AIM To investigate the mechanisms underlying the potential contribution of the heme oxygenase/carbon monoxide(HO/CO) pathway in the constipating effects of granisetron. METHODS For in vivo studies, gastrointestinal motility was evaluated in male rats acutely treated with granisetron [25, 50, 75 μg/kg/subcutaneous(sc)], zinc protoporphyrin IX [Zn PPIX, 50 μg/kg/intraperitoneal(ip)] and hemin(50 μmol/L/kg/ip), alone or in combination. For in vitro studies, the contractile neurogenic response to electrical field stimulation(EFS, 3, 5, 10 Hz, 14 V, 1 ms, pulse trains lasting 10 s), as well as the contractile myogenic response to acetylcholine(ACh, 0.1-100 μmol/L) were evaluated on colon specimens incubated with granisetron(3 μmol/L, 15 min), Zn PPIX(10 μmol/L, 60 min) or CO-releasing molecule-3(CORM-3, 100, 200, 400 μmol/L) alone or in combination. These experiments were performed under co-treatment withor without atropine(3 μmol/L, a muscarinic receptor antagonist) or NG-nitro-L-Arginine(L-NNA, 100 μmol/L, a nitric oxide synthase inhibitor).RESULTS Administration of granisetron(50, 75 μg/kg) in vivo significantly increased the time to first defecation(P = 0.045 vs vehicle-treated rats), clearly suggesting a constipating effect of this drug. Although administration of Zn PPIX or hemin alone had no effect on this gastrointestinal motility parameter, Zn PPIX co-administered with granisetron abolished the granisetron-induced constipation. On the other hand, co-administration of hemin and granisetron did not modify the increased constipation observed under granisetron alone. When administered in vitro, granisetron alone(3 μmol/L) did not significantly modify the colon’s contractile response to either EFS or ACh. Incubation with Zn PPIX alone(10 μmol/L) significantly reduced the colon’s contractile response to EFS(P = 0.016) but had no effect on contractile response to ACh. Co-administration of Zn PPIX and atropine(3 μmol/L) abolished the Zn PPIX-mediated decrease in contractile response to EFS. Conversely, incubation with CORM-3(400 μmol/L) alone increased both the contractile response to EFS at 10 Hz(10 Hz: 71.02 ± 19.16 vs 116.25 ± 53.70, P = 0.01) and the contractile response to ACh(100 μmol/L)(P = 0.012). Co-administration of atropine abolished the CORM-3-mediated effects on the EFS-mediated response. When granisetron was co-incubated in vitro with ZnP PIX, the ZnP PIX-mediated decrease in colon contractile response to EFS was lost. On the other hand, co-incubation of granisetron and CORM-3(400 μmol/L) further increased the colon’s contractile response to EFS(at 5 Hz: P = 0.007; at 10 Hz: P = 0.001) and to ACh(ACh 10 μmol/L: P = 0.001; ACh 100 μmol/L: P = 0.001) elicited by CORM-3 alone. L-NNA co-administered with granisetron and CORM-3 abolished the potentiating effect of CORM-3 on granisetron on both the EFSinduced and ACh-induced contractile response.CONCLUSION Taken together, findings from in vivo and in vitro studies suggest that the HO/CO pathway is involved in the constipating effects of granisetron. METHODS For in vivo studies, gastrointestinal motility was evaluated in male rats acutely treated with granisetron [25, 50 , 75 μg / kg / subcutaneous (sc)], zinc protoporphyrin IX [Zn PPIX, 50 μg / kg / intraperitoneal (ip)] and hemin (50 μmol / L / kg / ip) studies, the contractile neurogenic response to electrical field stimulation (EFS, 3, 5, 10 Hz, 14 V, 1 ms pulse trains lasting 10 s) as well as the contractile myogenic response to acetylcholine (ACh, 0.1-100 μmol / L) were evaluated on colon derivatives with granisetron (3 μmol / L, 15 min), Zn PPIX (10 μmol / L, 60 min) or CO-releasing molecule- L) alone or in combination. These experiments were performed under co-treatment with or without atropine (3 μmol / L, a muscarinic receptor antagonist) or NG-nitro-LA (5, 75 μg / kg) in vivo significantly increased the time to first defection (P = 0.045 vs vehicle-treated rats) clearly suggesting a constipating effect of this drug. However, the administration of Zn PPIX or hemin alone had no effect on this gastrointestinal motility parameter, Zn PPIX co-administered with granisetron abolished the granisetron-induced constipation. On the other hand, co-administration of hemin when administered in vitro, granisetron alone (3 μmol / L) did not significantly modify the colon’s contractile response to either EFS or ACh. Incubation with Zn PPIX alone (10 μmol / L ) significantly reduced the colon’s contractile response to EFS (P = 0.016) but had no effect on contractile response to ACh. Co-administration of Zn PPIX and atropine (3 μmol / L) abolished the Zn PPIX-mediated decrea se in contractile response to EFS. Conversely, incubation with CORM-3 (400 μmol / L) alone increased both contractile response to EFS at 10 Hz (10 Hz: 71.02 ± 19.16 vs 116.25 ± 53.70, P = 0.01) Co-administration of atropine abolished the CORM-3-mediated effects on the EFS-mediated response. When granisetron was co-incubated in vitro with ZnP PIX, the ZnP PIX-mediated decrease in colon contractile response to EFS was lost. On the other hand, co-incubation of granisetron and CORM-3 (400 μmol / L) further increased the colon’s contractile response to EFS (at 5 Hz: P = 0.007; : P = 0.001) and to ACh (ACh 10 μmol / L: P = 0.001; ACh 100 μmol / L: P = 0.001) elicited by CORM-3 alone. L-NNA co- administered with granisetron and CORM-3 abolished the potentiating effect of CORM-3 on granisetron on both the EFS induced and ACh-induced contractile response. CONCLUSION Taken together, findings from in vivo and in vitro studies suggest that the HO / C O pathway is involved in the constipating effects of granisetron.