THE RAT COSTO‐UTERINE MUSCLE: A PREPARATION OF SMOOTH MUSCLE CONTAINING A HOMOGENEOUS POPULATION OF β‐ADRENORECEPTORS

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Abstract
Some histological features of the costo-uterine muscles of the rat were described together with the effects of some sympathomimetic amines upon contractions evoked by field electrical stimulation of isolated preparations. Light and electron microscopy confirmed that the costo-uterine muscle of the rat consists of smooth muscle bundles, arranged longitudinally and interspersed with collagen. There were close contacts between individual cells within bundles. No axon profiles were observed. Histochemical techniques revealed only sparse catecholamine fluorescence at the border of the tissue in association with blood vessels. Isoprenaline, fenoterol, adrenaline [epinephrine], noradrenaline [norepinephrine], salbutamol and phenylephrine consistently inhibited contractions evoked by field stimulation (40 V, 30 Hz, 2 ms for 5 s every 200 s). Tyramine was without effect upon electrically evoked contractions in concentrations of up to 200 .mu.mol/l. The slopes of the log concentration-response curves of effective amines were similar, and all of these were capable of producing 100% inhibition of contraction. The potencies of the amines, relative to isoprenaline = 100 were: fenoterol 135; salbutamol 16; adrenaline 16; noradrenaline 0.7; phenylephrine 0.1; and tyramine < 0.001. Propranolol, added to preparations in the absence of inhibitors of amine uptake and .alpha.-adrenoreceptors, competitively antagonized the effects of the amines. Schild plots had slopes which did not differ significantly from -1, and the mean pA2 [competitive antagonistic activity] values fell within a narrow range, e.g., 8.65 with salbutamol and 9.20 with fenoterol. Mean pA2 values for propranolol with noradrenaline and isoprenaline were unaffected by the combined presence of phentolamine (10 .mu.mol/l), cocaine (10 .mu.mol/l) and corticosterone (10 .mu.mol/l). The potencies of the agonists were also unaffected by the presence of these drugs. Evidently, the rat costo-uterine muscle is largely devoid of symathetic terminals and of .alpha.-adrenoreceptors, and .beta.-adrenoreceptors within the tissue are homogeneous, possibly of the .beta.2-subtype.