We aimed to characterize 5-HT receptors mediating contraction and relaxation to 5-HT in dog proximal stomach longitudinal muscle (LM) strips. 2. Of the tryptamine analogues tested, 5-HT was the most potent contractile agent at basal length, while 5-CT was the most potent relaxant of PGF(2alpha)-induced contraction. Neither the contractions to 5-HT, nor the relaxations to 5-CT were influenced by tetrodotoxin, illustrating that action potential propagation is not involved. 3. The 5-HT-induced contraction was antagonized by mesulergine (0.03 to 0.3 microM) and ketanserin (2 - 20 nM), but the antagonism was not of a simple competitive nature, indicating multiple receptor involvement. Ketanserin (3 to 30 nM) and mesulergine (30 nM) competitively antagonized the alpha-Me-5-HT-induced contraction (pK(B): 8.83+/-0.09 and pA(2): 8.25+/-0.06 respectively). These affinity values are in line with literature affinities of ketanserin and mesulergine at 5-HT(2A) receptors in various bioassays. 4. The 5-CT-induced inhibition of PGF(2alpha)-induced contraction was competitively antagonized by mesulergine (pK(B) estimate: 8.52+/-0.12) and by the selective 5-HT(7) receptor antagonist SB-269970 (pK(B) estimate: 9.36+/-0.14). Both pK(B) estimates are in line with literature affinities of these compounds for 5-HT(7) receptors. Mesulergine (30 nM) and SB-269970 (10 nM) shifted the relaxant curve to 5-HT parallel to the right in the presence of ketanserin (0.3 microM) (pA(2) estimates of 8.08+/-0.10 and 8.75+/-0.14 respectively), indicative of 5-HT(7) receptor involvement. 5. It is concluded that 5-HT induces dog proximal stomach (LM) contraction via smooth muscle 5-HT(2A) receptors and relaxation via smooth muscle 5-HT(7) receptors.
