Overview of the nature of vasoconstriction in arterial grafts for coronary operations

Many vasoconstrictors (spasmogens) may cause arterial graft spasm; however, there is lack of an overview of the nature of vasoconstriction in grafts. This study was designed to investigate the response of three major arterial grafts currently used for coronary artery bypass grafting to various vasoconstrictor substances. Segments of three arterial grafts (gastroepiploic [GEA], n = 28; internal mammary [IMA], n = 213; inferior epigastric [IEA], n = 24) taken from patients undergoing coronary artery bypass grafting were studied in organ baths under a physiologic pressure. Cumulative concentration-contraction curves were established for the following vasoconstrictor substances: endothelin-1, U46619, prostaglandin F_2α, norepinephrine, methoxamine, phenylephrine, 5-hydroxytryptamine, and potassium chloride (K⁺). In IMA, the highest contraction force was induced by U46619 (5.69 ± 0.48 g), endothelin-1 (4.43 ± 0.4 g), PGF_2α (6.29 ± 1.42 g), and K⁺ (4.58 ± 0.5 g). Internal mammary artery is highly sensitive to endothelin-1 (EC₅₀, -8.13 ± 0.08 log M) and U46619 (EC₅₀, -8.21 ± 0.21 log M) (lower than any other vasoconstrictors, p < 0.001). Next sensitive vasoconstrictors were PGF_2α and norepinephrine. 5-Hydroxytryptamine induced significantly higher contraction force in the IMA without endothelium (2.8 ± 0.64 g versus 1.4 ± 0.23 g, p < 0.05). In GEA and IEA, endothelin-1 and U46619 were more potent vasoconstrictors (EC₅₀ for endothelin-1: -8.06 ± 0.02 log M in GEA and -8.22 ± 0.04 in IEA; for U46619: -8.49 ± 0.24 log M in GEA and -8.25 ± 0.09 in IEA) than that for norepinephrine (-6.86 ± 0.11 log M for GEA and -6.59 ± 0.18 log M for IEA, p < 0.0001), although all of them (and K⁺) evoked a strong contraction. In summary, the present study reveals that there are basically two types of vasoconstrictors that are important spasmogens in arterial grafts. Type I (endothelin, prostaglandins (thromboxane A₂ and prostaglandin F_2α), and α₁-adrenoceptor agonists) are the most potent vasoconstrictors and they strongly contract arterial grafts even when endothelium is intact. Type II vasoconstrictors (such as 5-HT) only induce a weak vasoconstriction when endothelium is intact. However, those vasoconstrictors probably play an important role in the spasm of arterial grafts if endothelium is lost by surgical handling.