CARDIAC I-123 METAIODOBENZYLGUANIDINE UPTAKE IN ANIMALS WITH DIABETES-MELLITUS AND OR HYPERTENSION/

The aim of the present study was to evaluate the use of the noradrenal ine analogue iodine-123 metaiodobenzylguanidine ([I-123]MIBG) for the assessment of cardiac sympathetic activity in the presence of diabetes mellitus and/or hypertension in animal models. One model used Wistar- Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) rendered di abetic at 12 weeks of age by an intravenous injection of streptozotoci n (STZ). The other model used lean and obese Zucker rats. In all group s basic haemodynamic values were established and animals received an i ntravenous injection of 50 mu Ci [I-123]MIBG. Initial myocardial uptak e and wash-out rates of [I-123]MIBG were measured scintigraphically du ring 4 h. After sacrifice, plasma noradrenaline and left cardiac ventr icular beta-adrenoceptor density was determined. The diabetic state, b oth in STZ-treated rats (direct induction) and in obese Zucker rats (g enetic induction), appeared to induce a lower cardiac density of beta- adrenoceptors, indicative of increased sympathetic activity, Cardiac [ I-123]MIBG then showed increased washouts, thereby confirming enhanced noradrenergic activity. This parallism of results led to the conclusi on that [I-123]MIBG wash-out measurements could provide an excellent t ool to assess cardiac sympathetic activity noninvasively. However, in hypertension (WKY vs SHR), both parameters failed to show parallelism: no changes in beta-adrenoceptor density were found, whereas [I-123]MI BG wash-out rate was increased. Thus, either [I-123]MIBG washout or be ta-adrenoceptor density may not be a reliable parameter under all circ umstances to detect changes in the release of noradrenaline. Changes i n the initial uptake of [I-123]MIBG were observed as well. This may be a good marker for the disappearance of cardiac innervation, but it se ems not to be a good parameter for distinguishing between loss of symp athetic innervation and enhanced uptake of noradrenaline in pathologic al conditions.