Relaxation and potentiation of cGMP-mediated response by ibudilast in bovine tracheal smooth muscle

Abstract.

The effects of ibudilast, an inhibitor of phosphodiesterases (PDEs), on tension, levels of guanosine 3',5'-cyclic monophosphate (cGMP) and adenosine 3',5'-cyclic monophosphate (cAMP) were investigated in bovine tracheal smooth muscle. We especially examined the combined effect of ibudilast with the cGMP-elevating agents on these parameters. Ibudilast was equipotent to attenuate the precontractions induced by both 0.3 µM methacholine and 40 mM K⁺. By contrast, the relaxant effects of sodium nitroprusside and salbutamol on 40 mM K⁺-contracted preparations were smaller than those on 0.3 µM methacholine-contracted ones. Neither N ^ω-nitro-L-arginine (100 µM), an inhibitor of nitric oxide synthase, nor ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one; 5 µM), an inhibitor of soluble guanylyl cyclase, affected the ibudilast-induced relaxation. The relaxations induced by ibudilast and diltiazem on 40 mM K⁺-contracted preparations were significantly attenuated when extracellular CaCl₂ was increased from 2.54 mM to 10 mM. Ibudilast (10 µM), which caused only minor effect by itself, significantly (P<0.05) shifted the concentration-response curves for the relaxant responses to sodium nitroprusside (SNP), atrial natriuretic peptide (ANP) and salbutamol to the left. On the other hand, ibudilast did not change the relaxant responses to diltiazem. Unlike ibudilast, diltiazem (3 µM) failed to affect the SNP- and salbutamol-induced relaxations. Ibudilast significantly (P<0.05) increased basal levels of cGMP and cAMP. Furthermore, ibudilast enhanced SNP (0.3 µM)- and ANP (0.3 µM)-induced cGMP accumulation and salbutamol (10 nM)-induced cAMP accumulation. Zaprinast (10 µM), a type 5 PDE inhibitor, enhanced both relaxation and cGMP accumulation induced by SNP and ANP without changing salbutamol-induced responses.

These findings suggest that blockade of voltage-gated Ca²⁺ channels is involved in the relaxing action of ibudilast in bovine tracheal smooth muscle. However, ibudilast potentiates relaxation responses to ANP and SNP by inhibition of PDE 5, not by blockade of Ca²⁺ channels. The enhancement of cGMP-mediated response may contribute to the therapeutic effects of ibudilast.