No abstract

Our previously published studies with monkeys describe a characteristic nocturnal haemodynamic pattern consisting of a monotonic decline in cardiac output and central venous pressure, and a concomitant rise in total peripheral resistance. These findings led us to hypothesize that there is a reduction in total plasma volume during the night. Since a fall in plasma volume should cause an increase in haemoglobin and plasma protein concentration, we designed this experiment to test the hypothesis that haematocrit levels would be significantly greater in the morning than in the evening. In a study of five monkeys, the levels of haematocrit were measured at 1700 h and at 0900 h the next morning for 14 days. The average morning haematocrit levels were 6.5% higher than the average evening values. Lower plasma volume and possibly greater blood viscosity could contribute to the pathophysiology of the morning increase in the incidence of 'silent' ischaemia and catastrophic vascular events such as sudden cardiac death, myocardial infarctions and strokes in man.

Aged, male C57BL/6J mice produce less heat than adults during cold, which may lead to a reduced ability to maintain core temperature, but the underlying mechanisms of the decreased heat production in aged mice are still unclear. We measured sympathetic nervous activity (SNA) to interscapular brown adipose tissue (IBAT) in aged mice and compared this activity with that of adults. Mice were anesthetized by urethane and isoflurane, and SNA was recorded from one of the fine nerves to IBAT. The animal's body caudal to the pelvic area was covered with a plastic bag containing iced-water to decrease colonic temperature 7 degrees C below control over 20 minutes. SNA to IBAT (IBAT-SNA) increased during cold in both groups, but aged mice had higher IBAT-SNA before and during cold. These findings indicate that neither the ability of aged mice to detect cold nor their ability to generate sympathetic outflow to BAT is deficient.