Binding and functional characterization of alpha-2 adrenoceptors in isolated swine myometrium

1. Alpha-adrenoceptors in the uterus of cyclic mature pigs were investigated for their binding characteristics as shown by a radioligand study employing [3H]-prazosin and [3H]-rauwolscine, and the alpha-adrenoceptors were studied for their functional roles on motility as shown mechanically in vitro. 2. Noradrenaline (10 nM-1 microM) caused a tetrodotoxin-resistant contraction in the cornual longitudinal muscle. The response was decreased by yohimbine (10 nM-1 microM) but not by prazosin (1 microM). 3. Clonidine, xylazine, phenylephrine and methoxamine also elicited a concentration-dependent contraction of the cornual longitudinal muscle. The ranking according to the potency was clonidine > xylazine >> phenylephrine > or = methoxamine. On the other hand, these alpha-adrenoceptor agonists did not elicit contraction of the cornual circular muscle even at 100 microM. 4. Noradrenergic contraction induced by transmural stimulation was significantly decreased by yohimbine (10-100 nM) but not by prazosin (1 microM). 5. The swine myometrium contained both alpha-1 (13%) and alpha-2 (87%) adrenoceptors. The distribution of alpha-adrenoceptors was muscle layer-specific, i.e. the number of both types of adrenoceptors in the longitudinal muscle was four times higher than that in the circular muscle. 6. These results show that although swine myometrium contains both alpha-1 and alpha-2 adrenoceptors, the alpha-2 type is predominant and largely responsible for mediating the excitatory response to both exogenous and endogenous noradrenaline in the cornual longitudinal muscle.