C A R B A C H O L I N D U C E S N I T R I C O X I D E G E N E R A T I O N I N G U I N E A - P I G G A L L B L A D D E R

Abstract

Objective: Acetylcholine is one of the majorcontractile transmitters in gallbladder; whereas, itis also innervated by non-adrenergic noncholinergicnerves which mediate relaxation. Itwas postulated that nitric oxide (NO) whichactivates soluble guanylate cyclase to increasecyclic GMP (cGMP) levels in the target cells maybe involved in these processes. This study wasdesigned to investigate whether muscarinicreceptor stimulation via carbachol (CCh) inducesNO-mediated cGMP synthesis in guinea-piggallbladder.Methods: cGMP levels were measured viaradioimmunoassay in gallbladder slicesincubated with carbachol (CCh) (10 6-10 3 mol/l)in the presence and absence of NO synthase(NOS) inhibitor L-NAME or muscarinic antagonistscopolamine. The effect of L-NAME and anotherNOS inhibitor L-NMMA on carbachol-inducedcontractions were also investigated in an in vitroorgan bath.Results: Carbachol stimulated cGMP formationin guinea-pig gallbladder slices significantly.CCh-induced cGMP formation was abolished byboth L-NAME (1mmol/l) and scopolamine (108-106 mol/l). In contraction experiments, L-NAME(3x10-4 mol/l) did not produce any change in theresting tension of the strips and theconcentration-response curves to carbachol,whereas L-NMMA (3x10-4 mol/l) induced a slight,but significant contraction (6.3+1.9 % ofcarbachol (106 mol/l)-induced response). Whencarbachol was added to the bath after L-NMMAinducedincrease in tension reached itsmaximum, the amplitude of the contractile response at the end was 108.0+4.0 % of the control.Conclusion: Therefore, it may be concluded that muscarinic receptor activation by carbachol stimulates NO production in guinea-pig gallbladder and the subsequent increase in cGMP counteracts with carbachol-induced contractions. However, it seems likely that an intact enteric nervous system is required for such an interaction to be prominent. The muscarinic receptor subtype(s) involved in this interaction remains to be elucidated.Key Words: cGMP, NO, Guinea-pig,gallbladder, L-NAME, L-NMMA