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 (E_max) in response to Ach in PE-precontracted ITA rings (P<0.0001). Protamine (50–800 μg/ml), free hemoglobin (0.1 and 0.5 μM), nitric oxide (NO) blocker N^ω-nitro--arginine methyl ester (L-NAME, 100 μM) or soluble guanylate cyclase inhibitor methylene blue (10 μM) administration did not cause a significant alteration on basal tonus of endothelium-intact or -denuded ITA rings. Protamine (50–800 μg/ml) induced concentration-dependent relaxation responses in ITA rings precontracted by either PE or high KCl. There was no difference in sensitivity or maximal response to protamine between the endothelium-intact and -denuded rings. Incubation of endothelium-intact or -denuded ITA rings with L-NAME or free hemoglobin or methylene blue did not cause a significant inhibition on relaxation responses to protamine. ITA ring contractions induced by stepwise addition of calcium to high KCl solution with no calcium were almost completely inhibited by protamine (P<0.0001). Conclusions: It was suggested that protamine induced relaxation responses in human ITA rings is not NO- or endothelium-dependent but seems to depend on the interactions of protamine with calcium influxes and/or calcium release from intracellular stores in this tissue.