Ouabain binding, Na+-K+-ATPase activity, and 86Rb uptake of canine arteries

In a previous report [Am. J. Physiol. 245 (Heart Circ. Physiol. 14): H598-H603, 1983] we measured potassium-induced relaxation of canine femoral and renal arteries as an index of sodium pump function and concluded that it may not be an accurate measure. In this report, the sodium pumps of femoral and renal arteries were examined using three separate approaches to compare sodium pump function more directly. The number of pump sites in subcellular fractions was measured using [3H]ouabain binding. No differences were observed (femoral artery 14.5 +/- 4.8 pmol/g wet wt; renal artery 17.4 +/- 6.5). Similarly, there was no difference in ouabain-inhibitable Na+-K+-ATPase activity in NaI-treated microsomal fractions of these arteries (femoral artery 3.10 +/- 0.65 mumol Pi X mg-1 X h-1; renal artery 3.42 +/- 0.23). Finally, sodium pump function was measured using the ouabain-inhibitable 86Rb uptake method. The rate of ouabain-inhibitable 86Rb uptake was found to be identical for the two arteries, as were the dissociation constants of the two arteries for rubidium. However, in the presence of 3.5 microM norepinephrine, the rate of 86Rb uptake in the renal artery was greater than that of the femoral artery. We conclude that, under resting conditions, the sodium pumps of femoral and renal arteries are identical, whereas in the presence of norepinephrine, the renal artery sodium pump is not rate limiting with respect to contractility. These data have significant implications with regard to the use of potassium-induced relaxation as an index of sodium pump activity.