Life in the fat lane: seasonal regulation of insulin sensitivity, food intake, and adipose biology in brown bears.

Grizzly bears (Ursus arctos horribilis) have evolved remarkable metabolic adaptations including enormous fat accumulation during the active season followed by fasting during hibernation. However, these fluctuations in body mass do not cause the same harmful effects associated with obesity in humans. To better understand these seasonal transitions, we performed insulin and glucose tolerance tests in captive grizzly bears, characterized the annual profiles of circulating adipokines, and tested the anorectic effects of centrally administered leptin at different times of the year. We also used bear gluteal adipocyte cultures to test insulin and beta-adrenergic sensitivity in vitro. Bears were insulin resistant during hibernation but were sensitive during the spring and fall active periods. Hibernating bears remained euglycemic, possibly due to hyperinsulinemia and hyperglucagonemia. Adipokine concentrations were relatively low throughout the active season but peaked in mid-October prior to hibernation when fat content was greatest. Serum glycerol was highest during hibernation, indicating ongoing lipolysis. Centrally administered leptin reduced food intake in October, but not in August, revealing seasonal variation in the brain's sensitivity to its anorectic effects. This was supported by strong phosphorylated signal transducer and activator of transcription 3 labeling within the hypothalamus of hibernating bears; labeling virtually disappeared in active bears. Adipocytes collected during hibernation were insulin resistant when cultured with hibernation serum but became sensitive when cultured with active season serum. Heat treatment of active serum blocked much of this action. Clarifying the cellular mechanisms responsible for the physiology of hibernating bears may inform new treatments for metabolic disorders.

Cardiac response to exercise in normal-weight and obese, Hispanic men and women: implications for exercise prescription.

AIM: The effects of obesity on cardiac function during incremental exercise to peak oxygen consumption (VO(2peak)) have not been previously described. The purpose of this study was to compare submaximal and maximal cardiac function during exercise in normal-weight and obese adults. METHODS: Normal-weight (n = 20; means ± SE: age = 21.9 ± 0.5 years; BMI = 21.8 ± 0.4 kg m(-2)) and obese (n = 15; means ± SE: age = 25.1 ± 5.2 years; BMI = 34.1 ± 01.0 kg m(-2)) participants were assessed for body composition, VO(2peak) and cardiac variables (thoracic bioimpedance analysis) at rest and at heart rates (HR) of 110, 130, 150 and 170 beats min(-1) and maximal HR during incremental cycling exercise to exhaustion. Differences between groups were assessed with mixed-model ancova with repeated measures. Cardiac variables were statistically indexed for body surface area and resting HR. VO(2) and arteriovenous oxygen difference (a-vO(2)) were statistically indexed for fat-free mass and resting HR. RESULTS: Significant main effects for group indicated obese participants had higher cardiac output (Q) index and stroke volume (SV) index but lower ejection fraction (EF) and a-vO(2) index during incremental exercise to exhaustion compared with their normal-weight peers, despite similar submaximal and maximal VO(2) and absolute power outputs (P < 0.05). CONCLUSIONS: Our findings suggest that although Q index and SV index were higher in obese, young adults, EF and a-vO(2) index were significantly lower when compared to matched, normal-weight adults.

A review of the stroke volume response to upright exercise in healthy subjects.

Traditionally, it has been accepted that, during incremental exercise, stroke volume plateaus at 40% of Vo(2)max. However, recent research has documented that stroke volume progressively increases to Vo(2)max in both trained and untrained subjects. The stroke volume response to incremental exercise to Vo(2)max may be influenced by training status, age, and sex. For endurance trained subjects, the proposed mechanisms for the progressive increase in stroke volume to Vo(2)max are enhanced diastolic filling, enhanced contractility, larger blood volume, and decreased cardiac afterload. For untrained subjects, it has been proposed that continued increases in stroke volume may result from a naturally occurring high blood volume. However, additional research is needed to evaluate the importance of blood volume, or other mechanisms, that influence the stroke volume response to exercise in untrained subjects.

Non-linear relationships between central cardiovascular variables and VO2 during incremental cycling exercise in endurance-trained individuals.

AIM: The purpose of this study was to examine the relationships between the central cardiovascular variables (cardiac output, stroke volume and heart rate) and oxygen uptake (VO2) during continuous, incremental cycle exercise to maximal aerobic capacity (VO2max). METHODS: Twenty-one moderately to highly trained males (n=19) and females (n=2) participated in the study. A baseline maximal exercise test was performed to measure VO2max. Following the initial VO2max test, cardiac output was measured (CO2 rebreathing technique) at rest and 3 times during each of 4 exercise trials (2 submaximal tests to 90% VO2max and 2 maximal tests). Stroke volume and arteriovenous O2 difference were calculated using standard equations. RESULTS: Significant non-linear relationships were found between all central cardiovascular variables and VO2 (P<0.01). A plateau in cardiac output at VO2max was identified in 3 subjects. Stroke volume plateaued at an average of 37+/-12.5% of VO2max in 18 subjects and increased continuously to VO2max in 3 subjects. The arteriovenous O2 difference progressively increased to VO2max in 17 subjects and revealed a plateau response in 4 subjects. CONCLUSIONS: Our data suggest that there is a significant non-linear relationship between the central cardiovascular variables and VO2 during incremental exercise to VO2max. Furthermore, depending on the person, VO2max may be limited by cardiac output (evidence of cardiac output[Q] plateau) or peripheral factors (continued increase in Q).

Is there a physiologic basis for creatine use in children and adolescents?

The purported ergogenic benefits of creatine for the adult population have been well documented. In able-bodied children and adolescents, there is a paucity of data on creatine use and the purported ergogenic effects of creatine. Only 1 study to date has investigated the ergogenic properties of creatine in the adolescent population. The purpose of this review was to try to establish a rationale for creatine use in the child and adolescent population. The limited literature available in this area did not provide a strong enough rationale from either a physiologic or performance perspective for creatine supplementation in these populations. However, significantly more research is required before definitive conclusions can be made.

Production of extracellular matrix, fibronectin and steroidogenic enzymes, and growth of bovine granulosa cells in anchorage-independent culture.

A proportion of the granulosa cells from bovine antral follicles will survive, like stem cells, in anchorage-independent culture. To study these cells, bovine granulosa cells were isolated from medium-sized follicles (3-5 mm), plated out (in aliquots of 2.5 x 10(4) viable cells) onto a 1 mL agar base, and overlaid with 1 mL of methycellulose solution in culture medium (control). The cells were cultured (14 days) and then processed for histology (n = 14) or Western immunoblotting (n = 5). Under control conditions or after treatment with basic fibroblast growth factor (bFGF; 50 ng mL-1), a proportion of the granulosa cells divided to produce colonies; individual cells remained small. bFGF increased the number of cells harvested (15.8 +/- 7.3-fold, as measured indirectly by the relative amount of the nuclear La antigen), increased the average diameter of the colonies from 88.9 +/- 13.5 microns to 136.5 +/- 4.9 microns and stimulated the production of fibronectin 5.7 +/- 1.5-fold (P < 0.05). An extracellular matrix, which has previously been shown to be a basal lamina, was observed in 19.1% of the colonies (total of 350 colonies examined; n = 8 experiments). Cells treated with dibutyryl cAMP (1 mM) hypertrophied and had 50 +/- 28.7-fold and 102.6 +/- 55.8-fold higher levels of cholesterol side-chain cleavage cytochrome P450 (P < 0.001) and 3 beta-hydroxysteroid dehydrogenase (P < 0.01) respectively (n = 5). Thus, granulosa cells with characteristics of stem cells can divide and produce extracellular matrix, or be induced to differentiate when in culture without anchorage.

Basal lamina and other extracellular matrix produced by bovine granulosa cells in anchorage-independent culture.

Bovine granulosa cells from 3-7 mm follicles were cultured without anchorage in soft agar/methylcellulose solution for 14 days, with or without 50 ng/ml basic fibroblast growth factor. The granulosa cells divided to form colonies of cells. These were analysed by light and electron microscopy, immunohistochemistry and Western immunoblotting. In approximately 20% of the colonies extracellular matrix was clearly visible at the light-microscope level. Ultrastructurally the matrix resembled a basal lamina 30-100 nm thick and was composed of tangled fibres or cords. Unidentified spherical structures of less than 50 nm diameter were sometimes present and attached to this basal lamina. The basal lamina of follicles had similar features, except that the basal lamina produced in vitro was a large aggregate of many convoluted layers. The cells produced collagen type IV and the cellular form of fibronectin. Intercellular areas not associated with basal lamina were identified. Ruthenium red staining revealed these areas to be rich in proteoglycan granules. Free granules were clustered near the cell surface, and the lumina of these areas were rich in fibres decorated with ruthenium red. This material did not resemble follicular fluid of antral follicles. Thus, granulosa cells in anchorage-independent cultures have a follicular cell morphology and secrete two distinct extracellular matrices, one similar to the follicular basal lamina.

The physiology of the ovary: maturation of ovarian granulosa cells and a novel role for antioxidants in the corpus luteum.

During folliculogenesis the granulosa cells divide whilst in contact with each other, and so exhibit some of the characteristics of stem cells. In vitro we have shown that bovine granulosa cells from 3-7 mm follicles, like stem cells, divide without the need for a substratum, and produce colonies of cells. Growth factors, bFGF and IGF's, stimulate their division. These cells secrete and assemble a basal lamina, suggesting that the follicular basal lamina is produced by the granulosa cells. They have the morphological characteristics of follicular granulosa cells. Thus this system is ideal for studying the functions of immature granulosa cells because the cells do not spontaneously differentiate or luteinize into luteal cells, as occurs in culture on a substratum. On differentiation into luteal cells in vivo the cells express the steroidogenic enzymes for progesterone production and accumulate beta-carotene. During culture of bovine luteal cells we observed that a proportion of the steroidogenic enzyme cholesterol side-chain cleavage cytochrome P450 enzyme became chemically cross-linked to its electron donor, adrenodoxin. P450 enzymes produce oxygen free radicals and oxygen free radicals can cause cross-linking between proteins in close proximity. Cell protect against this damage by the use of antioxidant vitamins. Repleting the cultured luteal cells with beta-carotene reduced the amount of cross-linking. We conclude that the high levels of beta-carotene in corpora lutea are to protect against damage due to oxygen free radicals generated in the course of progesterone synthesis.

Concentrations of cytochrome P450 cholesterol side-chain cleavage enzyme and 3 beta-hydroxysteroid dehydrogenase during prostaglandin F2 alpha-induced luteal regression in cattle.

Prostaglandin F2 alpha (PGF2 alpha)-induced regression of the corpus luteum causes both plasma progesterone concentrations and luteal concentrations of mRNA encoding the steroidogenic enzyme 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) to fall in parallel. To investigate the hypothesis that a decline in the concentrations of mRNA encoding steroidogenic enzymes causes plasma progesterone to fall, the luteal concentrations of the enzymes 3 beta-HSD and cytochrome P450 cholesterol side-chain cleavage were measured during induced luteolysis. Holstein heifers were treated with PGF2 alpha (25 mg Lutalyse) on Day 6 or Day 7 of the oestrous cycle and corpora lutea were collected 0 h, 2 h, 12 h, and 24 h later (n = 6, 4, 4, and 4 respectively). Analyses of the steroidogenic enzymes were carried out by Western immunoblotting. The luteal concentrations of both steroidogenic enzymes did not decrease over the 24-h period. It is concluded that, although the concentrations of mRNA encoding steroidogenic enzymes may decline in response to PGF2 alpha, this does not lead to a sufficiently rapid reduction in the concentrations of the enzymes to precede, and thus cause, the decline in plasma progesterone concentrations. Thus, the mechanism for the initial decline in plasma progesterone concentrations during luteolysis is still not known.