The use of the radial artery (RA) in coronary artery bypass surgery has increased over the last few years. This is due to its superior patency compared with saphenous vein grafts. However, the RA is at increased risk of spasm compared with the internal mammary artery. This can have an adverse clinical outcome due to myocardial hypoperfusion. Recently Taggart et al. (2000) described the use of phenoxybenzamine (PhB) to prevent spasm in the RA. PhB binds irreversibly to the α1 adrenoceptor, the dominant adrenoceptor in the RA (He & Yang, 1998) and treatment with PhB renders the graft insensitive to catecholamines present in the circulation post-operatively (Taggart et al. 2000). However, circulating levels of other vasoconstrictors also rise in the post-operative period (Downing & Edmunds, 1992), which means that additional strategies may be required to prevent spasm.
Sections of RA, treated with 1 mg ml-1 (1.6 mM) PhB, were obtained surplus to surgery with ethical approval. Rings of RA, pretensioned at 30 mN for 1 h, were then relaxed to 10 mN for a further 30 min, before the addition of agonists. We then went on to evaluate the effect of 0.5 mg ml-1 (2.2 mM) glyceryl trinitrate (GTN), papaverine (0.5 mg ml-1; 1.5 mM) and 10 µM diltiazem against these responses. Responses were compared using Student’s unpaired t test (P < 0.05). Data are presented as means ± S.E.M.
Sections of RA treated in theatre with papaverine, not PhB, responded to noradrenaline with an EC50 of 1.8 ± 0.6 mM and a maximal tension of 34.0 ± 1.9 mN (n = 10). PhB-treated arterial sections were insensitive to a 2 or 20 µM application of noradrenaline (n = 35). PhB-treated RA did respond to 100 nM vasopressin (26.7 ± 2.1 mN; n = 11), endothelin-1 (23.0 ± 1.7 mN; n = 12) and angiotensin II (27.9 ± 2.0 mN; n = 21). In addition, PhB-treated RA also responded to isomolar KCl in a concentration-dependent manner, giving responses at 60 mM of 25.7 ± 1.4 mN (n = 31). Acute addition of papaverine and GTN effectively antagonised responses elicited by all of the vasoconstrictors tested. Diltiazem reduced responses to KCl but was much less effective against endothelin-1 and had no effect against angiotensin II or vasopressin.
We conclude that receptors other than the adrenoceptors may play an important role in the aetiology of spasm in the RA and that GTN or papaverine may be useful supplements to PhB.
All procedures accord with current local guidelines.