Jan Sevcik, Vladimir Ruzicka, Josef Slansky, Jiri Smejkal and Karel Masek
Institute of Pharmacology, Academy of Sciences of the Czech Republic,
Videnska 1083, 142 20 Prague 4, The Czech Republic
Abstract: The relaxations mediated by the activation of 5-HT
receptors in the guinea pig proximal colon were investigated. Longitudinal
strips were cut from the colon segment and placed into the bath. In the
presence of atropine (0.2 microM), the relaxations were evoked by adding
increasing concentrations of 5-HT (1 - 100 microM). Noncumulative concentration-response
curves were established in the absence and presence of either 5-HT or nitric
oxide synthase (NOS) antagonists. Selective 5-HT3 antagonists
tropisetron (10 and 100 nM) and ondansetron (1 microM) inhibited the relaxations
and shifted the concentration-response curves to the right. Similar effects
were observed in the presence of the NOS inhibitor NG-nitro-L-arginine
(3.2, 10, 32 microM) and partly reversed with L-arginine (100, 320 microM).
NG-nitro-D-arginine, serving as a negative control, was ineffective.
The relaxations were further inhibited in the presence of the soluble guanylate
cyclase blocker methylene blue (10 microM) or NO scavenger hemoglobin (32
microM). These results suggest that the 5HT3 receptor plays a role in neurogenic
relaxations of guinea pig proximal colon, which are at least partly mediated
via release of NO from nerve endings.
Keywords: Proximal colon (guinea pig), Longitudinal muscle, Relaxation,
5-HT3 receptor, Nitric oxide