The age-dependent effect of caffeine on the plasma noradrenaline content in white mice. 
The level of noradrenaline (NA) in the plasma of white mice, determined radioenzymatically as described by Henry et al.  changes with age.  In 45 days old animals NA in plasma is 10.3 +/- 4.4 ng/ml, whereas in 220 days old animals it was found to be 22.7 +/- 8.8 ng/ml.  In 45 days old animals NA in plasma is increased 30 min after caffeine (45 microgram/g) significantly to 109% of control level, whereas in the older animals it is raised insignificantly (14%).  The observed NA increase in plasma with increasing age might be a compensatory reaction due to diminished sensibility of the postsynaptic adrenergic receptors to NA.  The explanation for the missing raise in NA plasma level after caffeine in adult mice might be the following: Calcium ions are indispensable for the release of NA by nerve impulses and the stimulation of presynaptic beta-receptors of noradrenergic neurons by NA contributes to facilitations of NA release.  As in older animals under physiological conditions the release of NA is higher than in young animals it can be assumed that in the older ones the NA release from sympathetic neurons stimulated by caffeine cannot be further enhanced since the calcium receptors are fully occupied by released calcium ions and the presynaptic beta-receptors by released NA molecules.