[Efficacy of PGF2 alpha on milk ejection during the sexual cycle of the cow]

After 30 days of lactation, 15 non-pregnant French Friesian cows were divided into groups A and B, balanced by the milk yield and lactation number of the cows. The 7 cows of group A were given a series of intrajugular prostaglandin F2 alpha (Upjohn Dinolytic) injections every morning, administered in increasing logarithmic doses (1, 4, 16, 64, 256, 1 024 micrograms). The aim of the experiment was to determine the minimal dose that would induce an increase in intramammary pressure (IMP) during the sexual cycle and milk let-down through a cannula in the teat cup. The 8 cows of group B were given a single injection of 256 micrograms of PGF2 alpha every morning to determine the changes during the sexual cycle in the response parameters of IMP (latency and amplitude) or of milk let-down (latency and collected volume) through a cannula. The following results were obtained: 1. The minimal dose of prostaglandin inducing milk let-down varied during the cycle; it was lower (1 to 16 micrograms) during the luteal phase (D4 to D12) than during the perioestral phase (256 to greater than 1 024 micrograms from D - 2 to D + 3) (figs. 2, 3). 2. During daily administration of the same dose (256 micrograms) of PGF2 alpha no response was obtained during the perioestral phase, while during the luteal phase; - maximal IMP deflection reached 12 to 14 mmHg (figs. 5, 6); - the volume of milk ejected was on an average higher than 3 liters per quarter (figs. 5, 6); - the latency time between injection and let down response was about 1.8 times shorter between D10 and D12 than between D4 and D5 (fig. 7). 3. The cyclic changes in the milk ejection parameters caused by PGF2 alpha were closely related to the plasma progesterone level (figs. 2, 3, 5, 6). The coefficients of correlation between the progesterone level and the - threshold dose: r = - 0.809** (group A), - volume of ejected milk: r = + 0.872*** (group B), - deflection amplitude: r = + 0.805*** (group B) were always significantly higher than the threshold of 0.001. Various hypotheses concerning the endocrine control of cyclic variations in milk let-down under the effect of PGF2 alpha are discussed.