SYMPATHETIC TONE ASSESSED BY WASHOUT OF IODINE 125-LABELED METAIODOBENZYLGUANIDINE FROM THE MURINE LEFT-VENTRICLE - INFLUENCE OF IMMOBILIZATION STRESS AND INHIBITION OF THE RENIN-ANGIOTENSIN SYSTEM

Background. Because it is not metabolized as is norepinephrine (NE), m ost of the metaiodobenzylguanidine (MIBG) taken up by the heart is con sidered to be lost subsequently by release concomitant,vith sympatheti c stimulation, Therefore we examined whether the washout of MIBG is in fluenced by sympathetic tone, which we modulated by using immobilizati on stress or activation of the renin-angiotensin system (RAS). Methods and Results. In 175 male ddY mice, left ventricular radioactivity was counted 30 minutes or 4 hours after injection of 74 kBq (2 mu Ci) of iodine 125- or iodine 131-labeled MIBG (I-125- or I-131-MIBG). The was hout rates of MIBG were determined under immobilizatiou stress or unde r sodium loading or restriction in combination with losartan (10 mg/kg ) or cilazapril (1 mg/kg) pretreatment, Immobilization enhanced the wa shout of I-121-MIBG (80.9% vs 57.9% in the control animals); this was determined to be related to washout from the neuronal compartment, bec ause the nonneuronal component assessed through desipramine pretreatme nt was not affected, Pretreatment with losartan or cilazapril decrease d the facilitation of I-125-MIBG washout in sodium-restricted mice (40 .9% and 33.7%, respectively, vs 43.5% in the control animals), but not in sodium-loaded mice, Conclusions. Measurement of MIBG washout may b e feasible for determining the changes in sympathetic tone caused by i mmobilization stress or activation of the RAS.