A. Cabassi et al., AGE-RELATED-CHANGES IN INTERSTITIAL NOREPINEPHRINE - A MICRODIALYSIS STUDY IN SPONTANEOUSLY HYPERTENSIVE RATS, American journal of hypertension, 9(9), 1996, pp. 878-883
This study was aimed at evaluating the time course of interstitial nor
epinephrine (NE) concentrations in the white adipose tissue and at ass
essing NE release after local perfusion with tyramine hydrochloride (T
YR) in rats of different ages. Two groups of eight spontaneously hyper
tensive (SHR) and normotensive Wistar-Kyoto (WKY) rats, aged 14 to 16
weeks, were studied. The same animals were reexamined at the age of 52
to 54 weeks. A soft microdialysis probe was implanted subcutaneously
in the parascapular region and was perfused with Ringer solution (flow
rate: 2.0 mu L/min). After an equilibration period, NE levels were mo
nitored for 120 min, following which, TYR (0.1 nmol/min) was perfused
for 90 min. Dialysates from each 30 min collection period were analyze
d by HPLC using electrochemical detection. At 14 to 16 weeks, SHR show
ed higher NE concentrations in dialysates as compared to WKY (1124.0 p
g/mL v 541.4 pg/mL; P < .001) and a blunted response to TYR challenge.
The net output, estimated by subtracting basal values, was 86.0 pg NE
/h in SHR as compared to 212.5 pg NE/h in WKY (P = .005). Differences
in basal NE levels persisted in the same aged groups (P < .001) as wel
l as a blunted response to TYR. The net NE output was still lower in S
HR as compared to WKY (320.4 pg NE/h v 414.7 pg NE/h in WKY; P = .023)
. Basal levels of NE in SHR could be accounted for by either a higher
amount of the neurotransmitter stored into and released from vescicles
or by an increased firing rate of the sympathetic fibers. Since TYR i
s known to deplete axoplasmic but not vesicular NE available for neuro
transmission, the response of SHR to TYR challenge is consistent with
an increased turnover rate of NE. Aging was associated with an increas
ed response to TYR in both strains, thus suggesting an age-dependent d
ecline in turnover rates or changes in NE reuptake mechanisms.