Ⅲ-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 52 (2019)

RI imaging tracers for Na+ /K+ dynamics in a living body
S. Motomura,∗1 I. Kii,∗2 H. Haba,∗3 H. Yakushiji,∗1 Y. Watanabe,∗1 and S. Enomoto∗4
Sodium (Na) and potassium (K) are essential metalelements that play important roles in our body. Bothmetal elements exist as ions in the body and each constitutes approximately 0.2% of the body weight. Although they both are alkali metals located in group 1of the periodic table of elements, the ions Na+ and K+behave very diﬀerently in a living body. For example,
the concentration of Na+ is higher in the extracellular
ﬂuid than that in the intracellular ﬂuid, while the concentration of K+ is conversely higher in the cell thanthat in the extracellular ﬂuid.
The diﬀerent behaviors of Na+ and K+ are caused
by certain functions of some biomolecules that can signiﬁcantly distinguish the ions. An example of suchmolecule is Na+ /K+ -ATPase, which takes in two K+
ions into a cell and takes out three Na+ ions out of acell in one cycle. This mechanism is used to form themembrane potential in neurons, reabsorption of Na+ions in kidneys, etc.
The idea of the MetalloDiagnosis is to obtain thediagnostic information of molecular functions by observing the behavior of associated metal elements(Fig. 1). Various metal elements are controlled bysome biomolecules in a living body. Thus, the behaviorof the metal elements can be altered corresponding towhether the molecule is functioning normally or not. If
we can non-invasively visualize the diﬀerent behaviorof the associated metal elements, we can make noninvasive imaging diagnosis of the molecular functions.
We are trying to use our imaging apparatus calledGREI1) to realize the MetalloDiagnosis. The GREI isa kind of gamma-ray imaging camera with an imaging
principle called the Compton camera made of germanium semiconductor radiation detectors. The GREI
can take three-dimensional images of radio-tracers injected into living animals even by using a single GREI
imaging head without any rotation or movement.2)
Furthermore, the GREI can identify each radioisotope
injected simultaneously into an animal by distinguishing the gamma-ray energy in the energy range from100 keV to 2 MeV, owning to the excellent energy resolution of the semiconductor detectors.
To realize the MealloDiagnosis by GREI, we needto prepare radioisotopes of each metal element that
we are going to take images of the behavior in a living body. For the MetalloDiagnosis of Na and K, wechose 24 Na, 42 K, and 43 K that have the half-lives of15-, 12-, and 22-hours, respectively (Table 1). We es∗1

∗2
∗3
∗4

RIKEN Center for Biosystems Dynamics ResearchCompass to Healthy Life Research Complex Program,RIKEN
RIKEN Nishina Center
RIKEN Center for Life Science Technologies- 189 -Fig. 1. Concept of MetalloDiagnosis by GREI. Diagnostic information is obtained by observing the associatedmetals.
Table 1. Radioisotopes of Na and K for GREI imaging.

Nuclide24

Half life

Gamma-ray energy (keV)Na

15.0 h

1368.6, 2754.0

4212.4 h1524.743

22.3 h372.8, 617.5

KK

tablished the preparation schemes for these radioisotopes utilizing the nuclear reactions of nat Mg(d, x)24 Na
and nat Ca(d, x)42, 43 K. We have also established a delivery scheme for these radioisotopes from Wako to themolecular imaging facility in the RIKEN Kobe campus. It should be noted that we have made delivery of24Na, 42 K, and 43 K radioisotopes 6 times in FY2018.
In the RIKEN Kobe campus, it was the ﬁrst timethat we have used the radioisotopes of 24 Na, 42 K, and43
K for experiment. We have started GREI imaging experiment of living mice injected the Na and K
radioisotopes simultaneously. For each GREI experiment, approximately 1 MBq and 100 kBq of 24 Na and43K, respectively, were used for the time-course imaging of Na+ /K+ dynamics in a living mouse, and it waspossible to visualize the diﬀerent behavior of Na+ andK+ . This study is ongoing and the results are being
prepared to consider for publication.
References
1) S. Motomura et al., J. Anal. At. Spectrom. 28, 934(2013).
2) T. Ida, S. Motomura et al., Jpn. J. Appl. Phys. 58,016002 (2019).