Smooth muscle layer-dependent distribution of 5-hydroxytryptamine7 receptor in the porcine myometrium

1 To analyse the mechanisms of muscle layer-dependent inhibition of porcine myometrial contractility by 5-hydroxytryptamine (5-HT), the effects of 5-HT, 5-carboxamidotryptamine(S-CT), 5-methoxytryptamine (5-MeOT), forskolin and cyclic adenosine 3', 5'-monophosphate (cyclic AMP) analogues Ion spontaneous and stimulant-induced contractions were examined in longitudinal (LM) and circular muscles (CM). In addition, accumulation of cyclic AMP by 5-HT and distribution of 5-HT7 receptors in LM and CM layers were compared using biochemical and molecular approaches. 2 5-HT receptor agonists inhibited the spontaneous contractions of LM and CM (5-CT > 5-HT > 5-MeOT), but CM was more sensitive than was LM. The inhibition by the agonists was antagonized by methiothepin (100 nM). 3 Carbachol-, high-K+-, histamine- and Ca2+-induced contractions were inhibited by 5-HT with different responsivenesses (CM > LM). Even in the presence of 3-isobutyl-1-methylxanthine (IBMX), the inhibition by 5-HT in the CM was still more conspicuous than that in the LM. 4 Compared with the CM, the inhibition of spontaneous contraction by forskolin, dibutyryl-cyclic AMP and 8-bromo-cyclic AMP was marked in the LM. 5 5-HT (1 nM - 1 mu M) increased the cyclic AMP in both muscle layers, but the increment in the CM was higher than that in the LM whether IBMX was present or not. 6 LM and CM layers contained a single class of [H-3]-5-CT binding sites with a similar K-d value (0.21-0.24 nM). However, B-max (5-HT7 receptor concentration) in the CM (120.6 fmol mg(-1) protein) was higher than that in the LM (30.4 fmol mg(-1) protein). 7 The molecular study (reverse transcription polymerase chain reaction) demonstrated the expression of 5-HT7 receptor mRNA in the CM was higher than that in the LM. 8 These results suggest that the muscle layer-dependent difference in inhibition by 5-HT is not restricted to spontaneous contraction but applies to various contractions in the porcine myometrium. Different inhibition of the contractility by 5-HT is caused by muscle layer-related accumulation of cyclic AMP (CM > LM), due Co smooth muscle-layer dependent distribution (CM > LM) of 5-HT7 receptors.