A xanthine-based KMUP-1 with cyclic GMP enhancing and K+ channels opening activities in rat aortic smooth muscle

1 KMUP-1 (1, 3, 5 mg kg(-1), i.v.), a xanthine derivative, produced dose-de pendent sustained hypotensive and short-acting bradycardiac effects in anae sthetized rats. This hypotensive effect was inhibited by pretreatment with glibenclamide (5 mg kg (1), i.v.). 2 In endothelium-intact or denuded aortic rings preconstricted with phenyle phrine, KMUP-1 caused a concentration-dependent relaxation. This relaxation was reduced by endothelium removal, the presence of NOS inhibitor L-NAME ( 100 muM) and sGC inhibitors methylene blue (10 muM) and ODQ (1 muM). 3 The vasorelaxant effects of KMUP-1 was attenuated by pretreatment with va rious K ' channel blockers TEA (10 mM), glibenclamide (1 muM), 4-AP (100 mu M), apamin (1 muM) and charybdotoxin (ChTX, 0.1 muM), 4 Increased extracellular potassium levels (30-80 mM) caused a concentratio n-related reduction of KMUP-1-induced vasorelaxations. Preincubation with K MUP-1 (1, 10, 100 nM) increased the ACh-induced maximal vasorelaxations med iated by endogenous NO release, and enhanced the potency of exogenous NO-do nor SNP. 5 The vasorelaxant responses of KMUP-1 (0.01, 0.05, 0.1 muM) together with a PDE inhibitor IBMX (0.5 muM) had an additive action. Additionally, KMUP-1 (100 muM) affected cyclic GMP metabolism since it inhibited the activity o f PDE in human platelets. 6 KMUP-1 induced a dose-related increase in intracellular cyclic GMP levels in rat A10 vascular smooth muscle (VSM) cells, but not cyclic AMP. The inc rease in cyclic GMP content of KMUP-1 (0.1 - 100 muM) was almost completely abolished in the presence of methylene blue (10 muM), ODQ (10 muM), and L- NAME (100 pm). 7 In conclusion, these results indicate that KMUP-1 possesses the following merits: (1) stimulation of NO/sGC/cyclic GMP pathway and subsequent elevat ion of cyclic GMP, (2) K ' channels opening, and (3) inhibition of PDE or c yclic GMP breakdown. Increased cyclic GMP display a prominent role in KMUP- 1-induced VSM relaxations.