FIGURE 8. ^99mTc-Pertechnetate Uptake Reflects In Vivo Cellular Energetics.

(A) In vivo study design. In vivo dual isotope single-photon emission computed tomography/computed tomography (SPECT/CT) imaging was performed at 1 to 3 h and 24 h following epicardial transplantation of NIS⁺ CDCs encapsulated in hydrogels, following intravenous injection of ^99mTc-pertechnetate to track NIS⁺ CDCs and ²⁰¹Tl-Cl to visualize myocardium. NIS⁺ CDCs were treated with a reversible Akt inhibitor for 1 h prior to washout, dissociation, and encapsulation in hydrogels to demonstrate that reduction of cellular ATP levels leads to down-regulation of in vivo ^99mTc-pertechnetate uptake by transplanted NIS⁺ CDCs. (B) In vivo dual isotope SPECT/CT imaging. Representative coronal images of in vivo ^99mTc-pertechnectate uptake (by NIS⁺ CDCs) at 1 to 3 h and 24 h after epicardial transplantation of NIS⁺ CDCs in noninfarcted rat myocardium. ^99mTc-pertechnectate uptake by transplanted cells is represented in green and ²⁰¹Tl uptake by myocardium is represented in red. Encapsulation in hydrogels results in high ^99mTc-pertechnectate uptake at 1 to 3 h following transplantation/encapsulation (B1); reversible Akt inhibition resulted in lower ^99mTc-pertechnectate uptake at 1 to 3 h post-transplantation when compared with 24 h (B2). White arrows indicate transplantation site. (C) Summary of in vivo ^99mTc-pertechnectate uptake at 1 to 3 h and 24 h following encapsulation in hydrogels and reversible Akt inhibition. Encapsulation in hydrogels led to similar ^99mTc-pertechnectate uptake at 1 to 3 h and 24 h post-transplantation. In contrast, reversible Akt inhibition in CDCs prior to transplantation led to significantly lower ^99mTc-pertechnectate uptake at 1 to 3 h post-transplantation, when compared with 24 h. Statistical significance was determined using the Mann-Whitney U test. Data is presented as mean ± SD; n = 4. (D) Figure illustrating trends for in vivo ^99mTc-pertechnetate by transplanted NIS⁺ CDCs at 1 to 3 h and 24 h following intramyocardial transplantation of suspended NIS⁺ CDCs (historic data [3]), and following epicardial transplantation of NIS⁺ CDCs encapsulated in hydrogels with/without Akt inhibition (C). Cell suspension and Akt inhibition, which result in energetic stress, were associated with lower ^99mTc-pertechnetate uptake at 1 to 3 h when compared to the 24-h time point, whereas hydrogel encapsulation alone resulted in similar uptake at these 2 time points. Abbreviations as in Figure 1.