Age-related changes in norepinephrine release and its modulation by presynaptic alpha-2 adrenoceptors in the rat vas deferens.

Our objective was to extend the study of age-related changes in the modulation of norepinephrine release by presynaptic alpha-2 adrenoceptors. The experimental model utilized was the epididymal and prostatic portion of the rat vas deferens, because the two portions differ in age-related changes and pattern of sympathetic neurotransmission. The functional influence of presynaptic alpha-2 adrenoceptors was evaluated by quantifying the potentiating effect of yohimbine on the contraction induced by transmural stimulation. Yohimbine-induced potentiation was much more pronounced in the epididymal portion of young rats and was reduced by aging only in this portion. Potassium-induced release of 3H-norepinephrine was significantly higher in the epididymal than in the prostatic portion, and an age-related reduction in 3H-norepinephrine release occurred only in the epididymal portion. However, aging did not change the inhibitory effect of clonidine on the contraction induced by transmural stimulation (0.2 Hz or single pulse) in either portion. Thus the age-related decline in alpha-2 feedback regulation is probably not directly related to changes in prejunctional alpha-2 adrenoceptor activity but may be associated with age-related changes in the amount of norepinephrine that stimulates this receptor. However, the lower overflow of neurotransmitter in the epididymal portion did not translate into age-related changes in the contractile responses to transmural stimulation (2-20 Hz). In the prostatic portion, aging increased the functional neurotransmission, partially because of a reduction in neuronal norepinephrine uptake. The results suggest that age-related changes are specific for each portion and may be associated with the pattern of neurotransmission of the tissue.