Effects of age, gender, and senescence on beta-adrenergic responses of isolated F344 rat brown adipocytes in vitro.

We previously reported greater age-related attenuation of cold-induced thermoregulation and brown adipose tissue thermogenic capacity in male vs. female F344 rats. With onset of the rapid weight loss that occurs near the end of the lifespan, this age-related attenuation becomes severe. We refer to this "end-of-life" physiological state of older rats as senescence. Here, we measured oxygen consumption of isolated brown adipocytes and found no age (6 vs. 12 vs. 26 mo) or gender effects on maximal norepinephrine (NE)- or CL-316,243 (beta 3-adrenergic agonist)-induced responses. In contrast, brown adipocytes from 22- to 26-mo-old senescent rats (males and females) consumed 51-60% less oxygen during maximal stimulation with NE and CL-316,243 than did cells from 26-mo-old presenescent rats. This attenuation was associated with lower (65-72%) uncoupling protein 1 concentrations but no alterations in NE-induced cAMP levels or lipolysis. Our data indicate that senescence, but not chronological age, significantly impacts NE-/beta 3-mediated thermogenesis of isolated brown adipocytes and that this effect involves altered mitochondrial rather than altered membrane or cytosol events.

Tyrosine hydroxylase-containing neurons in the spinal cord of the chicken. I. Development and analysis of catecholamine synthesis capabilities.

1. The development of tyrosine hydroxylase-immunoreactive (TH-IR) neurons was examined in the spinal cord of the chick embryo and hatchling. 2. Two groups of TH-IR cells are described, both of which appear to reach their full complement in number relatively late in embryonic development. One group is comprised of numerous cells located ventral to the central canal which make direct contact with the lumen of the canal. The other group consists of large multipolar neurons that reside in the dorsal horn, more commonly along the outer margin of the gray matter within lamina I and II, and less frequently deeper in the dorsal horn within medial portions of laminae V, VI or VII. 3. TH-IR cells ventral to the central canal in the chick are comparable in location to dopamine (DA)-containing spinal cord cells in lower vertebrate species. In contrast, the dorsally-suited TH-IR cells in the chick are known only to occur in similar positions in higher vertebrates. Therefore, the chick is novel in that the presence of both groups of TH-IR cells appearing together in significant numbers within the spinal cord has not been shown in any other species studied to date. 4. The TH-containing cells in the chick cord do not appear to contain the catecholamine biosynthesis enzymes, DBH or PNMT. Moreover, using anti-DA immunocytochemistry, neither group of TH-IR cells demonstrated detectable levels of DA in control animals nor in animals pretreated with inhibitors of MAO (MAO-I). 5. However, a difference was noted though between the two TH-IR cell groups in terms of their responses to exogenously supplied L-DOPA, the immediate precursor to DA. With the administration of L-DOPA and a MAO-I to chick hatchlings, cells in the region ventral to the central canal stained intensely for DA. In contrast, the same treatment failed to produce DA-immunoreactive cells in the dorsal horn. 6. One reasonable hypothesis for these results is that the TH-IR cells ventral to the central canal contain an active form of AADC, the enzyme that converts L-DOPA to DA. With this interpretation, if these cells can produce DA from L-DOPA, yet do not appear to synthesize DA endogenously, it would appear that the TH enzyme contained in these cells occurs in an inactive form. Whether the TH enzyme in the dorsally located immunoreactive cells is also inactive is uncertain since it remains unclear whether they contain AADC.

Effects of age and gender on brown fat and skeletal muscle metabolic responses to cold in F344 rats.

Older male Fischer 344 (F344) rats do not maintain core temperature as well as do older females during cold exposure. To elucidate factors contributing to the decreased thermoregulatory ability of older males, the metabolic potentials of interscapular brown adipose tissue (IBAT) and skeletal muscle were evaluated at rest (26 degrees C) and during 4 h of cold (6 degrees C) in male and female F344 rats, aged 6, 12, and 26 mo. Compared with 26-mo-old females, cold-exposed 26-mo-old males exhibited a greater drop in core temperature and lower amounts of IBAT mitochondrial uncoupling protein (UCP) and IBAT thyroxine 5'-deiodinase (T5'D) activity. Unlike females, 26-mo-old males showed no cold-induced increase in total IBAT UCP or T5'D activity. In contrast, plasma norepinephrine was higher in cold-exposed 26-mo-old males vs. females, whereas plasma insulin and thyroxine did not differ with gender. Skeletal muscle oxidative capacity (measured by citrate synthase activity) and carbohydrate availability (measured by muscle glycogen and plasma glucose levels) did not differ between the 26-mo-old males and females. Our data suggest that altered regulation of IBAT UCP levels during cold exposure of aged rats, due at least in part to attenuated cold-induced IBAT T5'D activity, contributes to the gender difference in thermoregulatory ability of older males vs. females.