44). There were no significant differences between the responses to intra-arterial big ET-3 and ET-3 at these doses. Local intravenous ET-3 caused a constriction of 9%+/-2% at 5 pmol x min(-1) (P = .04) and 22%+/-8% at 10 pmol x min(-1) (P = .002) after 90 minutes. Big ET-3 at 50 pmol x min(-1) and 100 pmol x min(-1) did not affect hand vein tone. All responses were slowly progressive.
Conclusions: Based on vasoconstriction, measurable conversion of big ET-3 to ET-3 occurs in forearm resistance vessels but not in dorsal hand veins in vivo. An endothelin-converting enzyme, capable of converting exogenously administered big ET-3 to ET-3, appears to be present in upper limb resistance arteries but not in capacitance vessels in humans.