Early lactation in dairy cattle is a period of severe negative energy balance (NEB) characterized by reduced blood glucose and insulin concentrations and elevated blood GH concentrations. The liver is refractory to GH during NEB and this uncoupling of the GH-IGF axis results in diminished plasma concentrations of IGF-I. Our objectives were to examine the effects of insulin administration during the immediate postpartum period on plasma IGF-I and GH concentrations and to examine the hepatic expression of total GH receptors (all GH receptor transcripts), GH receptor 1A (GHR 1A) and IGF-I. In addition, we examined adipose tissue for total GH receptor and IGF-I mRNA levels to establish the effects of chronic hyperinsulinemia on an insulin-responsive peripheral tissue. Holstein cows (n=14) were subjected to either a hyperinsulinemic-euglycemic clamp (insulin; INS) or saline infusion (control; CTL) for 96 h starting on day 10 postpartum. Insulin was infused i.v. (1 µg/kg body weight per h), blood samples were collected hourly, and euglycemia was maintained by infusion of glucose. Insulin concentrations during the infusions were increased 8-fold in INS compared with CTL cows (2·33 0·14 vs 0·27 0·14 ng/ml (S.E.M.); P<0·001) while blood glucose concentrations were not different between treatments (45·3 2·2 vs 42·5 2·2 mg/dl; P>0·1). Plasma IGF-I increased continuously during the insulin infusion, and reached the highest concentrations at the end of the clamp, being almost 4-fold higher in INS compared with CTL cows (117 4 vs 30 4 ng/ml; P<0·001). Hepatic expression of GHR 1A and IGF-I mRNA was low in CTL cows, but was increased 3·6-fold (P<0·05) and 6·3-fold (P<0·001) respectively in INS cows. By contrast, in adipose tissue the changes in gene expression in response to insulin were reversed with decreases in both total GHR and IGF-I mRNA. The expressions of GHR 1A and IGF-I mRNA in liver tissue were correlated in INS (r=0·86; P<0·05), but not CTL cows (r=0·43; P>0·1). Insulin appears to be a key metabolic signal in coupling the GH-IGF axis, thus orchestrating a marked elevation in circulating IGF-I concentrations.

Background: Translocator protein (TSPO) has been considered a mitochondrial cholesterol transporter critical for steroid hormone production. TSPO knock-out mice were reported to be embryonic lethal. Results: TSPO knock-out mice are viable with no effects on steroidogenesis. Conclusion: TSPO is not essential for steroidogenesis and is not necessary for sustaining life. Significance: This study rectifies a serious inaccuracy in the current understanding that is critical for treating steroid hormone disorders.

Molecular events that regulate cellular biosynthesis of steroid hormones have been a topic of intense research for more than half a century. It has been established that transport of cholesterol into the mitochondria forms the rate-limiting step in steroid hormone production. In current models, both the steroidogenic acute regulatory protein (StAR) and the translocator protein (TSPO) have been implicated to have a concerted and indispensable effort in this cholesterol transport. Deletion of StAR in mice resulted in a critical failure of steroid hormone production, but deletion of TSPO in mice was found to be embryonic lethal. As a result, the role of TSPO in cholesterol transport has been established only using pharmacologic and genetic tools in vitro. To allow us to explore in more detail the function of TSPO in cell type-specific experimental manipulations in vivo, we generated mice carrying TSPO floxed alleles (TSPOfl/fl). In this study we made conditional knockout mice (TSPOcΔ/Δ) with TSPO deletion in testicular Leydig cells by crossing with an anti-Mullerian hormone receptor type II cre/+ mouse line. Genetic ablation of TSPO in steroidogenic Leydig cells in mice did not affect testosterone production, gametogenesis, and reproduction. Expression of StAR, cytochrome P450 side chain cleavage enzyme, 3β-hydroxysteroid dehydrogenase/Δ5-Δ4 isomerase type I, and TSPO2 in TSPOcΔ/Δ testis was unaffected. These results challenge the prevailing dogma that claims an essential role for TSPO in steroid hormone biosynthesis and force reexamination of functional interpretations made for this protein. This is the first study examining conditional TSPO gene deletion in mice. The results show that TSPO function is not essential for steroid hormone biosynthesis.

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Prolonged anovulation following parturition has a negative impact on fertility in dairy cows. Insulin plays an important role in ovarian function in many species, and is profoundly depressed in dairy cows during early lactation. We hypothesized that hypoinsulinemia during early lactation represents a key indicator of nutritional status, resulting in delayed ovulation. Holstein cows (n 5 10) were subjected to either a hyperinsulinemic -euglycemic clamp (INS) or saline infusion (CTL) for 96 h, beginning on day 10 after parturition during the first postpartum follicular wave. Insulin was infused continuously (0.3 mg/kg body weight per h) via a jugular catheter, and euglycemia was maintained by infusion of glucose. Circulating insulin concentrations were elevated 2.6-fold in INS cows compared with CTL cows (0.73 6 0.026 vs 0.28 6 0.026 ng/ml; P < 0.001). Insulin treatment did not affect (P > 0.05) luteinizing hormone (LH) pulse frequency, pulse amplitude or mean circulating LH. Circulating estradiol was elevated in INS cows (P < 0.01) and circulating testosterone also tended to be higher. The ratio of testosterone to estradiol was not different between treatments for the initial 30 h of infusion, but was significantly reduced thereafter in response to insulin (P < 0.01), suggesting that hyperinsulinemia increased follicular aromatase activity. Insulin treatment also resulted in reduced circulating nonesterified fatty acids, and increased circulating total and free insulin-like growth factor-I concentrations. Insulin infusion increased estradiol secretion by the dominant follicle of the first postpartum follicular wave in dairy cows, and this effect appears not to be mediated through changes in pulsatile LH release.

The objective of this study was to evaluate the effect of dietary starch content and monensin (MON) on metabolism of dairy cows during early lactation. Before parturition, primiparous (n=21) and multiparous (n=49) Holstein cows were fed a common controlled-energy close-up diet with a daily topdress of either 0 or 400mg/d monensin. From d 1 to 21 postpartum, cows were fed a high-starch (HS; 26.2% starch, 34.3% neutral detergent fiber, 22.7% acid detergent fiber, 15.5% crude protein) or low-starch (LS; 21.5% starch, 36.9% neutral detergent fiber, 25.2% acid detergent fiber, 15.4% crude protein) total mixed ration with a daily topdress of either 0mg/d monensin (CON) or 450mg/d monensin (MON), continuing with prepartum topdress assignment. From d 22 through 63 postpartum, all cows were fed HS and continued with the assigned topdress treatment until d 63. Cows fed HS had higher plasma glucose and insulin and lower nonesterified fatty acids (NEFA) than cows fed LS during d 1 to 21 postpartum. Cows fed LS had elevated early-lactation β-hydroxybutyrate (BHBA) compared with cows fed HS. Cows fed HS had greater insulin resistance and increased plasma haptoglobin in the early lactation period. There was no effect of MON on postpartum plasma NEFA. Cows fed MON had higher plasma glucose compared with CON cows, which was driven by a MON × parity interaction in which primiparous cows fed MON had greater plasma glucose concentrations than cows fed CON. Cows fed MON had lower plasma BHBA compared with CON, which was contributed to by a MON × parity interaction in which primiparous cows fed MON had lower BHBA concentrations than CON. Starch treatment had no effect on overall liver triglyceride content. Primiparous cows fed MON had increased liver triglyceride content compared with CON primiparous cows, and multiparous cows fed MON had decreased liver triglyceride content compared with CON cows. Multiparous cows fed LS with MON had higher liver glycogen content than multiparous cows fed the LS without MON, with no effect of MON treatment for multiparous cows fed HS. There was no effect of starch or MON treatment on liver capacity to oxidize propionate to CO2, and effects of starch on gluconeogenesis were not significant. Cows fed MON tended to have greater capacity to convert propionate to glucose than CON. Supplementation with MON increased the ratio of glucose to CO2, which indicated that cows fed MON had a greater propensity to convert propionate to glucose. Overall, cows fed more propiogenic diets in early lactation (high starch or monensin) exhibited improved energy metabolism during early lactation.

Most dairy cows develop the first dominant follicle postpartum within 2 wk after calving, but only about 40% of these follicles produce sufficient estradiol to stimulate ovulation despite having normal ultrasound appearance and growth. This study aimed to characterize metabolic, endocrine, and follicular fluid profiles of cows in which the first dominant follicle postpartum will become ovulatory and those with nonovulatory follicles. Luteinizing hormone pulse frequency, follicular fluid androstenedione, and follicular fluid estradiol concentrations were lower in nonovulatory cows suggesting that the function of theca cells is impaired. In addition, nonovulatory cows had more severe negative energy balance and greater insulin resistance postpartum. This study describes for the first time the steroid hormone profile of early postpartum follicles and shows that a steroidogenic defect most likely occurs in theca cells limiting the amount of androgen precursor available for estradiol production that impairs their ovulatory outcome.

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