Nocturnal increase in plasma cGMP levels in humans

The circadian dynamics of responses to cyclic guanosine 3',5'-monophosphate (cGMP) in in vitro experiments and the stimulating effects of the pineal hormone melatonin on cGMP levels both in vitro and in vivo provoked an investigation into the diurnal pattern of occurrence of this second messenger in human plasma and its correlation with plasma melatonin levels. Plasma cGMP levels were measured in 9 normal human subjects who were over 50 years of age. Samples were obtained hourly through a 20-h period (11 a.m. to 7 a.m.) that included the subjects' habitual hours of nocturnal sleep; physical activity was kept to a minimum during the daylight hours. The area under the time-plasma cGMP concentration curve showed a significant increase during the period of nocturnal sleep compared to that observed during the period of daytime wakefulness. The individual temporal pattern of the nocturnal rise in plasma cGMP differed among the subjects; however, the initial increase typically was observed soon after bedtime. No significant correlation was observed between individual nocturnal plasma melatonin levels and cGMP levels.