Effect of flax supplementation and growth promotants on lipoprotein lipase and glycogenin messenger RNA concentrations in finishing cattle.

Lipoprotein lipase (LPL) hydrolyzes triacylglycerols into monoacylglycerol and fatty acids, which are taken up by tissues and used for energy. Glycogenin is the core protein on which glycogen molecules are synthesized. There is one molecule of glycogenin per molecule of glycogen in skeletal muscle; therefore, glycogen storage is limited by the amount of glycogenin present in muscle. The objective of this study was to investigate the effect of feeding flaxseed, a source of PUFA, and administering a growth promoter on steady-state LPL and glycogenin mRNA content of muscle in finishing cattle. Sixteen crossbred steers (initial BW = 397 kg), given ad libitum access to a 92% concentrate diet for 28 d, were used in a four-treatment, 2 x 2 factorial experiment, with flaxseed supplementation (0 or 5% of dietary DM) and implanting (not implanted or implanted with Revalor-S) as the main effects. Muscle biopsies were obtained from the LM at 0, 14, and 28 d, and used to quantify LPL and glycogenin mRNA concentrations using real-time quantitative PCR. Implanting with Revalor-S did not affect LPL (P = 0.13) or glycogenin (P = 0.98) mRNA concentrations. A day x flaxseed interaction (P < 0.001) was observed for both LPL and glycogenin mRNA concentrations. No differences (P > 0.10) were observed between 0 and 5% flaxseed supplemented steers; however, at 28 d, nonflaxseed-fed steers had 4.1- and 5.7-fold increases (P < 0.001) over flaxseed steers for LPL and glycogenin mRNA concentrations, respectively. To further evaluate the effects of alpha-linolenic acid (alpha-LA) on LPL and glycogenin mRNA concentrations, muscle satellite cells were isolated from five finishing steers, and different alpha-LA concentrations were applied in culture. The RNA was isolated from the bovine satellite cells. Addition of alpha-LA numerically increased (P = 0.16) the LPL mRNA concentration 48% at 1 microM alpha-LA compared with the control. The expression of glycogenin was increased (P < 0.05) 50% at 1 microM alpha-LA compared with the control. These results suggest that flaxseed supplementation to finishing steers for 28 d decreased gene expression of both LPL and glycogenin compared with not feeding flaxseed. Alterations in local concentrations of these two proteins could affect the ability of muscle to use fatty acids and glucose for energy, and, ultimately, affect carcass quality.

Effects of flax supplementation and a combined trenbolone acetate and estradiol implant on circulating insulin-like growth factor-I and muscle insulin-like growth factor-I messenger RNA levels in beef cattle.

We evaluated effects of a 5% (dry matter basis) ground flaxseed supplement (flax) and a trenbolone acetate and estradiol-17beta implant, Revalor-S, on circulating IGF-I and muscle IGF-I messenger RNA (mRNA). Sixteen crossbred yearling steers (initial BW = 397 kg) were assigned randomly to one of four treatments: 1) flax/implant; 2) nonflax/implant; 3) flax/nonimplant; and 4) nonflax/nonimplant. Serum was harvested from blood collected on d 0 (before implant or flax addition), 14, and 28, and used in subsequent analyses of circulating IGF-I. Biopsy samples (0.5 g) were obtained from the longissimus muscle on d 0, 14, and 28. Total RNA was isolated from the muscle samples, and real-time quantitative-PCR was used to assess relative differences in IGF-I mRNA. Flax supplementation had no effect (P > 0.10) on circulating IGF-I concentrations. Following implantation, sera from implanted steers had 52 and 84% greater (P < 0.05) IGF-I concentrations than sera from nonimplanted steers on d 14 and 28, respectively. On d 28, local muscle IGF-I mRNA levels increased 2.4-fold (P < 0.01) in biopsy samples obtained from implanted compared with nonimplanted steers. Muscle biopsy samples from nonflax cattle had 4.4-fold higher (P < 0.01) levels of IGF-I mRNA than those from flax cattle on d 28. To determine whether a component of flax, alpha-linolenic acid (alphaLA), was directly responsible for IGF-I mRNA down-regulation, we incubated primary cultures of bovine satellite cells, from implanted and nonimplanted steers, in two concentrations of alphaLA (10 nM and 1 microM). An implant x dose interaction (P < 0.05) was observed for IGF-I mRNA concentrations in bovine satellite cells cultured for 72 h with alphaLA. Satellite cells from nonimplanted steers had similar (P > 0.10) IGF-I mRNA concentration regardless of the level of alphaLA exposure; however, satellite cells from implanted steers exposed to 10 nM and 1 microM alphaLA had 2.5- and 2.0-fold greater IGF-I mRNA levels, respectively, than cells from implanted steers that were not exposed to alphaLA (P < 0.05). Administration of a Revalor-S implant increased circulating IGF-I and local muscle IGF-I mRNA concentrations in finishing cattle. However, muscle IGF-I mRNA levels were decreased by flax supplementation. Muscle cell culture experiments suggested that alphaLA was not responsible for the IGF-I mRNA down-regulation.

Plasma corticosterone in stressed magnesium-deficient weanling rats.

A study was undertaken to learn whether the young Mg-deficient mammal can respond to major stress with increased levels of plasma corticosterone. Plasma corticosterone was determined in 48 weanling rats with dietary Mg deficiency and in 48 Mg-sufficient controls fed a Mg-supplemented diet, studying 12 animals at a time on experimental day 14. Each animal was studied once, either in an unstressed state or after the stress of audiogenic or strychnine seizures. Plasma corticosterone was determined using a radioimmunoassay; Mg was analyzed by atomic absorption spectrophotometry. On experimental day 14, unstressed Mg-deficient rats were tremulous, hyperirritable and showed slightly increased plasma corticosterone levels that exceeded controls levels. After strychnine shock, the mean plasma corticosterone levels of Mg-sufficient and Mg-deficient rats were both significantly increased over resting levels, and were not statistically different. Moreover, the spontaneous mortality rate that occurred during the experimental period in all Mg-sufficient rats was 0 compared to 27% among all Mg-deficient animals (p less than 0.0001). It was concluded that young rats deficient in Mg for 2 weeks could respond to major stress with levels of plasma corticosterone that were not significantly different from values of equally stressed Mg-sufficient controls. The deficient animals suffered a higher mortality, providing support for the concept that Mg deficiency increases stress-induced mortality in animals.

Twenty-four hour pattern of prolactin levels in female rats subjected to transection of the mesencephalic raphe or ablation of the suprachiasmatic nuclei.

The present study was undertaken to evaluate the 24-hour periodicity in serum prolactin levels subsequent to transecting the ascending serotonergic system or ablating the suprachiasmatic nuclei (SCN) of adult (185 g) female rats. The ascending serotonergic system was transected at the level of the rostral mesencephalon using a 1.5 mm wide knife and the SCN was ablated with a modified Halász-Pupp knife. The effect of raphe transection (RT) or SCN ablation on the circadian rhythm in nonstress serum prolactin levels was assessed 45 days after surgery by measuring with radioimmunoassay the serum prolactin concentration in serial blood samples obtained from the tail vein of lightly restrained rats. Serum prolactin concentration in control and RT animals showed circadian periodicity; peak levels occurred during the midafternoon, 4 h prior to the dark phase of the lighting regime (lights on at 4 a.m., off at 6 p.m.). The amplitude of the fluctuations of both groups varied markedly and were related to the estrous cycle. However, RT animals showed a reduced amplitude in the rhythm. In controls peak prolactin levels on days of proestrus and estrus were 15-20 times higher than on days of diestrus. In contrast, serum prolactin levels in SCN-ablated rats did not vary with a circadian periodicity but rather showed random, low amplitude fluctuations. Reproductive cyclicity was also abolished by SCN ablation, i.e., SCN-ablated rats presented persistent vaginal cornification. These data indicate that circadian periodicity in serum prolactin levels is not compromised by sectioning the ascending serotonergic fibers but is abolished after ablating the SCN.

Pentobarbital anesthesia: interference in the effects of other pharmacological agents and of electrical stimulation on prolactin secretion.

In adult male rats, a pretreatment regimen of serial injections of dexamethasone (1 mg/kg), morphine (20 mg/kg) and pentobarbital (40 mg/kg) was evaluated for use in conjunction with studies on the effects of hypothalamic electrical stimulation on prolactin secretion. Serum prolactin levels were measured before and 15 min after electrical and sham stimulation of the hypothalamic arcuate nucleus in rats subjected to either the pharmacological regimen or to pentobarbital anesthesia alone. Pentobarbital alone caused a transient rise in serum prolactin levels, which obscured any effect of electrical or sham stimulation; this interference was not overcome by the addition of dexamethasone and morphine treatment. Thus, the result indicate that the acute stimulatory affect of pentobarbital anesthesia on prolactin release may interfere with further manipulation of prolactin-controlling mechanisms, by either pharmacological or surgical means. Furthermore, the dexamethasone-morphine-pentobarbital pretreated rat does not provide an adequate preparation for studing the effects of electrical stimulation on prolactin secretion.

The effect of surgical isolation or ablation of the medial basal hypothalamus on serum prolactin levels in male rats.

Complete isolation of ablation of the medial basal hypothalamus (MBH) was performed in adult male rats, and non-stress prolactin levels were measured by radioimmunoassay in blood collected by decapitation at 24h, 2 weeks and 6 weeks following surgery. 24 h after surgery, medial hypothalamic ablation (MHA) markedly elevated prolactin levels compared to those of intact control rats; prolactin levels subsequently declined to normal resting values 2 to 6 weeks following surgery. Circulating prolactin levels following hypothalamic isolation were not different from those of controls at either acute or chronic sampling intervals. These data suggest that chronic isolation or destruction of the MBH does not alter basal prolactin secretion in male rats, provided that circadian and stress-induced variations in control animals are properly considered.