Muscarinic stimulation of tracheal smooth muscle cells activates large-conductance Ca2+-dependent K+ channel

Effects of seasonality and fasting on the plasma leptin and thyroxin levels of the raccoon dog (Nyctereutes procyonoides) and the blue fox (Alopex lagopus)

Plasma leptin and thyroxin concentrations of eleven raccoon dogs and eleven blue foxes were monitored for 6 months. Half of the animals were placed on a 3-week fast in November. Leptin levels were low in summer, but in October they rose significantly. In November, leptin concentrations decreased rapidly within a week although the body mass of the animals remained stable. There were no significant differences between experimental groups for raccoon dogs, but in blue foxes the fasting group had lower leptin levels than the control group. High thyroxin levels in summer decreased as autumn progressed, but thyroxin concentrations of the fasting groups increased at the end of the fast. Leptin levels of the raccoon dog and the blue fox are not determined only by the fat reserves of the animals, but they seem to reflect the autumnal deposition of fat at the onset of winter. Blue foxes have metabolic rates of active animals during the winter and higher leptin levels in December than raccoon dogs. The superficially hibernating raccoon dogs have low leptin levels after the onset of winter perhaps as an adaptation to fasting. J. Exp. Zool. 289:109-118, 2001.

Synchronicity of frequently sampled, 24-h concentrations of circulating leptin, luteinizing hormone, and estradiol in healthy women

Leptin, an adipocyte hormone, is a trophic factor for the reproductive system; however, it is still unknown whether there is a dynamic relation between fluctuations in circulating leptin and hypothalamic-pituitary-ovarian (HPO) axis hormones. To test the hypothesis that fluctuations in plasma leptin concentrations are related to the levels of luteinizing hormone (LH) and estradiol, we sampled plasma from six healthy women every 7 min for 24 h during days 8-11 of the menstrual cycle. Cross-correlation analysis throughout the 24-h cycle revealed a relation between release patterns of leptin and LH, with a lag of 42-84 min but no significant cross-correlation between LH and estradiol. The ultradian fluctuations in leptin levels showed pattern synchrony with those of both LH and estradiol as determined by cross-approximate entropy (cross-ApEn). At night, as leptin levels rose to their peak, the pulsatility profiles of LH changed significantly and became synchronous with those of leptin. LH pulses were fewer, of longer duration, higher amplitude, and larger area than during the day. Moreover, the synchronicity of LH and leptin occurred late at night, at which time estradiol and leptin also exhibited significantly stronger pattern coupling than during the day. We propose that leptin may regulate the minute-to-minute oscillations in the levels of LH and estradiol, and that the nocturnal rise in leptin may determine the change in nocturnal LH profile in the mid-to-late follicular phase that precedes ovulation. This may explain the disruption of hypothalamic-pituitary-ovarian function that is characteristic of states of low leptin release, such as anorexia nervosa and cachexia.