To delineate the physiological effects of aging on basal levels and temporal patterns of neuroendocrine secretions, the 24-h profiles of cortisol, thyroid-stimulating hormone (TSH), melatonin, prolactin, and growth hormone (GH) levels were simultaneously obtained at frequent intervals in eight healthy, active elderly men, age 67-84 yr and in eight young male adults, age 20-27 yr. The study was preceded by an extended period of habituation to laboratory conditions, and sleep was polygraphically recorded. Mean cortisol levels in the elderly were normal, but the amplitude of the circadian rhythm was reduced. Circulating levels of daytime and nighttime levels of both TSH and GH were greatly diminished in old age. In contrast, prolactin and melatonin concentrations were decreased during the nighttime only. The circadian rises of cortisol, TSH, and melatonin occurred 1-1.5 h earlier in elderly subjects, and the distribution of rapid-eye-movement stages during sleep was similarly advanced, suggesting that circadian timekeeping is modified during normal senescence. Despite perturbations of sleep, sleep-related release of GH and prolactin occurred in all elderly men. Age-related decreases in hormonal levels were associated with a decrease in the amplitude, but not the frequency, of secretory pulses. These findings demonstrate that the normal process of aging involves alterations in the central mechanisms controlling the temporal organization of endocrine release in addition to a reduction of secretory outputs.

The effects of adenine nucleotides and adenosine on DNA synthesis and cell growth have been studied in bovine aortic endothelial cells (BAECs). ATP produced a small but significant (+44%) increase of the fraction of BAECs whose nuclei are labeled by [3H]thymidine. This mitogenic effect was mimicked by ADP, the phosphorothioate analogues ATPyS and ADP,BS, and the nonhydrolyzable analogue adenosine 5'-(,6,y-imido)triphosphate (APPNP), whereas adenosine 5'-(a3,-methylene)triphosphate (APCPP), a selective agonist of P2x-purinoceptors, had no effect at 10 ,uM and a small one at 100 ,iM; this profile is consistent with the involvement of P2y-receptors. Adenosine induced a mitogenic response of a magnitude similar to that of ATP. This effect was not reproduced by R-phenylisopropyl adenosine, by 5'-Nethylcarboxamide adenosine, or by 2',5'-dideoxyadenosine, selective ligands of the A1-and A2-receptors and the P site, respectively, nor was it inhibited by 8-phenyltheophylline, an antagonist of both A1-and A2-receptors. The mechanism of this adenosine action thus remains unclear. ATP and ATPyS did not enhance the proliferation of BAECs cultured in the presence of fetal calf serum concentrations ranging from 0.5% to 10%. They inhibited the growthpromoting effect of basic fibroblast growth factor; among the various nucleotides tested, APCPP was the least effective to reproduce the action of ATP, suggesting the possible involvement of P2y-receptors. In conclusion, the action of ATP on the proliferation of BAECs is complex: an increase in the fraction of cells synthesizing DNA, no effect on the cell proliferation in the presence of serum, and inhibition of the growth-promoting effect of basic fibroblast growth factor. (Circulation Research 1992;70:82-90) A TP induces a rapid release of endotheliumderived relaxing factor (nitric oxide) and prostacyclin (prostaglandin 12) from vascular endothelial cells.' These actions, which are mediated by P2y-receptors, are obtained in a range of ATP concentrations (1-100 ,uM) comparable to those obtained in plasma after platelet degranulation. They might thus play an important role in the interaction between platelets and the endothelium.' In endothelial cells, as in other cells, P2y-receptors are coupled to a phospholipase C that hydrolyzes phosphatidylinositol bisphosphate.12 As a consequence of this initial event, ATP induces in aortic endothelial cells several biochemical responses that are often associated with a mitogenic action: enhanced phosphorylation of 27-kd proteins,3-5 stimulation of phosphati-

ADP (0.2-200 microM) stimulated the synthesis of prostacyclin (PGI2), as reflected by the release of 6-keto-prostaglandin F1 alpha (6-K-PGF1 alpha), in endothelial cells cultured from bovine aorta. This effect of ADP was mimicked by ATP, whereas AMP and adenosine were completely inactive. The release of 6-K-PGF1 alpha triggered by ADP was rapid and onset (within 5 min), transient (10 min) and followed by a period of refractoriness to a new ADP challenge. Growing and confluent cells were equally responsive to ADP. ADP stimulated the release of free arachidonic acid from the endothelial cells. ADP could thus exert two opposite actions on platelet aggregation in vivo: a direct stimulation and an inhibition mediated by PGI2. This last action might contribute to limit thrombus formation to areas of endothelial cell damage.

To delineate the effects of aging on basal and stimulated TSH secretion, we studied the 24-h profile of plasma TSH levels and the TSH response to TRH stimulation (200 micrograms TRH, iv) in eight healthy elderly men, aged 67-84 yr, and eight normal young men, aged 20-27 yr. Subjects with thyroid antibodies against microsomal or thyroglobulin antigens were excluded. During the 24-h study, blood was sampled at 15-min intervals. TSH levels were measured by an ultrasensitive immunoradiometric assay. Sleep was polygraphically monitored, and circadian and pulsatile TSH variations were quantified using specifically designed computer algorithms. In older men, the 24-h mean TSH concentration was approximately 50% lower than that in young men (0.78 +/- 0.37 vs. 1.43 +/- 0.41 microU/mL; P less than 0.01), but basal T3 levels were only slightly lower (93 +/- 12 vs. 115 +/- 16 ng/dL; P less than 0.02), while basal T4 levels were normal. The normal diurnal variation of TSH levels, with a nocturnal acrophase and an afternoon nadir, as well as the pulsatile nature of TSH release were preserved in elderly men. When expressed in microunits per mL, the amplitude of these temporal variations was reduced in elderly men compared to that in younger subjects. However, when expressed in relation to the mean TSH levels, the amplitudes of diurnal and pulsatile variations were similar in both groups of subjects. TRH-induced TSH secretion was lower in old than in young men (area under the curve, 15.9 +/- 6.3 microU/mL.10 min in elderly men vs. 42.0 +/- 16.6 microU/mL.10 min in young men; P less than 0.002). However, the TRH-induced elevations of T3 and T4 were of similar magnitude in both groups. These results indicate that in healthy elderly men, the overall 24-h TSH secretion is decreased, and the pituitary is less responsive to stimulation by TRH. However, the chronobiological modulation is preserved. These alterations could reflect an adaptative mechanism to the reduced need for thyroid hormones in old age. The thyroid keeps an intact capacity to respond to acute increases in TSH concentrations.

No abstract