Plasma prolactin, blood metabolites and milk production
in bromocryptine-treated crossbred goats
Mahendra Singh
*, R.S. Ludri
Dairy Cattle Physiology Division, National Dairy Research Institute, Karnal 132001, Haryana, India
Accepted 12 July 1999
Abstract
To determine the effect of bromocryptine on plasma prolactin, metabolites and milk production, six healthy crossbred goats in their second or third lactation were selected from the institute's goat herd. The goats were injected with bromocryptine @ 5 mg per day intramuscularly for a period of 5 days during 55±60 days of lactation (period I) and treatment was repeated after 14 days for an additional 5 days (period II). Blood samples were collected for 5 consecutive days before, during and after administration of bromocryptine during both periods of the study. Milk samples were collected on days coinciding with the days of blood sampling. During period I, bromocryptine administration decreased (P< 0.01) milk yields by 27.4%. After withdrawal of bromocryptine the yields increased (P< 0.01); however, during period II, bromocryptine treatment did not decrease milk yields. During period I bromocryptine treatment suppressed prolactin level (P< 0.01) which coincided with decrease in milk yield, but in period II such decreases in prolactin did not coincide with changes in milk yields. Fat content increased (P< 0.01) after bromocryptine administration in both periods. A decrease (P< 0.01) in protein and lactose content indicated a possible role of prolactin in the synthesis of these constituents by the mammary gland cell in goats. Blood glucose and NEFA were not affected by bromocryptine in period I but in period II an increase in blood glucose with a simultaneous decrease in NEFA was observed (P< 0.01).From this study it is concluded that prolactin has a role in the maintenance of milk secretion through protein synthesis by the mammary gland. However, it does not seem to have any direct role on milk fat synthesis.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Bromocryptine; Milk production and composition; Mid-lactation; Prolactin; Blood metabolites; Crossbred goats
1. Introduction
Information on bromocryptine suppression of pro-lactin and on milk production in Saanen goats during mid- (Forsyth and Lee, 1993) and late (Gabai et al.,
1992) lactation in temperate climate is available; however, comparable information under tropical con-dition is lacking. The present study was undertaken to determine the effect of bromocryptine administration on plasma prolactin and on the quality and quantity of milk in crossbred goats. In addition, we were inter-ested in seeing the effect of bromocryptine on blood glucose and plasma non-esteri®ed fatty acids (NEFA) levels which are important energy-yielding substrates for milk synthesis.
Small Ruminant Research 35 (2000) 255±262
*Corresponding author. Tel.:91-0184-259071; fax: 91-0184-250042.
E-mail address: ndri@x400.nicgw.nic.in (M. Singh).
2. Materials and methods
2.1. Selection and management of animals
Six crossbred goats (crosses between Alpi-neBeetal and SaanenBeetal) in their second or third lactation were selected from the institute's goat herd. These were kept loose in a goat pen with brick ¯ooring and were fed a ration containing green fodder and concentrate mixture. The green fodder was ad lib and consisted of berseem (Trifolium alexandrinum). The concentrate mixture had 70% TDN and 20% CP, and based on milk production was fed at the time of morning and evening milking as practiced in the institute's goat herd. The goats were hand-milked twice daily at 5:00 and 17:00 hours and the quantity of milk from individual goats was recorded at each milking.
2.2. Experimental treatment and analytical methods
Bromocryptine (2-bromo-alpha-ergocryptine; M/S Sigma Chemical, USA) was dissolved in a solution of alcohol and saline (60 : 40) and was administered at 08:00 hours intramuscularly @ 5 mg per day for 5 consecutive days during mid-lactation of 55±60 days. After an interval of 14 days, bromocryptine was again administered at the same level for 5 days. Jugular blood samples were collected at 08 : 00 hours in heparinized vacutainer tubes daily for 5 days before (ÿ5,ÿ4,ÿ3,ÿ2,ÿ1) during (1, 2, 3, 4, 5) and after (1,2,3,4,5) bromocryptine administration (period I). After an interval of 4 days the sampling was repeated (period II). Plasma was collected by centri-fugation of blood samples and stored in a freezer (ÿ208C) untill further analysis. Blood samples col-lected in separate tubes containing sodium ¯uoride were used for the estimation of glucose. Milk samples were collected on the same days as for blood sampling for the analysis of milk constituents. Aliquots of milk in proportion to yields from each milking of individual goats were composited. Minimum and maximum ambient temperatures were recorded for the duration of the study and the value of the temperature humidity index (THI) was calculated. Plasma prolactin was estimated using double antibody radioimmunoassay procedure (Singh and Ludri, 1998). Iodination of prolactin was done using carrier free sodium I125
supplied by Board of Radiation and Isotope Technol-ogy, Mumbai, India. First peak of labelled hormone was used for RIA of prolactin. Ovine prolactin (oPRL-AFP-4328 C) and its antiserum (NIDDK-oPRL-1-l) was obtained gratis from Dr. Philip F. Smith, National Hormone and Pituitary Program, MD, USA. The intraassay and interassay coef®cients of variation were 8.1% and 6.0% respectively, and the sensitivity of assay was 5 pg/tube. In milk samples, fat was deter-mined volumetrically by Gerber's method (ISI, 1958), protein by formaldehyde titration method (Singhal and Desraj, 1989) and lactose by picric acid method (Perry and Doon, 1950). Blood glucose was estimated by Nelson and Somogyi method as described by Oser (1965) and plasma NEFA was assayed by the extrac-tion method (chloroform : heptane : methanol, 49 : 49 : 2) of Shipe et al. (1980). Analysis of data was carried out using 2-way ANOVA according to Snedecor and Cochran (1980). The averages were compared with Duncan's multiple range test.
3. Results
Average values of milk yield, fat, protein and lactose have been presented in Table 1, and blood glucose and plasma NEFA in Table 2. The change in milk yield and prolactin concentration in relation to bromocryptine administration in the individual goats has been shown graphically in Figs. 1 and 2 to signify the degree of response in each goat. Plasma prolactin declined signi®cantly (P< 0.01) after bromocryptine administration in all the goats and in both periods of study. After the withdrawl of bromocryptine, plasma prolactin levels started rising gradually. The yields of milk during period I after bromocryptine administra-tion decreased signi®cantly (P< 0.01). After the ces-sation of bromocryptine injection, the milk yields of goats increased but still remained lower than those before bromocryptine treatment. The decreases in milk yield during and after bromocryptine treatment were 27.4% and 8.9%. During period II, bromocryp-tine administration did not decrease the yields, rather there was a signi®cant increase. The fat content of milk increased signi®cantly (P< 0.01) during bromo-cryptine administration and thereafter decreased in both periods. There was a non-signi®cant decline in protein percent in milk during period I, whereas in
Table 1
Average daily yield, fat, protein and lactose content of milk in period I and period II of mid-lactation before, during and after bromocryptine administrationa
Period Before Overall average
(S.E.)
During Overall average
(S.E.)
After Overall average
(S.E.)
ÿ5 ÿ4 ÿ3 ÿ2 ÿ1 1 2 3 4 5 1 2 3 4 5
Milk yield (Kg)
Pd.I 1.70 1.63 1.53 1.63 1.38 1.57a0.02 1.20 1.20 1.08 1.07 1.17 1.14b0.01 1.83 1.67 1.33 1.12 1.22 1.43c0.04 Pd.II 0.92 1.22 1.12 1.27 1.23 1.15a0.02 1.30 1.22 1.22 1.35 1.33 1.28b0.00 1.38 1.42 1.45 1.40 1.43 1.42c0.00
Fat (%)
Pd.I 3.47 3.98 3.80 4.38 4.00 3.93a0.05 4.11 4.47 4.37 4.88 4.58 4.48b0.05 4.18 4.23 4.20 4.25 4.05 4.18c0.01 Pd.II 4.08 4.30 3.67 3.83 3.82 3.94a0.04 4.20 4.75 5.30 5.06 4.23 4.71b0.08 4.52 4.53 3.85 4.23 4.63 4.35c0.05
Protein (%)
Pd.I 2.72 2.78 2.75 2.72 2.64 2.720.01 2.60 2.69 2.72 2.78 2.66 2.690.01 2.57 2.47 2.81 2.78 2.69 2.660.02 Pd.II 2.90 2.80 2.87 2.85 2.78 2.84a0.01 2.76 2.80 2.58 2.63 2.65 2.69b0.02 2.62 2.57 2.68 2.80 2.69 2.67b0.01
Lactose (%)
Pd.I 4.70 4.60 4.43 4.49 4.27 4.49a0.02 4.19 4.40 4.44 4.49 4.26 4.36b0.02 4.26 4.08 4.31 4.24 4.18 4.22c0.01 Pd.II 4.18 4.14 4.19 4.15 4.17 4.17a0.00 4.22 4.12 3.65 3.88 3.72 3.92b0.04 3.89 3.95 4.13 4.16 4.19 4.07c0.02
aValues within a row with different superscripts differ (P< 0.05); each period value is based on six goats.
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Table 2
Average daily blood glucose and plasma NEFA concentration in period I and period II of mid-lactation before, during and after bromocryptine administrationa
Period Before Overall average
(S.E.)
During Overall average
(S.E.)
After Overall average
(S.E.)
ÿ5 ÿ4 ÿ3 ÿ2 ÿ1 1 2 3 4 5 1 2 3 4 5
Blood glucose (mg%)
Pd.I 61.91 55.77 48.27 53.08 53.26 54.45a0.81 50.42 60.13 46.30 59.04 59.11 55.00a1.02 45.15 52.96 46.86 39.22 50.28 46.89b0.85 Pd.II 38.60 41.65 46.42 50.73 41.48 43.78a0.78 48.21 52.98 47.47 53.00 50.85 50.50b0.42 49.12 60.58 53.10 49.08 58.87 54.15c0.87
NEFA (mM/I)
Pd.I 0.11 0.19 0.23 0.11 0.17 0.160.00 0.21 0.15 0.24 0.18 0.10 0.170.00 0.14 0.23 0.26 0.21 0.27 0.220.00 Pd.II 0.40 0.26 0.17 0.09 0.07 0.20a0.02 0.07 0.15 0.13 0.12 0.14 0.12b0.00 0.16 0.12 0.09 0.12 0.16 0.13b0.00
aValues within a row with different superscripts differ (P< 0.05); each period value is based on six goats.
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Fig. 1. Milk yield (*-*) and prolactin (-) concentration during mid-lactation.
period II the decline was signi®cant (P< 0.01). Lac-tose in milk declined signi®cantly (P< 0.01) during both periods. In period I, lactose further decreased but in period II, it increased after the cessation of bromo-cryptine administration. The overall average blood glucose levels before, during and after bromocryptine administration remained highly variable (P< 0.01) in both periods and were not related to changes in milk yields as a result of bromocryptine administration. During period I in all the three treatment intervals, NEFA levels remained almost similar but in period II, a signi®cant (P< 0.01) decline during and after bro-mocryptine administration was observed.
4. Discussion
In the present study, bromocryptine treatment dur-ing period I resulted in decreased prolactin levels which coincided with decreased milk yields, but in period II, decline in prolactin was not concomitant to the changes in milk yield. The variable response to bromocryptine might be due to the fact that milking stimulus effect on prolactin release could not be controlled and therefore, in some of the goats levels of prolactin could not reach the minimum threshold value responsible for the decline in milk secretion. Due to the decline in milk yield, during period I of bromocryptine treatment, the fat content increased signi®cantly (P< 0.01; 3.93% vs. 4.48%) and there-after, recovered at a slow rate. This is in contrast with the earlier report of Flint and Gardner (1994) who reported that induction of prolactin de®ciency in goats leads to a reduction in milk volume, but without an effect on milk fat output. In the second period, the increase in fat percent was concomitant to decreasing NEFA levels suggesting enhanced rate of their utiliza-tion for milk fat synthesis. These differences in fat content indicate no direct role of prolactin on milk fat synthesis, but some other factors might be responsible for this difference. There was a decline in lactose and protein content of milk after bromocryptine injection in both periods.
The decrease in protein and lactose content of milk subsequent to the decline in prolactin suggests that prolactin regulates their synthesis in mammary gland. Both growth hormone (GH) and prolactin increase the number of protein and glucose transporters in rodent
mammary secretory cells (Schennan, 1994); however, prolactin has been reported to be more effective and operates through a different pathway than growth hormone. Forsyth and Lee (1993) reported non-sig-ni®cant change in milk yield of goats treated with bromocryptine (5 mg per day) for 8 days during lactation of 7413 day. However, compared with the week before treatment, milk yield was reduced in the week of bromocryptine treatment and following week, and then began to recover. Such trend of decline in milk yield in mid-lactation in the present study was observed in period I and not in period II when the treatment was repeated. Knight et al. (1995) adminis-tered bromocryptine during mid-lactation which sup-pressed prolactin and also decreased milk yield from 2.96 to 2.86 kg per day. A combined GH and prolactin blockade did not completely prevent milk secretion, which further suggests that mammary tissue is itself capable of synthesizing a number of hormones, including quite possibly both prolactin and GH (Sel-man et al., 1994; Steinmetz et al., 1993). Hence systemic hormonal de®ciency does not necessarily result in local intramammary insuf®ciency. This may be the probable reason that we did not ®nd a suppressive effect of bromocryptine on milk yield during period II in this study. Further, ruminant mam-mary tissue does not need a continuous `go' signal but instead is exquisitely sensitive to the `stop' signal provided by autocrine negative feedback factor FIL (Wilde et al., 1995). The changes in milk composition as affected by bromocryptine have not been reported in small and large ruminants. A suppressive effect of bromocryptine on prolactin and decrease in protein and lactose content of milk clearly indicates prolac-tin's role in the maintenance of milk secretion through protein synthesis by the mammary cells. In the present study there seems to be no direct role of prolactin on milk fat synthesis, however, it has a role in lactose and protein synthesis by the mammary gland.
Acknowledgements
we are also thankful to Dr. Philip J. Smith of National Hormone and Pituitary Program for providing the gift of ovine prolactin and its antiserum.
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