Responses to catecholamines of the rat isolated uterus throughout the natural oestrous cycle

The effects of noradrenaline (NA), adrenaline (Adr) and isoprenaline (Iso) on rat isolated uterus were studied, throughout the natural oestrous cycle. The beta-inhibitory effects of the catecholamines were measured as a percentage inhibition of a standard acetylcholine (ACh)-induced contraction. Iso produced approximately 80% maximum inhibition of the standard ACh-contraction in all 4 stages of the oestrous cycle. Adr and NA produced 80% maximum inhibition in oestrus only, and 50-60% maximum inhibition in proestrus, metoestrus and dioestrus. The differences in the degree of inhibition produced by the catecholamines were overcome (i.e. the maximum inhibition produced by Adr and NA was increased to become equal to that produced by Iso) when experiments were repeated in the presence of the uptake inhibitors desmethylimipramine (DMI) and normetanephrine (NMN), but not in the presence of an alpha-adrenoceptor antagonist (azapetine 10(-7) M). Variations in the uptake of 3H-Iso and 3H-Na were observed in the different stages of the oestrous cycle. Small alpha-adrenoceptor mediated motor responses to NA and Adr were observed only in the presence beta-antagonist (propranolol 10(-5) M), in uteri from rats in oestrus, metoestrus and dioestrus, but not proestrus. It is concluded that in the rat isolated uterus, beta-inhibitory receptors predominate throughout the natural oestrous cycle, although the existence of alpha-excitatory receptors has been shown in 3 of the 4 stages.