Table 3. Comparison of Rb transport by human RBCs measured by ICP-MS and ⁸⁶Rb tracer.

	ICP-MS						86Rb tracer
	by wet weight			Fe- based mass			by wet weight
	Rb content nMol Rb/g	RSD %	K transport rate nMol K/g-min	Rb content nMol Rb/g	RSD %	K transport rate nMol K/g-min	Rb content nMol Rb/g	RSD %	K transport rate nMol K/g-min
no ouabain (n = 6)	79.0 ± 6.8	8.6	21.2 ± 1.8	112.8 ± 1.6	1.6	23.5 ± 0.4	78.5 ± 15.0	19.1	16.4 ± 3.1
+ouabain (n = 6)	15.1 ± 1.2	8.2	5.22 ± 0.3	23.5 ± 2.0	1.6	4.9 ± 0.1	23.4 ± 2.3	9.9	4.9 ± 0.5
NET transport by Na,K-ATPase		16.0			18.6			11.5

All measurements were run in parallel under identical conditions using the same pooled batch of RBCs. The batch was pooled from 4 units of leucocyte-depleted RBCs, washed, and re-suspended in Rb-free buffer at a hematocrit of 50%. The incubation mix contained 150 μL RBCs, 5 mM K, 200 μM RbCl, and Uptake Buffer without or with 1 mM ouabain, in a volume of 1 ml. Radioisotope tracer assays contained, in addition, 4 nCi/ml ⁸⁶Rb. Uptake was carried out for 2 h at 37 C. K transport rate was computed by multiplying the Rb transport rate by the molar ratio of K to Rb in the Uptake buffer. The endogenous Rb concentration of the pooled, untreated RBCs was 2,374 ng/g and was subtracted in all calculations of exogenous Rb uptake.