Smooth Muscle Layer-Specific Variations in the Autonomic Innervation of Bovine Myometrium

Abstract

1. To clarify the autonomic innervation regulating longitudinal muscle (LM) and circular muscle (CM) motility in the bovine uterus, functional (nerve stimulation, adrenergic drug responsiveness) and biochemical studies (catecholamine content, radioligand binding) were conducted on parous luteal-phase myometrium. 2. Electrical field stimulation (EFS; 60 V, 0.5-msec duration) caused tetrodotoxin (1 μM)-sensitive contractions in a frequency-dependent manner (0.5–20 Hz) in both LM and CM layers. 3. The EFS-induced LM contractions were potentiated by propranolol and conspicuously decreased by phentolamine, yohimbine, idazoxan or guanethidine, but were unaffected by prazosin or atropine. 4. On the other hand, CM contractions were only slightly decreased by phentolamine, idazoxan, yohimbine and guanethidine, but were insensitive to propranolol, prazosin or atropine. 5. The noradrenaline content in LM was about five times higher than that in CM. 6. Noradrenaline, adrenaline, clonidine, xylazine, UK14,304 and phenylephrine caused concentration-dependent contractions of both smooth muscle layers. 7. Clonidine, UK14,304 and xylazine were more potent contractile agents than noradrenaline and phenylephrine. 8. The contractile response to noradrenaline was competitively antagonized by yohimbine, but not by prazosin. 9. Binding studies using [ 3H]-prazosin and [ 3H]-rauwolscine revealed that the bovine myometrium contained both α 1- and α 2-adrenoceptors, but the α 2-type receptor was dominant in both LM (94% of α-adrenoceptors) and CM (88%) layers. 10. The distribution of α-adrenoceptors was muscle layer-specific; that is, the concentration of α 1-receptors in LM was the same as in CM, but the concentration of α 2-receptors in LM was 2.6 times higher than that in CM. 11. The results of the present study indicate that there are layer-specific variations in the functional innervation of the parous bovine myometrium (exclusive adrenergic innervation in LM and adrenergic [minor] plus nonadrenergic, noncholinergic innervation [major] in CM), and that α 2-adrenoceptors, which were responsive to the excitatory response of endogenous and exogenous noradrenaline, were dominant in both muscle layers of the bovine myometrium.