Actions of estrogen on pulsatile, nyctohemeral, and entropic modes of growth hormone secretion.
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The neuroendocrine mechanisms by which estradiol drives growth hormone (GH) secretion in the human are poorly defined. Here we investigate estrogen's specific regulation of the 24-h pulsatile, nyctohemeral, and entropic modes of GH secretion in healthy postmenopausal women. Volunteers (n = 9) received randomly ordered placebo versus estradiol-17beta (1 mg micronized steroid twice daily orally) treatment for 7-10 days and underwent blood sampling at 10-min intervals for 24 h to capture GH release profiles quantitated in a high-sensitivity chemiluminescence assay. Pulsatile GH secretion was appraised via deconvolution analysis, nyctohemeral GH rhythms by cosinor analysis, and the orderliness of GH release patterns via the approximate entropy statistic. Mean (+/-SE) 24-h serum GH concentrations approximately doubled on estrogen treatment (viz., from 0.31 +/- 0.03 to 0.51 +/- 0.07 microgram/l; P = 0.033). Concomitantly, serum insulin-like growth factor-I (IGF-I), luteinizing hormone, and follicle-stimulating hormone concentrations fell, whereas thyroid-stimulating hormone and prolactin levels rose (P < 0.01). The specific neuroendocrine action of estradiol included 1) a twofold amplified mass of GH secreted per burst, with no significant changes in basal GH release, half-life, pulse frequency, or duration; 2) an augmented amplitude and mesor of the 24-h rhythm in GH release, with no alteration in acrophase; and 3) greater disorderliness of GH release (higher approximate entropy). These distinctive and dynamic reactions to estrogen are consistent with partial withdrawal of IGF-I's negative feedback and/or accentuated central drive to GH secretion. (+info)
Use of orchiectomy and testosterone replacement to explore meal number-to-meal size relationship in male rats.
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Because food intake is a function of meal number and meal size and because gender-related hormones are involved in feeding regulation, we explored effects of orchiectomy and testosterone replacement on the relationship between meal number and size and changes in resulting feeding patterns in adult male rats, randomized into orchiectomy and sham-operation groups. A rat eater meter measured feeding indexes for 1 wk before and 2 wk after castration and during 8 days of testosterone replacement. Orchiectomy leads to an immediate change in the meal number-to-size relationship, resulting in 1) change in pattern of feeding; 2) a significant decrease in dark-phase meal number; 3) a significant increase in dark-phase meal size, but insufficient to offset decrease in meal number, so total food intake significantly decreased during dark phase; 4) no significant change in light-phase meal number; and 5) an increase in meal size leading to an increased food intake during light phase, which offset decreased food intake in dark cycle and resulted in no net significant change in food intake after orchiectomy. Testosterone replacement acutely reversed effects of orchiectomy on meal number-to-meal size relationship, restoring feeding pattern. Data suggest that androgens immediately influence the meal number-to-meal size relationship. The speed of onset seen after orchiectomy suggests that the influence of testosterone on food intake may also occur partially via a nongenomic effect. (+info)
Light-induced uncoupling of multioscillatory circadian system in a diurnal rodent, Asian chipmunk.
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Responses of the circadian locomotor rhythm to a single light pulse were examined in a diurnal rodent, Asian chipmunk, by exposing it to a 1-h light pulse of 2,000 lx under constant conditions. A light pulse given at the beginning and end of the subjective night produced a phase delay and advance shifts, respectively. When pulsed around the midpoint of the subjective night, the circadian rhythm was shifted as much as 12 h in most animals or became arrhythmic in some. In the latter case, an additional light pulse restored the circadian rhythm. Some animals were unresponsive to light. The phase response curve is categorized as type 0. A large phase-shift was sometimes followed by splitting of an activity band into two components. These results are best explained by an assumption that the chipmunk circadian system is composed of two mutually coupled major oscillators, each of which is constituted by multiple oscillators. Our results suggest that light affects the oscillatory coupling not only of the major oscillators but also of constitutional oscillators. (+info)
Endogenous thermoregulatory rhythms of squirrel monkeys in thermoneutrality and cold.
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Whole body heat production (HP) and heat loss (HL) were examined to determine if the free-running circadian rhythm in body temperature (Tb) results from coordinated changes in HP and HL rhythms in thermoneutrality (27 degrees C) as well as mild cold (17 degrees C). Squirrel monkey metabolism (n = 6) was monitored by both indirect and direct calorimetry, with telemetered measurement of Tb and activity. Feeding was also measured. Rhythms of HP, HL, and conductance were tightly coupled with the circadian Tb rhythm at both ambient temperatures (TA). At 17 degrees C, increased HP compensated for higher HL at all phases of the Tb rhythm, resulting in only minor changes to Tb. Parallel compensatory changes of HP and HL were seen at all rhythm phases at both TA. Similar time courses of Tb, HP, and HL in their respective rhythms and the relative stability of Tb during both active and rest periods suggest action of the circadian timing system on Tb set point. (+info)
Effects of feeding frequency and voluntary salt intake on fluid and electrolyte regulation in athletic horses.
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The effect of feeding frequency and voluntary sodium intake (VSI) on fluid shifts and plasma aldosterone concentration (PAC) were studied at rest and after exercise in six athletic horses. The horses were fed twice a day (2TD) and six times a day (6TD) for 25 days for each protocol, according to a changeover design. VSI was measured by weighing each horse's salt block daily. Feeding 2TD or 6TD caused no major alterations in fluid shifts, but in the 2TD treatment there was a postprandial increase in plasma protein concentration and osmolality that lasted <1 h. PAC and VSI were not affected by feeding frequency. VSI ranged from 0 to 62 mg x kg body weight-1 x day-1 and caused significant alterations in PAC. At VSI <26 mg x kg body weight-1 x day-1, a diurnal rhythm for PAC was noted. Water intake, fecal concentrations of sodium and potassium, and packed cell volume during exercise were influenced by VSI. The response to exercise did not differ between treatments. In conclusion, VSI, but not feeding frequency, has significant effects on fluid and electrolyte regulation in athletic horses. (+info)
An extraretinally expressed insect cryptochrome with similarity to the blue light photoreceptors of mammals and plants.
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Photic entrainment of insect circadian rhythms can occur through either extraretinal (brain) or retinal photoreceptors, which mediate sensitivity to blue light or longer wavelengths, respectively. Although visual transduction processes are well understood in the insect retina, almost nothing is known about the extraretinal blue light photoreceptor of insects. We now have identified and characterized a candidate blue light photoreceptor gene in Drosophila (DCry) that is homologous to the cryptochrome (Cry) genes of mammals and plants. The DCry gene is located in region 91F of the third chromosome, an interval that does not contain other genes required for circadian rhythmicity. The protein encoded by DCry is approximately 50% identical to the CRY1 and CRY2 proteins recently discovered in mammalian species. As expected for an extraretinal photoreceptor mediating circadian entrainment, DCry mRNA is expressed within the adult brain and can be detected within body tissues. Indeed, tissue in situ hybridization demonstrates prominent expression in cells of the lateral brain, which are close to or coincident with the Drosophila clock neurons. Interestingly, DCry mRNA abundance oscillates in a circadian manner in Drosophila head RNA extracts, and the temporal phasing of the rhythm is similar to that documented for the mouse Cry1 mRNA, which is expressed in clock tissues. Finally, we show that changes in DCry gene dosage are associated predictably with alterations of the blue light resetting response for the circadian rhythm of adult locomotor activity. (+info)
Dopamine mediates circadian rhythms of rod-cone dominance in the Japanese quail retina.
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A circadian clock modulates the functional organization of the Japanese quail retina. Under conditions of constant darkness, rods dominate electroretinogram (ERG) b-wave responses at night, and cones dominate them during the day, yielding a circadian rhythm in retinal sensitivity and rod-cone dominance. The activity of tyrosine hydroxylase, the rate-limiting enzyme in dopamine synthesis, also exhibits a circadian rhythm in the retina with approximately threefold higher levels during the day than at night. The rhythm of tyrosine hydroxylase activity is opposite in phase to the circadian activity of tryptophan hydroxylase, the first enzyme in the melatonin biosynthetic pathway. We tested whether dopamine may be related to the physiological rhythms of the retina by examining the actions of pharmacological agents that effect dopamine receptors. We found that blocking dopamine D2 receptors in the retina during the day mimics the nighttime state by increasing the amplitude of the b-wave and shifting the retina to rod dominance. Conversely, activating D2 receptors at night mimics the daytime state by decreasing the amplitude of the b-wave and shifting the retina to cone dominance. A selective antagonist for D1 dopamine receptors has no effect on retinal sensitivity or rod-cone dominance. Reducing retinal dopamine partially abolishes rhythms in sensitivity and yields a rod-dominated retina regardless of the time of day. These results suggest that dopamine, under the control of a circadian oscillator, has a key role in modulating sensitivity and rod-cone dominance in the Japanese quail retina. (+info)
Effect of the number of peak expiratory flow readings per day on the estimation of diurnal variation.
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BACKGROUND: The number of peak expiratory flow (PEF) readings required per day to assess diurnal variation accurately is not known; published studies have used between two and seven PEF readings per day. This study compares the diurnal variation calculated using 2-10 PEF readings per day. METHODS: All days with 10 readings were selected from a database of PEF records. For each day, diurnal variations calculated using 2-9 of the readings available were compared with that calculated using the full 10 PEF readings. Diurnal variation calculated using all 10 readings was taken as the true diurnal variation. When less than 10 readings were used the readings were evenly spaced over waking hours. Diurnal variation was calculated as maximum--minimum/predicted. RESULTS: Two hundred and 25 days with 10 readings per day were selected from PEF records provided by 63 individuals. When only two PEF readings per day were used, the limits of agreement suggested a possible underestimate of true diurnal variation, calculated using all 10 readings, of 1.23-15.10%. The possible underestimate fell to 0.27-3.96% when calculated using four evenly spaced readings. Analysis of the timing of the highest PEF reading of the day was undertaken for rest and work days. This showed a mean (SD) timing of 13:56 (4:56 hours) for rest days and 11:47 (5:59 hours) for work days. CONCLUSIONS: Clinically significant underestimates of true diurnal variation may be seen when only small numbers of PEF readings per day are used in its calculation. At and above four readings the results suggest that the underestimate becomes increasingly insignificant in terms of the diagnosis and treatment of asthma. Analysis of the timing of the highest PEF reading of the day showed a wide variation, precluding the ability to capture the true diurnal variation with just two or three carefully timed PEF readings per day. The authors suggests that at least four readings per day should be performed, evenly spaced during waking hours, to obtain an accurate assessment of diurnal variation in PEF. (+info)