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RESEARCH |
Animal Reproduction Laboratory, Dairy Cattle Physiology Division, National Dairy Research Institute, Karnal-132 001 (Haryana), India
Correspondence should be addressed to M Mondal, National Research Centre on Mithun (ICAR), Jharnapani, Medziphema, Nagaland-797 106, India; Email: mohan_mondal{at}rediffmail.com
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Abstract |
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Introduction |
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Growth hormone (GH) is a major regulator of growth and development during postnatal life, by influencing key metabolic pathways of intermediary metabolism (Nalbandov 1963, Etherton & Kensinger 1984, Breier & Gluckman 1991). Exogenous GH for enhancement of growth had been tried in farm animals by many workers in a wide variety of species, but repeated exogenous GH has met with limited success due to refractoriness of GH synthesis and release from adenohypophysis because of negative feedback effects of insulin-like growth factor-I released from the liver in response to exogenous GH (Berelowitz et al. 1981, Yamashitu et al. 1986). A direct administration of a neurohormone, namely GH-releasing factor (GRF), in its synthetic or recombinant form, took forefront as an alternative measure for enhancement of growth because of being active in a wide range of species, and thus GRF treatment could potentially be used to accelerate growth of animals of commercial importance (Gelato & Merriam 1986). Research in this direction has been carried out on a short-term basis by many workers in recent years (Lapierre et al. 1992, Enright et al. 1993, Ringuet et al. 1994, Binelli et al. 1995, Kazmer et al. 2000).
The practical applicability of long-term GRF administration for growth enhancement in animals depends on its potential to maintain sustained increased levels of plasma GH. The potential of exogenous long-term GRF administration on the level of plasma GH and subsequent changes of plasma luteinizing hormone (LH), if any, has not been documented so far in Indian livestock in general, and buffaloes in particular. The objective of the present study was, therefore, to assess the patterns of changes of GH and LH release after long-term GRF administration in Murrah buffaloes.
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Materials and Methods |
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Measurement of body weight
All the animals were weighed twice a week at 3–4 day intervals on a platform of an electronic balance at 0800 h before any feed was offered. The average body weight of the two observations in a specified week was used for the calculations.
Selection of GRF dosage
Dosages greater than 10 µg/100 kg body weight, when administered i.v., may represent supraphysiological concentrations in cattle, as several studies have failed to demonstrate increasing GH response when using such dosages (Kazmer & Zinn 1998). Lapierre et al. (1990) also suggested that the maximal response of GRF was achieved at comparatively reduced dosages. We, therefore, used the dose of 10 µg/100 kg body weight i.v. in buffalo calves.
Preparation of GRF solution
Synthetic bovine GRF (bGRF(1–44)-NH2; Product code #G0644, Sigma-Aldrich Co., St Louis, MO, USA) was purchased as a formulated lyophilized substrate. For the experiment, GRF solution was prepared by dissolving GRF in sterile distilled water at 4°C. The amount of GRF solution required was calculated a day prior to injection by taking body weights of individual animals of the treatment group, as these animals were to be administered at a dose rate of 10 µg GRF/100 kg body weight i.v.
Treatment
Control and treatment group animals were administered i.v. with either normal saline or an equal volume of GRF solution containing 10 µg GRF/100 kg body weight at an interval of 15 days until 18 injections were completed (9 months).
Blood sampling
Blood samples (3.5 ml) were collected by means of an indwelling jugular catheter prior to and after the first and last injection of GRF at -60, -45, -30, -15, -10, -5 min and +5, +10, +15, +30, +45, +60 min, and thereafter at intervals of 15 min up to 8 h post-injection, in heparinized tubes (20 IU heparin/ml of blood). Blood sampling began at 0600 h on each day. The tubes were put in an ice bucket and carried back to the laboratory immediately after collection. Blood samples were also collected twice a week (at 3–4 day intervals) from all animals by means of jugular venipuncture at 0830 h after taking the body weight throughout the experiment for plasma progesterone monitoring to assess whether either group had begun ovarian cycles. All the samples were centrifuged within 30 min of collection at 500 x g for 30 min and plasma was separated. The plasma samples thus obtained were properly labelled and stored at -20°C until hormone analysis. All experimental protocols and animal care met Institutional Animal Care and Use Committee (IACUC) regulations. Before catheterization, local anaesthesia (Xylocaine) was given and after removal of catheters the animals were treated with antibiotic (oxytetracycline) for 3 days.
Hormone assays
GH assay
GH was assayed by a highly sensitive enzyme immunoassay (EIA) using a second-antibody technique as described by Prakash et al. (2003). The lowest GH detection limit significantly from zero concentration was 50 pg/100 µl plasma, which corresponded to 0.5 ng GH/ml plasma. Intra- and inter-assay coefficients of variation determined using pooled plasma containing 2.0 and 64.0 ng/ml were found to be 2.62 and 0.75% and 3.83 and 4.12% respectively.
LH assay
Quantification of plasma LH was carried out by an EIA developed and validated in our laboratory (Prakash et al. 2002). The sensitivity of the assay for LH in plasma at the minimum detection limit was 6.25 pg/well per 20 µl or 0.31 ng/ml plasma. The intra- and inter-assay coefficients of variation of plasma LH were 4.0 and 9.7% respectively.
Progesterone assay
Plasma progesterone was estimated in ether-extracted samples in duplicate by an RIA procedure developed in our laboratory as detailed by Prakash & Madan (1986) with slight modification. Hundred-microlitre plasma samples were taken for ether extraction. The sensitivity of the assay for progesterone by the extraction procedure at the minimum detection limit was 4 pg/tube, the 50% binding limit being 70 pg/tube. The intra- and inter-assay coefficients of variation of plasma progesterone were 6.7 and 11.1% respectively. Extraction efficiency of the plasma and assay buffer were 98.2 and 98.8% respectively.
Statistical analysis
Mean concentrations of LH and frequency of LH pulses (pulses per 9 h) were calculated for each sequential blood sample. An LH pulse was defined as an increase in LH concentration that exceeded the previous nadir by two intra-assay standard deviations (Schillo et al. 1988). The data for hormonal and weight-gain parameters were analysed by an ANOVA for repeated-measures technique with a post-test for linear trends to compare hormonal changes and weight gain across time using Graphpad InStat 3.0 software. To test the effects of treatment and sampling time on hormonal parameters and weight gain, an ANOVA technique was used separately for pre- and post-GRF administration and treatment x time interaction was also taken into consideration.
Area under the GH response curve (AUC) was calculated by using the Graphpad Prism 2.01 software package 1995.
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Results |
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and 2). Following the first injection, GH concentrations were maintained at a higher level for 150 min post-treatment and thereafter became similar to the control group GH concentration, but GH concentrations in the last intensive bleeding samples were found to be maintained at higher levels for longer (240 min post-injection), although the peak values of the two did not differ significantly (P > 0.05) being 89.0 ± 5.9 and 89.1 ± 20.5 ng/ml (means± S.E.M.) for the first and last GRF injections respectively (Figs 1 and 2).
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Plasma LH changes associated with GRF administration
In the blood plasma samples after the first GRF injection, the mean plasma LH of the control group of animals remained basal (around 0.31 ng/ml) throughout the collection period (Fig. 3A). In the treatment group of animals also, the mean plasma LH level remained low until 285 min post-injection, rising thereafter to a peak value (0.41 ± 0.07 ng/ml) at 345 min post-injection and falling to the basal level (0.31 ng/ml) at 375 min post-injection (Fig. 3A). There was a second rise in plasma LH thereafter to a peak value of 0.49 ± 0.12 ng/ml at 435 min (Fig. 3A). When the data of post-treatment LH levels of treatment and control groups were analysed, it was found that the overall mean plasma LH level of the treatment group was significantly higher (P < 0.01) than the control group of animals for the first GRF injection.
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). In the pre-treatment period, the mean LH levels of animals of the treatment group were significantly higher (P < 0.01) than those of the control group of animals (Fig. 3B). Three distinct peaks of LH were found after GRF administration in the treatment group of animals and the peak values were 0.97 ± 0.22 ng/ml at 150 min, 0.98 ± 0.36 ng/ml at 300 min and 0.94 ± 0.31 ng/ml at 420 min post-injection. No such peaks of plasma LH were observed in controls, in which the plasma LH remained almost at basal level throughout the collection period, and the value ranged from 0.31 ± 0.00 to 0.45 ± 0.05 ng/ml (Fig. 3B).
The mean plasma LH concentrations around the last injection of GRF administration were significantly higher (P < 0.01) than those recorded at the time of the first injection in GRF-treated buffaloes (Fig. 3A and B). Correspondingly, the plasma LH concentrations in controls were also higher (P < 0.01) around the last injection vis- à -vis the first injection (Fig. 3A and B). The hormone concentration exhibited a higher pulsatility with greater amplitude after the last injection as compared with that recorded after the first injection (Fig. 3A and B). Although pulses of LH were recorded also in controls following the last injection, these were fewer and of lower magnitude than those seen in GRF-treated animals (Fig. 3B).
Representative examples of individual buffalo plasma LH concentration patterns from control and treatment groups for the first and last GRF injections are presented for two buffaloes from each group in Figs 4 and 5 respectively. No LH pulse was recorded in controls but the frequency of LH pulses ranged between one and three per 9 h sampling period in GRF-treated animals during the first GRF injection (Fig. 4). During the last GRF injection, the frequency of LH pulses ranged between one and three, and three and seven per 9 h sampling period for control and treatment animals respectively (Fig. 5). Statistical analysis of LH pulse frequencies indicated that there was a treatment x time interaction (P < 0.01).
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. From week 2 onwards plasma progesterone levels in treated animals increased and reached a peak level of 0.74 ± 0.03 ng/ml at week 6 declining thereafter to 0.48 ± 0.03 ng/ml at the 19th week of experimentation. There was a rise in mean progesterone levels from week 19, again to a peak of 0.69 ± 0.06 ng/ml at week 23, showing minor fluctuations thereafter. No distinct trend was found in plasma progesterone in controls, where progesterone levels were found to be within the range of 0.40 ± 0.05 to 0.61 ± 0.05 ng/ml from week 1 onwards until the end of the last intensive sampling. Overall, mean progesterone of the treatment group (0.60 ± 0.01 ng/ml) was found to be significantly higher (P < 0.01) than control group (0.52 ± 0.01 ng/ml). No individual animal from either group had plasma progesterone concentrations 
1.0 ng/ml for at least two subsequent samples collected at 3–4 day intervals.
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. At week 1, the body weights of the treatment and control groups of animals did not differ significantly (P > 0.01), but the body weight of the treatment group showed a significantly increasing trend (P < 0.01) as GRF injections started in the first week of the experiment compared with the control group, and this trend continued until the last injection of GRF.
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Discussion |
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The present study showed that higher plasma GH levels in GRF-treated animals were maintained for 150 min post-first injection and 240 min post-last injection compared with controls (Figs 1 and 2). Similar results on the duration of post-GRF elevated plasma GH treatment were obtained in bovines by Enright et al. (1989, 1993). Moseley et al. (1984) observed that GH levels declined to baseline within 120–240 min in bovines post-injection with different doses of GRF (100, 300 and 1000 µg/312 kg body weight). In contrast, Simpson et al. (1992) found an elevated GH concentration until 5 h post-treatment in primiparous heifers treated with 12.5 µg/kg s.c. injections of GRF.
AUCs for GH response curves increased in GRF-treated bovines (Moseley et al. 1984, Petitclerc et al. 1987, Enright et al. 1989, 1993, Lapierre et al. 1990, Simpson et al. 1992, Ringuet et al. 1994, Kazmer & Zinn 1998, Kazmer et al. 2000, Wiener et al. 2000), ovines (Hart et al. 1985, Della-Fera et al. 1986, Kensinger et al. 1987, Wheaton et al. 1988, Byrem et al. 1989, Beermann et al. 1990, Godfredson et al. 1990), and swine (Takano et al. 1985, Dubreuil et al. 1990) over placebo. The present study revealed similar observations in GRF-administered growing Murrah buffaloes. An interesting finding of the present investigation is that although the experimental animals became older from the first through to the 18th GRF injection (9 months of treatment), still responsiveness to repeated administration of GRF was not affected, but rather became higher in terms of the GH response AUC. Long-term treatment with GRF was found to enhance the responsiveness to subsequent administration of GRF in humans and rodents (Heiman et al. 1984). The results of the present investigation also agree with those of Hongerholt et al. (1992), who found that the time of the GH peak and the height of the peak in response to exogenous GRF was not affected by chronic GRF administration in bovines. In another study (Ringuet et al. 1994), twice daily s.c. administration of GRF for 246 days at 5 µg/kg body weight in Holstein dairy heifers resulted in an increase in GH concentrations throughout the trial and all heifers responded to GRF until the last day of the experiment, suggesting that GRF may be used to induce daily GH release without loss of responsiveness over an extended period of time in young dairy heifers. Similarly, exogenous intermittent administration of GRF enhanced secretion and circulating concentrations of GH in ovines without them becoming refractory to GRF (Hart et al. 1985, Della-Fera et al. 1986, Kensinger et al. 1987, Wheaton et al. 1988, Byrem et al. 1989, Beermann et al. 1990, Godfredson et al. 1990). Chronic repeated administration of GRF in growing pigs did not induce desensitization of somatotroph cells. On the contrary, an increased responsiveness was observed as the days of treatment advanced (Takano et al. 1985, Dubreuil et al. 1990) and this could be the result of stimulation of GH synthesis and release by GRF (Cella et al. 1985). In contrast, Lapierre et al. (1990) observed decreased (P < 0.01) GH responsiveness with days of treatment in dairy calves, but this decrease was due to ageing rather than the chronic treatment period. On all the days of blood sampling a peak of endogenous GH was always registered, suggesting chronic GRF injections for 3 months did not cause any refractoriness of endogenous GH release. The GRF-induced GH response was also found to decrease with age in rats (Sonntag et al. 1983, Ceda et al. 1986, Cuttler et al. 1986) and cattle (Johke et al. 1984), and this decrease could be due to a decrease in sensitivity of somatotrophs to GRF with ageing (Ceda et al. 1986), an increase in sensitivity of somatotrophs to somatostatin with ageing (Cuttler et al. 1986) and/or to an age-related accumulation of somatostatin in somatotrophs (Brazeau et al. 1986).
Plasma LH in blood samples collected prior to and after GRF administration
In the present investigation, plasma LH was found to be significantly higher in blood samples collected around the first and last injections of GRF in treated animals than in controls. Higher pulse frequency was recorded during the last injection of GRF and the basal LH levels in treated animals were also higher. The LH levels of control animals in both the intensive bleeding samples were significantly (P < 0.05) lower, and plasma LH concentrations in blood samples collected around the first injection remained close to the basal level (0.31 ng/ml). The plasma LH concentrations in controls were also higher (P < 0.01) around the last injection vis- à -vis the first injection. The hormone concentration exhibited a higher pulsatility with greater amplitude after the last injection as compared with that recorded after the first injection. Although pulses of LH were recorded also in controls following the last injection in individual animals, these were fewer and of lower magnitude than those seen in treated animals. The results bring out clearly a greater pituitary responsiveness to GRF treatment in terms of LH release, which may also be an indication that GRF treatment could enhance the maturity process in buffaloes in terms of ovarian steroidogenesis. From the early postnatal stimulation of gonads by the hypothalamic and pituitary hormones, progesterone plays a key role in the changes leading to puberty. Puberty is determined by monitoring plasma progesterone concentrations; when plasma progesterone becomes >1.0 ng/ml for at least two subsequent samples collected at 3–4 day intervals it is said that the animal has attained puberty (Ringuet et al. 1994, Salama et al. 1994, Melvin et al. 1999). Although the plasma progesterone of GRF-treated animals was found to be always higher than the control group of buffaloes in this study, the level did not reach 1.0 ng/ml even in a single sample collected at 3–4 day intervals for any animal from either group, suggesting that no animal in this experiment had reached puberty by the time of the last injection of GRF. The GRF-treated buffaloes were advancing towards puberty, which is also indicated by their higher body weight gain (Fig. 7). In both the groups, the source of progesterone may be luteinized tissue within the ovary located beneath the ovarian surface (Berardinelli et al. 1979) and the role of these short luteal phases in the pubertal process is unclear.
The results of the present investigation are in accord with the study of Jimenez-Krassel et al. (1999), who also found higher serum LH in the cattle treated with GRF. Exogenous GH tended to increase LH pulse frequency over control pigs (Gilbertson et al. 1991), which further supports the result of the present investigation. The pulse frequency and amplitude of LH found in the present investigation showed an increasing pattern with age, which is essentially similar to that reported in bovines (Dodson et al. 1988).
In contrast, Moseley et al. (1984) reported that LH release in response to different doses of GRF in steers was not significant. Similarly, exogenous GRF administration did not affect endogenous LH release in rats (Wehrenberg & Ling 1983) and man (Thorner et al. 1983). Endogenous LH secretion was also not affected by exogenous somatotrophin administration (McShane et al. 1989, Hall et al. 1994). The interesting results from the present investigation may be due to (i) species differences, (ii) age at which GRF was applied and its duration, and (iii) the study being carried out in intact animals.
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Conclusion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionConclusionAcknowledgementsReferences |
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Acknowledgements |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionConclusionAcknowledgementsReferences |
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Footnotes |
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References |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionConclusionAcknowledgementsReferences |
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