Regional alterations of myocardial norepinephrine transporter density in streptozotocin-induced diabetic rats: implications for heterogeneous cardiac accumulation of MIBG in diabetes

Cardiac scintigraphic studies using iodine-123 labeled metaiodobenzylguanidine ([¹²³I]MIBG) have previously demonstrated the heterogeneous myocardial accumulation of radioactivity in diabetes. In this study, we investigated the myocardial regional distribution of [¹²⁵I]MIBG and the effects of regional myocardial blood flow, myocardial norepinephrine (NE) content, and norepinephrine transporter (NET) function on regional [¹²⁵I]MIBG accumulation in streptozotocin-induced diabetic (STZ-D) rats. Dual-isotope autoradiographic studies using [¹²⁵I]MIBG and technetium-99m labeled hexakis (2-methoxy-2-isobutylisonitrile) (^99mTc-MIBI), a tracer for the measurement of myocardial blood flow, were carried out to investigate the changes in regional myocardial blood flow in STZ-D rats. Uptake of [¹²⁵I]MIBG was similar between the anterior wall and the inferior wall in control rats. On the other hand, in STZ-D rats, uptake of [¹²⁵I]MIBG in the inferior wall was significantly less than that in the anterior wall. Uptake of ^99mTc-MIBI was not significantly different between the anterior and inferior walls in control or STZ-D rats, indicating that myocardial blood flow did not change regionally in either control or STZ-D rats, and that the blood flow was not responsible for the heterogeneity of the distribution of [¹²⁵I]MIBG in STZ-D rats. In STZ-D rats, cardiac NE concentrations determined using an HPLC-electrochemical detection (ECD) system were significantly increased in both the anterior and the inferior wall, although there was no significant difference in NE concentration between the anterior and inferior walls in control or STZ-D rats. Furthermore, the density and affinity of NET were investigated by studying the binding of [³H]desipramine to cardiac membranes. The B_max values of the NET in the anterior wall were not significantly different between control and STZ-D rats, but the B_max value of the NET in the inferior wall was significantly lower in STZ-D rats than in controls. In conclusion, myocardial MIBG uptake was reduced in the inferior wall of STZ-D rats compared with control rats; this decrease was correlated with the decrease in NET density, but was not dependent on the regional myocardial blood flow and NE concentration. These results suggest that regional fluctuations in NET levels in the inferior wall contribute to heterogeneous MIBG accumulation in diabetes.