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Distribution of Thallium-201 Injected into Rats Following Stress: Imaging, Organ to Plasma Uptake Ratios, and Myocardial Kinetics

Wood Veterans Administration Center, Marquette University, and Medical College of Wisconsin, Milwaukee, Wisconsin

Correspondence: For reprints contact: Joseph G. Llaurado, Nuclear Medicine Service/115, VA Ctr., Wood, WI 53193.

ABSTRACT

Male rats were stressed by forced swimming for 2 hr, after which 0.2 mCi Tl-201 were given i.v. to each rat. Some animals were imaged in vivo; when a pinhole collimator was used, the heart, liver, and kidneys could be distinguished. Animals were killed 30 min or 3 hr after injection for tissue sampling and in in vitro study of myocardial kinetic. Results for tissue sampling (kidneys, heart, adrenals, lungs, aorta, liver, spleen, thymus, seminal vesicles, muscle, testes, brain, fat, erythrocytes, and fur) are expressed as the ratio of activity in organ to that in plasma. Kidneys and heart showed the largest uptake; considerable uptake was also found in adrenals. At 3 hr kidneys, heart, and adrenals already showed a diminution of uptake as compared to 30 min. At 3 hr, muscle, brain, and fur showed a marked increase in uptake; testes and fat exhibited a dramatic increase. It appears that organ uptake is greater in stressed than in unstressed animals. In the myocardium study a three-compartment model (extracellular, intracellular, and subcellular) was found to describe adequately the kinetics of Tl-201. Transport rate constants (k_ij) and relative compartment sizes for Tl-201 distribution were determined by a continuous outflow method. In the intracellular compartments 2 and 3, each inflow k_ij was larger than the corresponding k_ji leaving from the same compartment, thus indicating that Tl-201 movement is directed predominantly into cellular structures. Values for k_ij furnish a norm that may be used comparatively to study quantitatively the kinetics of different tracers. The ratio of compartment sizes indicated that more than 70% of Tl-201 is in the intracellular compartments. For these kinetic studies there was no statistically significant difference between results obtained at 30 min and 3 hr after Tl-201 injection.