Influence of aging on tissue-specific noradrenergic activity at rest and during nonexertional heating in rats

The purpose of this study was to test the hypothesis that sympathetic nervous system (SNS) activity to internal organs during nonexertional heat stress is exaggerated with advancing age. Norepinephrine (NE) synthesis was blocked with alpha-methyl-DL-p-tyrosine, and NE turnover rates were determined in the left ventricle, renal cortex, liver, and adrenal gland in groups of conscious unrestrained mature (12-mo-old) and senescent (24-mo-old) Fischer 344 rats before and after whole body heating. Animals were maintained in thermoneutral control conditions (n = 10 per group) or exposed to an ambient temperature of 42 degrees C until colonic temperature (TCO) reached a designated level (39.5 or 41.0 degrees C; n = 10 per group at both temperature levels). NE turnover rates were similar in all four tissues sampled for both mature and senescent rats in the resting condition. Over the course of the heating period, progressive increases in arterial blood pressure and heart rate were observed, with the magnitudes of the pressor and tachycardic responses similar between age groups. At TCO of 39.5 degrees C, calculated NE turnover rates were unchanged from control levels for the senescent and mature animals, whereas animals heated to 41.0 degrees C demonstrated marked elevations in NE turnover in all four tissues for both age groups. In addition, the increase in NE turnover rate at this TCO was significantly higher for senescent rats compared with their younger counterparts in the renal cortex (410 vs. 278%) and liver (557 vs. 170%).(ABSTRACT TRUNCATED AT 250 WORDS)