What is the source of anticontractile factor released by the pedicle of human internal thoracic artery?

Perivascular tissue (PVT) surrounding human internal thoracic artery (ITA) releases an unidentified anticontractile factor. The exact source of perivascular tissue-derived relaxing factor (PVRF) is unknown, although the adventitia and adipose tissue have both been suggested as primary candidates, hence the name adventitia or adipocyte-derived relaxing factor (ADRF). To look for the source of ADRF, we examined the dilatory response of human ITA to PVT aliquots in their histological composition.

We studied isolated ITA segments from 20 patients subjected to coronary artery surgery. The vessels were skeletonized in vitro. ITA rings and PVT were incubated in separate isolated organ baths. The arterial rings were suspended on stainless steel wire hooks in the organ bath chamber. Vessel wall tension was measured with an isometric force transducer. Skeletonized ITA segments were precontracted with 10(-5.5) M phenylephrine. The 5 ml PVT aliquots were next transferred to the ITA tissue bath, resulting in its relaxation. Subsequently, the whole PVT used during experiment was fixed in paraformaldehyde and subjected to histological examination. Tissue was paraffin-embedded, sectioned and stained with haematoxylin and eosin. The paraffin blocks containing PVT were cut into slices every 800 μm to create three-dimensional model. Every PVT specimen was evaluated morphometrically using the Image Pro Plus software to assess the content of three basic kinds of tissues. The ITA relaxation to PVT aliquots was correlated to the histological composition of the PVT.

Phenylephrine elicited a 37.82 mN (Q1 = 26.49; Q3 = 46.31) contraction of the ITA. The addition of PVT aliquots to the skeletonized ITA induced a 54.17% (Q1 = 16.73; Q3 = 68.21) relaxation. The median PVT weight was 786 mg (Q1 = 562; Q3 = 976). The PVT composition was as follows: 30.5% (Q1 = 18.5; Q3 = 55.2) adipose tissue, 53.5 (Q1 = 24.6; Q3 = 66.5) muscular tissue and 13.5% (Q1 = 9.9; Q3 = 20.0) connective tissue. This translated into 197.7 mg of adipose tissue (Q1 = 142.2; Q3 = 393.2), 378.9 mg (Q1 = 178.8; Q3 = 537.0) of muscular tissue and 92.4 mg (Q1 = 68.6; Q3 = 185.8) of connective tissue. Neither PVT mass (r = 0.2, P = 0.92) nor adipose tissue (r = -0.2, P = 0.34), muscular tissue (r = 0.3, P = 0.18) or connective tissue (r = -0.2, P = 0.41) content correlated with ITA relaxation response to PVT aliquots.

Adipose tissue from the pedicled ITA graft is an unlikely source of ADRF.