Starvation-Induced Decreased Sensitivity of Resting Metabolic Rate to Triiodothyronine

275

107

A B S T R A C T Diet-induced alterations in thyroid hormone concentrations have been found in studies of long-term (7 mo) overfeeding in man (the Vermont Study). In these studies ofweight gain in normal weight volunteers, increased calories were required to maintain weight after gain over and above that predicted from their increased size. This was associated with increased concentrations of triiodothyronine (T3

Section: Discussionsupporting

confidence: 60%

Dietary-induced alterations in thyroid hormone metabolism during overnutrition.

Danforth¹,

Horton²,

O’Connell³

et al. 1979

275

107

“…Such redundancy would appear to be useful as a "failsafe" mechanism in protecting the nutrient-deprived animal. In this connection it is interesting to note that in starvation there is a reduction in oxygen consumption that is clearly not dependent on a decrease in T3 production, since a comparable reduction is observed in starved hypothyroid animals maintained on a constant daily dose of T3 (43). Similar response patterns have been observed in hepatic nuclear proteins (44).…”

Section: Discussionsupporting

confidence: 59%

Starvation and hypothyroidism exert an overlapping influence on rat hepatic messenger RNA activity profiles.

Carr¹,

Seelig²,

Mariash³

et al. 1983

A B S T R A C T To assess the effect of starvation and to explore the potential interrelationship of starvation and thyroid status at the pretranslational level, we have analyzed by two-dimensional gel electrophoresis, the hepatic translational products of starved and fed euthyroid and hypothyroid rats. 5 d of starvation resulted in a statistically significant change in 27 of 240 products visualized, whereas hypothyroidism caused a change in 20, both in comparison with the fed euthyroid state. Of considerable interest was that 68% of all changing messenger (m)RNA sequences were common to the hypothyroid and starved groups and showed the same directional shift. Further, both starvation and hypothyroidism yielded comparable decreases in total hepatic cytoplasmic RNA content. Although it has been well established that the level of circulating triiodothyronine (T3) and the level of hepatic nuclear receptors fall in starvation, this reduction cannot account for the observed decrease of total hepatic RNA nor for all of the alterations in the concentrations of specific mRNA sequences. Thus, administration of T3 to starved animals in a dose designed to occupy all nuclear T3 receptors fails to prevent the fall in total RNA and the majority of starvation-induced changes in the level of mRNA sequences. Moreover,The results of this study were reported in part at the Na- (1)(2)(3)(4)(5)(6)(7)(8) and also a decrease in hepatic nuclear T3 receptors in the rat (8)(9)(10)(11)(12). Associated with the effect of starvation on T3 receptor capacity and content are alterations in the incremental response of two hepatic enzymes, malic enzyme (ME) and a-glycerol phosphate dehydrogenase (a-GPD) to a given dose of T3 (8). Starvation and hypothyroidism are known to impair protein synthesis (13)(14)(15) and to result in a decrease in hepatic protein and RNA content (16,17).

“…Fasting, on the other hand, reduces resting oxygen consumption slightly in the rat, -7% below prefasting control values in normal animals within 3 d and 15% within 5 d (30). Because this fall in metabolic rate was observed in hypothyroid animals and was associated with diminished metabolic responsiveness to exogenous triiodothyronine, the hypometabolic effect of fasting in the rat cannot be secondary to fasting-induced changes in thyroid hormones.…”

mentioning

confidence: 90%

Effect of Diet and Cold Exposure on Norepinephrine Turnover in Brown Adipose Tissue of the Rat

Young¹,

Saville²,

Rothwell³

et al. 1982

336

153

A B S T R A C T Brown adipose tissue (BAT)is an important site of adaptive changes in thermogenesis in the rat. The sympathetic nervous system, which richly supplies BAT, is thought to play an important role in the regulation of BAT thermogenesis because catecholamines stimulate and beta adrenergic blocking agents inhibit oxygen consumption in this tissue. The present studies were carried out to assess directly sympathetic activity in BAT in response to cold exposure and to changes in dietary intake, both of which alter heat production in the rat. Sympathetic activity was determined from the rate of norepinephrine (NE) turnover in interscapular brown adipose tissue (IBAT) after preliminary experiments validated the use of NE turnover techniques in IBAT. Acute exposure to 4°C increased NE turnover in IBAT 4-to 12-fold compared with ambient temperature controls, depending upon the interval over which the turnover measurement was made, while in the heart NE turnover doubled in response to the same cold stimulus. In animals exposed to cold continuously for 10 d before study, NE turnover measurements in IBAT and in the heart were elevated comparably to those obtained during acute exposure. Alterations in feeding were also associated with changes in NE turnover in IBAT. Fasting for 2 d decreased NE turnover in IBAT (-35% from 29.2±4.2 ng NE/h to 18.9±5.9) and in heart (-52%). In animals fed a "cafeteria" diet, a model of voluntary overfeeding in the rat, NE turnover was increased in both IBAT (+108% from 24.8±4.5 ng NE/h to 51.7±6.8) and heart (+66%). Because ganglionic blockade exerted a greater effect on NE turnover in IBAT in cafeteriafed rats than in controls, the increase in NE turnover in IBAT with this overfeeding regimen reflects enhanced central sympathetic outflow. Thus NE turnover techniques can be satisfactorily applied to the