Intramammary pressure responses to intravenous injections of oxytocin were compared in the urethane-anaesthetized rat before and after various denervation procedures and before and after administration of various adrenergic blocking drugs. Intravenous administration of either phenoxybenzamine, propranolol or pentolinium after a period of time resulted in a more quickly ascending, higher amplitude pressure response but which usually returned more slowly — often oscillating — to the pre-existing basal pressure. Unilateral severance of the dorsal roots supplying the six abdomino-inguinal glands resulted in immediate though lesser increases in both the slope and amplitude of the intramammary pressure response. The responses were increased to a similar extent in contralateral as well as ipsilateral glands caudal to the point of root section whereas the responses of glands rostral to the section were unaffected. The effect of dorsal root section persisted unchanged in rats pretreated with either phenoxybenzamine or propranolol. The intramammary pressures also increased immediately when the segmental nerves were cut. The intramammary pressure was decreased in all glands tested after transecting the spinal cord between T-10 and T-11, then increased dramatically in slope and amplitude over the next 1 to 2 h in glands caudal to the level of section; the responses of glands rostral to the level of cord section were unchanged. The resulting high amplitude responses obtained after cord section usually oscillated and were slow to return to pre-existing basal pressures. The intramammary pressure profile changed in a manner similar to that following spinal cord section when 25% KCl was placed bilaterally on the cerebral cortices. The effect of KCl persisted in rats pretreated with phenoxybenzamine and pentolinium. The results obtained from these experiments suggest that a diffuse motor control of ductal and vascular tone exists in the rat mammary gland which appears to be inhibitory in function and to be sensitive to central as well as to peripheral influences.
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