Alterations of myocardial sympathetic innervation in response to hypoxia

The effects of altitude hypoxia on myocardial sympathetic nerve function were assessed in rats using metaiodobenzylguanidine (MIBG). Methods: To estimate the change in uptake-1 function induced by hypoxia, three sets of rats were submitted to 5-, 7- and 21-day hypoxia (hypobaric chamber at 410 Torr) and one set of control rats was injected with 25 mu Ci of I-123-MIBG. Four hours later, the rats were killed and I-123 activity was counted in both ventricles. The proportion of MIBG fixed in the myocardium through the norepinephrine (NE) transporter (uptake-1) was evaluated indirectly in 5-day hypoxic and controls rats by the injection of desipramine before I-123-MIBG administration. Myocardial perfusion was evaluated in 5-day hypoxic rats and controls by Tl-201 injection. Results: Myocardial I-123-MIBG activity was 0.253% +/- 0.036% kg dose/g(-1) in controls and was decreased (0.188% +/- 0.029% kg dose/g(-1), p = 0.001) in 5-day hypoxic rats. This decrease was not related to a change in cardiac perfusion. The decrease in MIBG uptake existed before the appearance of cardiac hypertrophy. Desipramine decreased MIBG uptake by 48% in controls and 17% in hypoxic rats, suggesting that the decrease predominantly affected MIBG uptake by the NE transporter. Conclusion: Chronic hypoxia leads to a decrease in myocardial NE uptake-1 function. This finding suggests that altered tissue oxygen supply could play a role in the decreased cardiac MIBG uptake reported in human cardiomyopathies

Preparation and pharmacological characterization of [76Br]-meta-bromobenzylguanidine ([76Br]MBBG)

[76Br]-meta-Bromobenzylguanidine ([76Br]MBBG) was prepared from the iodinated analog (MIBG) and [76Br]NH4 using a Cu+-assisted halogen exchange reaction. [76Br]MBBG was produced in a 60–65% radiochemical yield with a specific activity of 20 MBq/nmol. In rats, biodistribution kinetic studies showed a high uptake of [76Br]MBBG in heart tissues with its maximum of 5% ID/g at 2 h p.i.; whereas 4 h p.i., the maximum of the heart-to-lung concentration ratio of 8 was observed. Metabolic studies in rats indicated that [76Br]MBBG was rapidly metabolized in plasma. However in heart tissue, 25 h p.i., 85% of the radioactivity still represented unchanged radiotracer. Pharmacological studies in rats showed that the myocardial uptake of [76Br]MBBG was similar to that of norepinephrine. After pretreatment of the rats, the uptake of [76Br]MBBG was reduced 4 h p.i. to the following values: after desipramine (DMI) to 37%, after dexamethasone (DXM) to 88% and after 6-hydroxydopamine (6-OHDA) to 16%. These preliminary results suggest that [76Br]MBBG can be useful for the assessment of heart catecholamine reuptake disorders with PET