Detecting K+ Fluxes in vivo Using the Stable Isotopes 41K and 39K
Authors: Ha Darren , Higgins John A. , Youn Jang H. , McDonough Alicia Ann 
Published: April 2020

Extracellular potassium (K+) is tightly regulated to maintain membrane potential. While kidneys ultimately match output to input, extrarenal regulation of K+ distribution between intracellular (ICF, 98%) and extracellular (ECF, 2%) stores takes place after a meal or during fasting. For example, skeletal muscle altruistically donates K+ to the ECF during chronic low K+ intake. The molecular mechanisms responsible for the shift (less K+ uptake and/or more K+ efflux) are not clearly understood. Advances in mass spectrometry now allow for the detection and quantitation of two naturally occurring, stable K+ isotopes: 41K and 39K with natural abundances of 6.73% and 93.26%, respectively.AIMSTo learn whether there is isotope preference of K+ transporters: 1). determine which tissues transfer K+ from ICF to ECF during 10 days of zero‐K+ diet (0K), and 2). whether tissue 41K and 39K are differentially depleted during 0K diet.METHODS
Baseline study of 41K/39K in 4 Wistar rats fed standard chow.
K depletion. Male Wistar rats were divided into 3 groups (3/group) and fed diets for 10 days with 1% K+ (1K), 0% K+ (0K), or 0K followed by 2 day K+ restoration with 1K (1K‐R). Urine and feces were collected O/N (16 hr) in metabolic cages prior to termination and tissue collection. [Na+] and [K+] were measured by flame photometry. 41K/39K tissue ratios were determined by inductively coupled plasma mass spectrometry.RESULTS
Baseline.41K/39K ratios at baseline were expressed relative to the ratio in diet, defined as zero. Ratios were higher in feces and lower in urine, higher in muscle and liver and lower in brain, suggesting that 41K moves slower than 39K during: intestinal absorption, influx across BBB, renal secretion, and efflux across plasma membranes.
K depletion. Plasma K+ decreased 50% (0K) and restored to 80% of 1K value in 1K‐R. The following tissues lost K+ (as %) during 0K feeding: gastrocnemius (G, 23%) > tibialis anterior (TA, 13%) = extensor digitorum longus (EDL,12%) > soleus (S, 8%); tissue Na+ increased reciprocally with same rank order (Table
). The 41K/39K ratio (in ppt, relative to ratio in chow) between 1K and 0K diet decreased: G (1.05 to 0.08), and EDL (0.74 to 0.04). Liver, heart, cerebrum, and RBC did not lose or gain K+; the 41K/39K ratio between 1K and 0K increased in liver (0.52 to 0.82) and RBC (−0.6 to 0.18)CONCLUSIONSDuring 10 day 0K diet, skeletal muscles shift K+ from ICF to ECF, and we learned that there is an accompanying decrease in the muscles’ 41K/39K ratios, suggesting a preferential shift (net efflux) of 41K from ICF to ECF during 0K diet. In contrast, liver and RBC did not lose or gain K+ yet exhibited an increase in their 41K/39K ratios, suggesting that the lower plasma [K+] milieu may differentially increase passive efflux and active influx of 41K vs. 39K.Support or Funding InformationUniversity Kidney Research Organization Project Grant, and NIH NIDDK DK083785
Tissue [K+] and [Na+] during K+ depletion‐repletion

[K+] mmole/gm tissue wet weight

[Na+] mmole/gm tissue wet weight

Tissue

1K

0K

1K‐R

1K

0K

1K‐R

Plasma

5 ± 0.9

2.6 ± 0.3

4.1 ± 0.3

136 ± 3

135 ± 3

138 ± 1

Gastrocnemius

118 ± 1

91 ± 5

118 ± 4

28 ± 2

43 ± 3

35 ± 3

Tibialis Ant.

117 ± 2

101 ± 4

122 ± 2

32 ± 2

45 ± 3

31 ± 4

Liver

92 ± 5

84 ± 4

87 ± 2

29 ± 2

34 ± 2

27 ± 1

RBC

96 ± 3

97 ± 1

97 ± 1

14.7 ± 0.4

18.2 ± 1.8

15.1 ± 0.7