The inhibitory action of protamine on human internal thoracic artery contractions: the effect of free hemoglobin

Objective: We investigated the mechanism of the protamine action and the effects of free hemoglobin on protamine-induced responses in endothelium-denuded and-intact human internal thoracic artery (ITA) rings precontracted with phenylephrine (PE) or high KCl. Methods: Samples of redundant ITA obtained from patients undergoing a coronary artery bypass graft surgery were cut into 3 mm wide rings and suspended in 20 ml organ baths. Isometric tension was continuously measured with an isometric force transducer connected to a computer-based data acquisition system. Results: Acetylcholine (Ach, 10−8–10−5 M) caused a concentration-dependent relaxation of PE-precontracted ITA rings. Free hemoglobin (0.1 and 0.5 μM) produced a concentration-dependent and significant decrease in sensitivity (pD2) and maximal contractility (Emax) in response to Ach in PE-precontracted ITA rings (P