CONTRACTION OF LACTATING RAT MAMMARY GLAND IN RESPONSE TO DIRECT MECHANICAL STIMULATION

Experiments were performed to determine whether the mammary gland myoepithelium of the lactating rat responds to direct mechanical stimulation. Rat pups on postpartum day 14 obtained 8.6 and 12.2% of the total extractable milk in the mammary glands in 31–41 min from anesthetized spinal cord-sectioned and sham-operated mothers, respectively. The mammary glands of both groups responded with a rise in intramammary pressure following mechanical stimulation of the skin overlying the cannulated mammary gland. The responses occurred after a latent period of 1–2 sec, attained an amplitude of 1–13 cm H2O, and lasted 8–18 sec. Summation of the responses did not occur. Temporary fatigue of the response could be induced with repetitive stimuli. Stimulation of mammary glands contralateral to or adjacent to the cannulated gland never produced a response in the cannulated gland. The exposed mammary gland responded to lightly applied mechanical stimulation with alveolar contractions confined to the area of stimulus contact. Each rat responded to intravenous oxytocin (25 mU) with a 10–29 cm H2O intramammary pressure rise of 4–31 min duration.

THE ACTION OF SODIUM 4-HYDROXYBUTYRATE ON SPINAL REFLEXES

The actions of sodium 4-hydroxybutyrate, gamma-aminobutyric acid and meprobamate have been studied in unanaesthetized animals, in local anaesthetic tests, on isolated organ preparations, on convulsions induced by picrotoxin and strychnine, and on monosynaptic (patellar) and polysynaptic (plantar) reflexes of the spinal cord. Sodium 4-hydroxybutyrate induced a sleep-like state with three unusual features: the righting reflex was remarkably persistent, respiration was good throughout and recovery was abrupt. gamma-Aminobutyric acid was inactive and meprobamate caused flaccid paralysis with loss of the righting reflex. None of the agents affected the responses of the rat diaphragm either to direct stimulation of the muscle or to indirect stimulation through the phrenic nerve. Only meprobamate reduced the responses of theguinea-pig isolated ileum preparation, showed local anaesthetic action and had an anticonvulsant action. All three compounds were capable, after intravenous or topical application, of blocking plantar reflexes in doses which did not affect the patellar reflex. The spinal animal responded in the same way, to the same dose of sodium 4-hydroxybutyrate, as the decerebrate preparation. Topical application to the motor cortex had no effect on spinal reflexes. We conclude that sodium 4-hydroxybutyrate acts preferentially on the internuncial neurones in the spinal cord but differs from meprobamate in its other actions. The similarity between the actions of sodium 4-hydroxybutyrate and of gamma-aminobutyric acid provides furtherevidence in support of the hypothesis that sodium 4-hydroxybutyrate is involved in the gamma-aminobutyric acid metabolic pathways.