MDL 27,032 [4-propyl-5-(4-pyridinyl)-2(3H)-oxazolone], an active site-directed inhibitor of protein kinase C and cyclic AMP-dependent protein kinase that relaxes vascular smooth muscle.

MDL 27,032 [4-propyl-5-(4-pyridinyl)-2(3H)-oxazolone] is a novel vasodilator whose mechanism of action has not been elucidated. We investigated whether smooth muscle relaxation by MDL 27,032, in vitro, may involve an alteration in the activity of protein kinase C, cyclic AMP (cAMP)-dependent protein kinase or myosin light chain kinase by investigating the effects of MDL 27,032 on cyclic nucleotide phosphodiesterases (PDEs) and protein kinase activities. Strips of dog femoral artery or saphenous vein contracted with phorbol 12-myristate 13-acetate (PMA) were relaxed by 100 microM concentrations of MDL 27,032, as well as by other known inhibitors of PDEs [3-isobutyl-1-methylxanthine and papaverine], myosin light chain kinase (W-7) and protein kinase C (H-7 and polymyxin B). In contrast to 3-isobutyl-1-methylxanthine and papaverine, MDL 27,032 was either inactive or weak as an inhibitor of purified PDE types I, II, IVa and IVb. Similarly, it was a weak inhibitor of myosin light chain kinase. However, MDL 27,032 was a significantly more potent inhibitor of protein kinase C and cAMP-dependent protein kinase in cytosolic extracts of dog vein. Kinetic experiments utilizing purified rat brain protein kinase C revealed that inhibition with MDL 27,032 was competitive with Mg(++)-ATP (Ki 24 microM) and noncompetitive with phospholipid, diacylglycerol, PMA, calcium or substrate proteins. Inhibition of the catalytic subunit of cAMP-dependent protein kinase was also competitive with Mg(++)-ATP (Ki 14.3 microM). Similar results were obtained with MDL 27,032 and H-7 on both enzymes.(ABSTRACT TRUNCATED AT 250 WORDS)