The Measurement of Lactational Performance in the Rat in Studies of the Endocrine Control of Lactation

Abstract

A serious difficulty in lactational studies on small laboratory animals is the lack of a satisfactory means of milking them and thus directly measuring their milk yields. At present, lactational performance in rats can only be inferred from litter growth and survival data determined under carefully standardized conditions. Comparison of litter-growth curves (and survival data) for different groups of rats gives some idea of their relative lactational performances but it is not possible to calculate daily milk yields from such curves since the weight losses due to the excreta and insensible perspiration are unknown.Attempts to determine true milk yields by the complicated and not entirely unobjectionable (see discussion by Enzmann) procedures used by Enzmann [1933] for mice and by Brody & Nisbet [1938] for rats, which involve determination not only of the daily litter weight gains but also of the daily losses due to the excreta and perspiration, are clearly impracticable in experiments carried out on any considerable scale. Thus when, in the course of studies of the relation between the adrenal cortex and lactation [Cowie & Folley, 1946a, b], we wished to determine quantitatively the lactational responses of adrenalectomized rats to treatment with adrenal cortex hormones, we had to devise a quantitative criterion of lactation which was derived solely from the ordinary litter-growth curve. THE EQUATION OF THE LITTER-GROWTH CURVEDaggs [1935] proposed a 'lactation index' for rats based on the claim (in which respect he followed Brody [1927]) that the points obtained by plotting the logarithm of the mean weight of the young against time fall along two straight lines which intersect at approximately í = 10 days. The numerical index used by Daggs has, however, little theoretical significance and cannot be used for quantitative comparisons even if the premises on which it is based are true.We have constructed a mean growth curve (Fig.

The time course of cardiovascular changes in lactation in the rat

“…Fig. 4 shows that the growth rate of the litter was approximately linear between days 7 and 20 (see also Cowie & Folley, 1947) and that this was closely paralleled, between days 5 and 15, by the growth of mammary tissue. The milk yield of the rat is known to increase progressively through lactation (Brody & Nisbet, 1938;Grosvenor & Turner, 1958;Weihe, 1968) 104 ANN HANWELL AND J. L. LINZELL and since mammary blood flow/g tissue remains fairly steady between days 5 and 15 (Fig.…”

Observations on the Suckling Stimulus and Lactation in the Rat

“…Litters were not retained in the experiment after the eyes had opened; this ensured that the young supplying the suckling stimulus were always wholly dependent upon the mother's milk. The exacting (Cowie & Folley, 1947). In a few experiments, as indicated, foster young outside this age-range were used.…”

“…Several indices based on litter growth (Daggs, 1935;Brody & Nisbet, 1938;Cowie & Folley, 1947;Schultze, 1954) (Uyldert, 1946 (Bruce, 1958 On the assumption that the weight increase of young rats, entirely dependent on their mother's milk, represents about 50 to 60% of the milk yield (Brody & Nisbet, 1938;Cowie & Folley, 1947;Cross & Harris, 1952), these females weighing about 320 g produced from 4 to 6 kg milk each during a single lacta¬ tion with no stimulation other than vigorous suckling (Table 2). …”

Milk yield, suckling behaviour and milk ejection in the lactating rat nursing litters of different sizes.

“…Growth rate is linear between days 5 and 11 post partum (Cowie & Folley, 1947) and linear regression lines were calculated for milk yield in this period.…”