M. Rekik et al., VIP-INDUCED RELAXATION OF GUINEA-PIG INTESTINAL SMOOTH-MUSCLE CELLS -SEQUENTIAL INVOLVEMENT OF CYCLIC-AMP AND NITRIC-OXIDE, British Journal of Pharmacology, 118(3), 1996, pp. 477-484
1 A possible interaction between cyclic AMP and nitric oxide (NO) in m
ediating the relaxant effect of vasoactive intestinal polypeptide (VIP
) on intestinal smooth muscle cells has been investigated. The effects
of the inhibitor of NO synthesis, N-G-nitro-L-arginine methyl ester (
L-NAME), have been studied on VIP-, forskolin-, and 8 bromo-cyclic AMP
- induced relaxation of cells, dispersed by enzymatic digestion of mus
cle strips from the circular layer of guinea-pig ileum. 2 VIP alone di
d not modify the length of isolated muscle cells. By contrast, when th
e cells were contracted by cholecystokinin octapeptide, CCK8 (10 nM),
VIP inhibited this contraction, inducing a concentration-dependent rel
axation of the cells. Maximal relaxation was induced by 1 mu M VIP (EC
(50)=408.2+/-16.7 pM). 3 N-ethylmaleimide, inhibitors of adenylate cyc
lase or somatostatin, abolished the relaxing effect of VIP. (R)-p-cAMP
s, an antagonist of cyclic AMP on protein kinase A also inhibited the
VIP-induced relaxation by 92.1+/-6.3%. Inhibitors of nitric oxide synt
hase (NOS), L-NAME and L-NMMA, partially inhibited VIP-induced relaxat
ion. The effect of L-NAME was reversed by L-arginine but not by D-argi
nine. 4 (R)-p-cAMPS and L-NAME also inhibited the cell relaxation indu
ced either by forskolin which directly stimulates adenylate cyclase ac
tivity or 8-bromo-cyclic AMP, an analogue of cyclic AMP. 5 When cells
were incubated for 30 min with dexamethasone 10 mu M, a glucocorticoid
known to decrease the synthesis of iNOS, the relaxing effect of a max
imal concentration of VIP was decreased by 52+/-4% and L-NMMA had no f
urther effect on this residual VIP-induced relaxation. Milrinone, a ph
osphodiesterase type III inhibitor, potentiated the relaxant effect of
VIP. 6 These data demonstrate that the intracellular pathway mediatin
g the relaxant effect of VIP in intestinal smooth muscle cells include
s the sequential activation of adenylate cyclase, protein kinase A, ac
tivation of NOS and finally production of NO and cyclic GMP. NO could
in turn regulate the cyclic AMP-dependent pathway of cell relaxation.