The nitric oxide donor SIN-1 is free of tolerance and maintains its cyclicGMP stimulatory potency in nitrate-tolerant LLC-PK1 cells

Purpose. Using an established cell culture model, the present study investi gates whether linsidomine (SIN-1), a spontaneous donor of nitric oxide and active metabolite of the antianginal drug molsidomine, induces tolerance to its own cyclic GMP stimulatory action or shows a diminished response after tolerance induction with glyceryl trinitrate. Methods. Incubations with nitric oxide donors were carried out in LLC-PK1 k idney epithelial cells. Intracellular levels of cyclic GMP, the vasodilator y second messenger of nitric oxide, were determined by radioimmunoassay. Results. A 5-h preincubation with glyceryl trinitrate (0.01-100 mu M) led t o complete inhibition of a subsequent cyclic GMP stimulation by glyceryl tr initrate but left the cyclic GMP response to SIN-1 unaltered. Similarly, cy clic GMP elevations by the spontaneous nitric oxide donors sodium nitroprus side and spermine NONOate were not affected after pretreatment with glycery l trinitrate. Moreover, pretreatment with SIN-1 (1-1000 mu M) had no signif icant effect on SIN-1-dependent cyclic GMP stimulation. Conclusions. Our results show that in LLC-PK1 cells, SIN-1 is free of toler ance induction and not cross-tolerant to glyceryl trinitrate. This may be d ue to the spontaneous nitric oxide release from SIN-1, which in contrast to nitric acid esters does not require enzymatic bioactivation and may theref ore be unaffected by nitrate tolerance.