Effect of age on substance P levels in the rat mesenteric artery and vein.

The content of substance P in the vasculature of Fischer 344 rats 6 to 27 months of age was determined by radioimmunoassay. Substance P could not be measured in the subclavian vein, femoral artery and vein or cerebral arteries of 6 month old rats. Levels of substance P in the mesenteric artery and vein of 6 month old rats averaged 4.8 +/- 1.0 and 8.3 +/- 1.8 pmole/g wet weight (n = 6-7), respectively. At 20 months of age, substance P levels in the vein were significantly increased. At 27 months, levels of substance P in both mesenteric artery and vein were increased to almost twice the values found in younger animals. This increased substance P content could reflect increased nerve density, or, more likely, an increased substance P content in each nerve ending. In the latter case, it is not possible to distinguish between increased content due to decreased nerve activity or increased content which would result in increased amounts of substance P released with each nerve impulse.