Reproduction in Domestic Animals

GH and IGF-I in Cattle and Pigs 35has inadequate feed. In all cases, first parity sows are afocus of metabolic studies because their capacity for feedconsumption is less and they are more likely to becatabolic (van den Brand et al. 2001).Weaning is associated with changes in GH, insulin,glucose and IGF-I but these changes may be confoundedby the lower feed level after weaning andother management considerations. Weaning is stressfulfor sows and the combination of stress and thediscomfort of a fully engorged mammary gland maydepress appetite. Loss of appetite and decreased feedintake reduce blood insulin and IGF-I. Growthhormone concentrations typically decrease after weaning(Mejia-Guadarrama et al. 2002; Govoni et al.2007) because suckling stimulates GH release in pigs(Rushen et al. 1993). If sows are underfed duringlactation (negative energy balance) then weaning mayimprove insulin and IGF-I status (Koketsu et al. 1998;Mejia-Guadarrama et al. 2002). If sows are well-fedduring lactation then weaning may have lesser effectson insulin and IGF-I (Koketsu et al. 1998; Mejia-Guadarrama et al. 2002). Regardless of their nutritionalstatus during lactation, there appears to be anormalization of IGF-I within 3 days after weaning.Sows will typically undergo a short period of lowIGF-I immediate after weaning (associated with loss ofappetite, lower feeding level and stress) and then haveelevated IGF-I as they progress toward oestrus. Theincrease in IGF-I is not necessarily a metabolicresponse because there are stimulatory effects ofoestradiol on liver IGF-I synthesis and secretion(White et al. 2003).The porcine GHRSurprising little is known about the porcine GHR. Ina study of pigs from birth to 1 year of age, the liverGHR mRNA increased with increasing age but muscleGHR mRNA failed to change (Schnoebelen-Combeset al. 1996). The tissue-specific pattern of GHRexpression that was identified in the growing pigsuggested that GHR gene expression was controlledby different mechanisms in liver and muscle; asituation that is known to occur in cattle (see previoussection). Likewise, other investigators have reportedtissue-specific effects of nutrition and GH on theexpression of GHR in liver, muscle and adipose tissuebut a simple theory that explains the physiologicalresponse has not evolved (Dauncey et al. 1994;Brameld et al. 1996; Combes et al. 1997). To ourknowledge there has been only one study of GHRvariants in pigs. In that study, both GHR 1A andGHR 1B mRNA were found in pig liver (Liu et al.2000a). The GHR 1B mRNA but not the GHR 1AmRNA was detected in muscle. The pig, therefore,was like other species because GHR 1B was found ina variety of tissues, whereas GHR 1A was a liverspecificmRNA. Beyond this information, we knownothing about the regulation of specific GHR variantsin pig liver. We do not know if liver GHR expressionchanges during lactation or if negative energy balancein late lactation uncouples the somatotropic axisthrough a GHR-dependent mechanism.Unique aspects of the somatotropic axis during porcinelactationSows are typically catabolic during lactation and loseweight and body condition. Despite the catabolic natureof lactation, the somatotropic axis remains coupled inlactating sows and IGF-I is elevated during lactation.Greater blood IGF-I concentrations, however, do notsuppress blood GH during the sow lactation perhapsbecause nursing piglets stimulate GH secretion andsupersede the IGF-I negative feedback. In the dairycow, the act of machine milking will increase bloodprolactin concentration but, based on limited data, thereappears to be little change in blood GH in response tomachine milking (Negrao and Marnet 2002). The releaseof GH in response to suckling in the beef cow has notbeen studied extensively.Mechanisms that Link the Somatotropic Axis toReproductionSwine and cattle share many aspects of the underlyingbiology controlling ovarian follicular growth. Forexample, in both species follicular development iscontrolled by a combination of LH and FSH. Theinhibition of LH secretion by lactation prevents preovulatoryfollicular development in sows before weaning(Varley and Foxcroft 1990). After weaning, basal LHconcentrations and numbers of LH peaks increase(Shaw and Foxcroft 1985). The increase in LH pulsatilityis believed to drive follicular growth towardovulation. Concentrations of FSH initially increaseafter weaning, but then decline as preovulatory folliclesdevelop (Shaw and Foxcroft 1985). Although LH isclearly important for follicular growth, a high correlationbetween intensity of LH secretion and interval tooestrus has not been found for sows.In cows, follicular development begins shortly aftercalving with a transient increase in FSH and thedevelopment of a dominant follicle. A major componentof the process is the secretion of LH during the earlypost-partum period (Lucy 2003). Low LH pulsatility isassociated with anoestrus. The primary causes ofanoestrus are different for beef and dairy cows but theessential features are similar. In beef cows, sucklinginhibits LH pulsatility and the lack of LH pulsatilityleads to anoestrus (Williams and Griffith 1995). Thissituation is analogous to the sow (suckling inhibits LHsecretion). Anoestrus in dairy cattle is viewed as lesscommon (compared with beef) because the post-partuminhibition of LH may be less in animals that are notsuckled.Systemic interactions of metabolic hormones with thereproductive axisChanges in metabolic hormones like insulin and IGF-Iare dynamic in post-partum cows and sows. Insulin andIGF-I concentrations are initially depressed but graduallyincrease post-partum in cows (Lucy 2003). BloodIGF-I concentrations increase between weaning andoestrus in sows (Mejia-Guadarrama et al. 2002). Thefact that sows are weaned and have elevated IGF-IÓ 2008 The Author. Journal compilation Ó 2008 Blackwell Verlag