• Porcine myometrium;
  • 5-HT7 receptor;
  • cyclic AMP;
  • [3H]5-carboxamidotryptamine binding;
  • reverse transcription polymerase chain reaction
  • 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(5-CT), 5-methoxytryptamine (5-MeOT), forskolin and cyclic adenosine 3′, 5′-monophosphate (cyclic AMP) analogues on 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.

  • 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).

  • 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.

  • 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-HT (1 nM–1 μM) 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.

  • LM and CM layers contained a single class of [3H]-5-CT binding sites with a similar Kd value (0.21–0.24 nM). However, Bmax (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).

  • 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.

  • 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 to smooth muscle-layer dependent distribution (CM>LM) of 5-HT7 receptors.

British Journal of Pharmacology (2000) 130, 79–89; doi:10.1038/sj.bjp.0703290