Age-related changes in the sensitivity of sympathetic nerves to altered extracellular calcium in tail arteries of F-344 rats.

There is a significant age-related increase in stimulation evoked norepinephrine release from adrenergic nerves in tail arteries of F-344 rats, that is not accounted for by a change in function of prejunctional alpha 2-adrenergic receptors. To further explore the mechanism of this age-related change in transmitter release, we investigated the effect of altered extracellular calcium. With short stimulation trains the effects of altered extracellular calcium were significantly greater in 20-month-old tail arteries compared to 6 months. At the highest calcium concentration (7.5 mM) when alpha 2-adrenergic receptors were active or blocked, there was a significant decline in fractional norepinephrine release only in arteries of 20-month-old animals. With long stimulation trains the effects of altered extracellular calcium were not as pronounced as with short trains, but the effect of calcium was still significantly greater in 20 month old tail arteries. With alpha 2-adrenergic receptors blocked 6 month tail arteries were insensitive to altered calcium while 20-month tail arteries remained sensitive. One explanation for these findings may be an age-related change in the efficiency of intracellular calcium buffering mechanisms leading to greater calcium transients in the nerves of older animals.