Age-related changes of myocardial norepinephrine transporter density in rats: implications for differential cardiac accumulation of MIBG in aging.

The myocardial regional distribution of [(125)I]MIBG was examined in various aged (7-, 18-, 42-, 47-, and 65-week-old) rats and compared with the effects of regional myocardial blood flow and norepinephrine transporter (NET) function on regional [(125)I]MIBG accumulation in aged rats. In 7- and 18-week-old rats, the accumulation of [(125)I]MIBG was higher in the inferior wall than anterior wall. However, in more than 42-week-old rats, the uptake of MIBG was lower in the inferior wall than that the anterior wall. The uptake of [(99m)Tc]MIBI was greater in the inferior wall than the anterior wall in 18-week-old rats, but was reduced in the inferior wall compared to the anterior wall in 42- and 47- week-old rats. Furthermore, the in vitro binding studies of [(3)H]desipramine to cardiac membranes showed that the B(max) value of NET was larger for the inferior wall than the anterior wall in 7-week-old rats, but was smaller for the inferior wall than the anterior wall in 47-week-old rats. The K(D) values for both walls were significantly larger in 47-week-old than 7-week-old rats. These results indicated that myocardial MIBG accumulation was lower in the inferior wall than the anterior wall of older rats, and that this differential MIBG accumulation in aging was related to the regional changes in myocardial blood flow and NET functions in the inferior wall.