Category: Puberty

Although the stimulatory effects of progestins reported in this and other studies are novel, they do not contradict the concept that physiological levels of progesterone organize the estrous cycle via negative feedback action on LH release. It is important to note that the doses of progestins used to enhance onset of puberty are probably much less than those normally experienced by the heifer or cow during the luteal phase of the estrous cycle. Studies dealing with the use of progestins to synchronize estrus indicate that the doses of MGA and norgestomet used to enhance puberty fail to inhibit pulsatile release of LH during the follicular phase of the estrous cycle. Anderson et al. examined the effects of several doses of norgestomet on LH secretion and onset of puberty. They reported that even though large doses of norgestomet inhibited LH secretion, LH pulse frequency was increased following withdrawal of the progestin regardless of dose. On the basis of these results, it appears that the stimulatory effects of progestins on LH secretion are greatest after removal of the steroid. The possibility that these stimulatory effects have physiological significance warrants attention. buy diabetes drugs

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DISCUSSION(2)

Our observation that pulsatile LH secretion is increased following MGA withdrawal is similar to that of Anderson and coworkers, who described an increase in LH pulse frequency in prepubertal heifers after norgestomet withdrawal. Recently, Anderson et al. proposed a mechanism whereby progestins might enhance pulsatile release of LH in prepubertal heifers. According to their hypothesis, progestins effect a decrease in estrogen receptors within neuronal systems that mediate the negative feedback actions of estradiol on GnRH secretion. This group has since provided support for this hypothesis by demonstrating that norgestomet reduces the number of estrogen receptor-containing neurons in the hypothalami of prepubertal beef heifers. ventolin inhaler

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Administration of MGA to prepubertal heifers for 8 days increased the number of heifers attaining puberty within 10 days after treatment. This effect was associated with an increase in pulsatile LH secretion and growth of the largest ovarian follicle. Our data support the claim that progestin treatment hastens the onset of puberty by enhancing the pulsatile release of LH after the withdrawal of progestin. birth control pills

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RESULTS(2)

Analysis of covariance of mean LH concentrations revealed a treatment X time interaction (p = 0.0001). On Days 0 and 7, mean LH was not different (p = 0.071 and 0.112, respectively) between CON animals and MGA-treated animals (Fig. 3). On Day 9, mean LH was higher (p = 0.003) in MGA-treated animals than in CON heifers (Fig. 3). Regression analysis revealed that LH concentrations on Days 0, 7, and 9 increased in a quadratic manner (p = 0.005) in MGA-treated heifers, but did not change {p = 0.742) in CON animals. After removal of the two CON animals that attained puberty during MGA treatment, mean LH on Day 7 was no longer different between groups (1.21 ± 0.21 vs. 0.85 ± 0.11 ngml-1; CON vs. MGA; p – 0.128). ventolin inhalers

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Effect of MG A on Puberty Induction

Before MGA treatment, corpora lutea or elevated concentrations of progesterone were not detected in any heifers. Within 10 days of MGA withdrawal, all of the MGA-treated heifers had ovulated, whereas only 4 (44%) of the CON heifers had ovulated (p = 0.013). Of the 4 CON heifers that attained puberty, 2 became pubertal during treatment (Day 3), and 2 became pubertal 8 days after the treatment period. Distributions of estimated age at puberty for the two groups are shown in Figure 1. On Days 3-11, two of the CON heifers and none of the MGA-treated heifers attained puberty (p — 0.171). On Days 12 to 14, two MGA-treated heifers attained puberty (p = 0.95). On Days 15-16, an additional 2 CON heifers and 3 MGA heifers reached puberty (p = 0.472). By Day 17, 10 days after steroid withdrawal, the last 3 MGA-treated heifers attained puberty.

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MATERIALS AND METHODS(4)

Approximately 5 ml of blood was drawn for analysis of progesterone concentrations. Each sample was collected into a tube containing 100 |xl of 0.16 M EDTA. Blood was kept on ice until centrifugation (1770 X g) at 4°C for 17 min. Blood plasma was pipetted off and stored at -20°C until assayed for progesterone using a solid-phase RIA (Coat-A-Count; Diagnostic Products Corporation, Los Angeles, CA) originally validated for use in cattle by Holt et al. and Patterson et al.. Inter- and intraassay coefficients of variation for this RIA were 3.7% and 6.9%, respectively. The sensitivity of the assay was 0.1 ng-assay tube-1, the lowest standard that inhibited binding of labeled progesterone by 5%.

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In addition to sequential blood sampling, blood samples were drawn via jugular venipuncture 12 days and 1 day before initiation of MGA and on Mondays, Wednesdays, and Fridays for 3 wk after MGA withdrawal. These samples were used to monitor circulating concentrations of progesterone and thus determine luteal function. asthma inhalers

In order to detect changes in ovarian structures, all heifers were subjected to transrectal ultrasonography 12 days and 1 day before treatment, as well as on Days 3 and 6 after initiation of MGA treatment. Ultrasonography was continued on Days 8, 10, 14, 21, and 28 after initiation of treatment. Transrectal ultrasound was accomplished by use of a LS-1000 Real-Time Linear Scanner with a 7.5-MHz intrarectal transducer (Tokyo Keiki LS-1000; Products Group International, Boulder, CO). Only follicles with a diameter of 5 mm or greater were noted and recorded.

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