Charles W. Turner

Nucleic Acids of the Mammary Glands of Rats.

Summary A study of the nucleic acid concentrations (DNA and PNA) in the developing and functioning mammary glands of rats is reported. DNA increases in the early part of pregnancy and increases only slightly in the lactating gland. PNA increases throughout pregnancy and continues to increase in the lactating gland reaching a maximum value at 21-22 days of lactation. Following enforced weaning the PNA content of the gland falls rapidly. These findings are discussed in regard to growth and function of the glands.

Effect of Lactation upon Thyroid Secretion Rate in the Rat.

Summary 1) Thyroid hormone secretion rate (TSR) was determined in lactating and non-lactating rats by administration of graded levels of L-thyroxine and measuring by radioiodine technics, subsequent suppression of thyroidal I131 output. 2) The average TSR of 2.2 μg/100 g L-thyroxine for lactating rats during the period of intense lactation (8–14 days postpartum) was significantly greater than the average of 1.3 μg/100 g for non-lactating rats of the same strain, age and weight range. These data are in accord with the concept that a hyperthyroid state is generally desirable for optimal milk secretion.

Release and restoration of pituitary lactogen in response to nursing stimuli in lactating rats.

Summary 1) The effects of nursing stimuli upon release and subsequent restoration of lactogenic hormone from the hypophysis has been studied in 2 strains of lactating rats on 14th day postpartum. 2) Wistar lactating rats produced litters which, when 14 days old, were almost as heavy as those of the much larger Sprague-Dawley mothers. There was 15% more pituitary gland/100 g in Wistar rats which, after 10 hours isolation of mother and litter assayed 30% more lactogen than pituitaries of Sprague-Dawley mothers (3.50 and 2.44 units/100 g, respectively). The Reece-Turner pigeon intradermal method of assay was used. Following 30 minutes nursing there was a greater discharge of lactogen (33%) in Wistar (1.16 units/100 g) than in Sprague-Dawley rats (0.78 unit/100 g). About one-half prenursing level was restored in both strains within 2 1/2 hours postnursing but in one strain tested (Sprague-Dawley) restoration to prenursing level did not occur even 9 1/2 hours postnursing. The influence of amounts of lactogen released in the two strains is discussed in relation to lactational performance.

Evidence for Adrenergic and Cholinergic Components in Milk Let-Down Reflex in Lactating Rat.

Summary The amount of milk obtained by litters of lactating rats treated with various ergot alkaloids, Dibenamine or atropine, was compared with control values previously obtained. All ergot drugs tested significantly blocked the let-down response. Methylergo-novine (Methergine) was the most potent inhibitor, followed by the dihydroergotoxine complex (Hydergine) and dihydroergotamine (DHE 45), in that order. Results indicate action on central nervous system is probably responsible for some, or perhaps all, milk letdown inhibitory action of ergot drugs. This observation was based primarily on results with Methergine, a non-sympatholytic compound. Dibenamine evoked symptoms of central excitation which persisted for 30-45 minutes after injection though milk let-down was not inhibited in those animals who nursed during this period. Milk let-down, however, was significantly inhibited when the drug was injected 2 hours prior to nursing, after which time adrenergic blockade has occurred. Atropine induced a highly significant blockade of the let-down response when injected 150-220 minutes before nursing, but failed to block the response when administered 15-60 minutes before nursing. Oxytocin administered after any of these drugs restored normal milk let-down, indicating blockade was central rather than peripheral. Since Dibenamine. Hydergine and DHE 45 are powerful adrenergic blocking agents and atropine a cholinergic blocking agent, it is postulated both adrenergic and cholinergic links are present in the neurohumoral arc responsible for discharge of oxytocin from the posterior pituitary gland.

Thyroid Hormone and Mammary Gland Growth in the Rat.

Summary 1) The effect of mild hyperthyroidism upon mammary gland growth has been studied in adult ovariectomized rats treated with estradiol benzoate (EB) and progesterone (P) for 19 days. 2) Using total DNA as index of mammary growth, glands of animals receiving daily injections of 1 μg EB, 3 mg P and 3 or 6 μg/100 g thyroxine did not differ from that of controls. Upon doubling EB and P levels, a significant increase in total DNA was observed. 3) It is suggested that thyroid hormone may be a limiting factor in mammary gland growth when E and P secretions are adequate, but when TSR is optimal, E and P secretion may limit growth.

Effect of reserpine on oxytocin and lactogen discharge in lactating rats.

Summary 1) The effect of reserpine upon release of oxytocin and lactogenic hormone from the hypophysis has been studied in 14-day postpartum lactating rats. Oxytocin discharge, evaluated by milk let-down yield per timed nursing, was significantly inhibited. Results indicate the drug inhibits oxytocin discharge by suppression of central sympathetic centers involved in the let-down reflex. 2) Decrease of pituitary lactogen content (prenursing level) by approximately 48% indicates reserpine stimulates release of lactogenic hormone. It is possible that the effect is mediated by suppression of an inhibitory hypothalamic center controlling lactogen discharge or by direct action of drug upon adenohypophysis.

Effect of Ovariectomy and Replacement Therapy Upon Thyroxine Secretion Rate of Rat.

Summary 1) Effect of ovariectomy and replacement with 1 μg estradiol benzoate (EB) and/or 3 mg progesterone (P) upon daily thyroxine secretion rate (TSR) has been studied in mature rats. 2) Ovariectomy reduces TSR approximately 33% within 2-6 days postcastration. Increasing the interval to approximately 90 days between ovariectomy and TSR determination did not result in a further reduction in TSR. EB alone and with P prevented decrease in TSR following ovariectomy but P alone had no effect. 3) It is suggested that ovariectomy reduces the circulating estrogen level which, in turn. may reduce thyrotropin secretion and, subsequently, lower daily TSR.

Effects of oxytocin and blocking agents upon pituitary lactogen discharge in lactating rats.

Summary 1) The effect of various compounds upon release of lactogenic hormone from the hypophysis in response to nursing stimuli has been studied in 14-day postpartum lactating rats. 2) Atropine, Dibenamine and Nembutal effectively blocked pituitary lactogen discharge since 30 minutes nursing evoked no decline (2.35, 2.39 and 2.46 Reece-Turner units/100 g, respectively) from previously obtained control prenursing level (2.44 R-T units/100 g). The normal level following 30 minutes nursing was previously determined as 1.66 R-T units/100 g. Since Dibenamine is a potent adrenergic blocking agent and atropine a cholinergic blocking agent, it is suggested cholinergic and adrenergic links are involved in the reflex arc responsible for lactogen discharge from rat pituitary gland. 3) Oxytocin injected i.v. into Nembutal-anaesthetized lactating rats in physiological doses failed to alter pituitary lactogen (2.49 R-T units/100 g) from the control prenursing level. Amounts of oxytocin 30 and 60 x this dose produced but a slight discharge. These results suggest oxytocin has no stimulatory effect upon pituitary lactogen release in the lactating rat, and are contrary to the recent hypothesis that oxytocin is a humoral link involved in lactogen discharge.

Effect of growth hormone and oxytocin upon milk yield in the lactating rat.

Summary (1) We have investigated the effect of growth hormone (GH) and oxytocin alone, and in combination, upon lactation in the rat. Amount of milk obtained by a litter of 6 during 30 minutes nursing on day 14 postpartum, expressed as percent litter body weight, was used as the index of response. (2) Control yields averaged 3.8% with normal distribution. Removal of milk with aid of .1 USP unit/kg oxytocin i.v. resulted in 50% higher yield with more uniformity among individual values. (3) GH injected subc., from days 7-13 at dose of 1 mg/rat/day evoked a 41.1% increase in milk yield. Yields obtained in GH-treated lactating rats with aid of oxytocin were greater and more uniform than with GH or oxytocin alone. The significance of this is discussed. (4) GH had no effect upon pituitary, thyroid or adrenal weight/100 g body weight. GH caused a significant increase in percent weight gain of mothers but not their litters. Litter growth rate as an index of lactation in the rat is discussed.

Bioassay of Galactin, the Lactogenic Hormone

The increase in the crop weight of the common pigeon, while a valuable qualitative test, was found unsatisfactory in the quantitative assay of galactin due to the variability of groups of birds to the same amount of hormone. As the further purification of galactin requires a reliable bioassay, a more accurate method was sought. Instead of weight, the minimum proliferation of the crop gland was found more satisfactory. This degree of crop growth is characterized by the presence of transverse strands or lobules of cellular development and considerable opaqueness when the crop is extended. In the assay of the estrogenic hormone, Coward and Burn 1 made use of the variation in the response of rats as indicated by the percentage showing a positive reaction at various levels of administration. The adaptability of this method of bioassay to galactin has been studied. A characteristic sigmoid curve with maximum sensitivity in the region of 50% response was obtained (Fig. 1). An assay method has therefore been devised using minimum proliferation combined with the principle of percentage response. The details of the assay method follow: Common pigeons weighing from 260 to 340 gm. are used. The fight young birds and the very heavy birds are less uniform in their response. As no sex difference has been noted, both males and females may be used. Experience indicates that about 20 birds should be used. The extract is injected once daily for 4 days just beneath the subcutaneous tissue into the breast muscle.† The crop glands are examined on the 5th day for proliferation.