Influence of age and cold stress on plasma catecholamine levels in rats.

We measured arteria plasma concentrations of norepinephrine (NE) and epinephrine (E) in pentobarbital-anesthetized 3- and 24-month-old male F344 rats in basal (control) conditions and following acute cold exposure (6 h at 4 degrees C). Basal levels of circulating NE and E were also determined in 3- and 24-month-old Sprague-Dawley animals. Basal NE did not change between 3 and 24 months of age in either strain of rat, whereas older animals of both strains had significantly higher basal plasma E concentrations compared to younger counterparts. Cold exposure increased plasma NE approximately 100 pg . ml-1 above respective basal levels in 3- and 24-month-old F344 rats, suggesting no age-related differences in sympathetic nervous system reactivity to cold stress. Plasma E in young cold-stressed F344 animals was elevated approximately 336 pg . ml-1 (287%) over basal levels, and approximately 370 pg . ml-1 (155%) over basal levels in older animals, resulting in cold-induced circulating E concentrations of 515 +/- 90 pg . ml-1, and 1040 +/- 122 pg . ml-1 in 3- and 24-month-old animals, respectively. Thus, arterial plasma E concentration in older rats is significantly elevated, both in basal conditions and in response to acute cold stress, suggesting enhanced adrenal medullary activity with advancing age.