He GW.  Yang CQ.  Vasorelaxant effect of the phosphodiesterase-inhibitor milrinone in the human radial artery used as coronary bypass graft.  Journal of Thoracic & Cardiovascular Surgery.  119(5):1039-45, 2000.


    The authors investigated the effect of the phosphodiesterase III inhibitor milrinone on the human radial artery.  Radial artery segments (n = 76) taken from 15 patients were studied in an organ chamber. Concentration-relaxation curves for milrinone were established in the radial artery precontracted with 3 vasoconstrictors (phenylephrine, K(+), and U46619). In radial artery rings incubated with therapeutic plasma concentrations of milrinone (7 and 70 micromol/L) for 10 minutes, concentration-contraction curves for the 3 vasoconstrictors were constructed.  The authors report that milrinone caused a submaximal relaxation in phenylephrine- (98.6% +/- 1.4%), K(+)- (89.1 +/- 4.5%), or U46619- (74.2 +/- 8.0%) precontracted radial arteries at -4.5 log(10) M. The EC(50) was higher against K(+) (-5.85 +/- 0.24 log(10) M, P =.02) or U46619 (-5. 21 +/- 0.61 log(10) M, P =.03) than phenylephrine (-6.68 +/- 0.11 log(10) M). Pretreatment with milrinone depressed the contraction by phenylephrine from 70.0% +/- 7.9% to 23.5% +/- 9.3% (P =.003) and by K(+) from 138.6% +/- 5.8% to 73.0% +/- 13.9% (P =.006) and shifted the EC(50) 3.8-fold higher (P =.03) for phenylephrine and 2.2-fold higher for K(+) (P =.01). Milrinone reduced the U46619 contraction at low concentration (-8.5 log(10) M) but had little effect on the maximal contraction.  The authors conclude that milrinone is a potent vasodilator for the radial artery, with possibly higher potency in alpha-adrenoceptor- and depolarizing agent K(+)-mediated, but less potency in thromboxane A(2)-mediated, contraction. Because it also has a positive inotropic effect, the authors suggest that milrinone may be particularly indicated for use in patients receiving radial artery grafts in coronary artery bypass grafting.