Studies on the reaction catalyzed by transport (Na, K) adenosine triphosphatase—II. In vitro and in vivo effects of phenoxybenzamine

Abstract

Phenoxybenzamine is a potent inhibitor of NaK-ATPase both in vitro (i 50 = 3.5 × 10 −5 M) and in vivo(3 mg/kg inhibits NaK-ATPase in 100 per cent of the animals tested). the inhibition of the enzyme, in vitro, was time- and temperature -dependent, being maximal at 37° for 30 min. the inhibition was irreversible and was not specifically influenced by incubation in the presence of Na, K, Mg, ATP or varying pH. Phenoxybenzamine did not alter the concentration of Na or K required for half-maximal activation of the enzyme. Incubation of the enzyme with phenoxybenzamine under conditions where the phosphorylated form of the enzyme exists gave an I 50 identicai to that observed when no phospho-enzyme is formed. K-NPPase activity was inhibited by phenoxybenzamine (I 50 = 5.1 × 10 −5 M) over the same concentration range as required for inhibition of the NaK-ATPase reaction. Neither norepinephrine nor phentolamine could prevent the inhibition by phenoxybenzamine in vitro. Ouabain did not alter the sensitivity of the enzyme for phenoxybenzamine.