[The problem of the cellular receptor for cardiac glycosides (author's transl)]

Subunit requirements for expression of functional sodium pumps in yeast cells

Na+/K(+)-ATPase from animal cell membranes is known to consist of an alpha-subunit and a beta-subunit. Amino acids within the alpha-subunit have been shown to participate in the catalytic functions of the enzyme and in the binding of cardioactive steroids. Although the function of the beta-subunit is not known, expression of both alpha- and beta-subunits is required for the functional enzyme. A putative third subunit, the gamma-subunit, has been suggested to be a part of the functional Na+/K(+)-ATPase complex, based on experiments showing that both the catalytic alpha-subunit and a small peptide of M(r) = 11,000 can be labeled by a photoreactive ouabain analog. Although the primary structure for the putative gamma-subunit from rat and sheep was recently deduced from cDNA clones, participation of this small protein in the catalytic activity of the Na+/K(+)-ATPase has not been demonstrated. In experiments described here, the heterologous expression of Na+/K(+)-ATPase in yeast cells was used to investigate whether the gamma-subunit is an essential component of the Na+/K(+)-ATPase. Yeast cells do not contain an endogenous Na+/K(+)-ATPase. The alpha- and beta-subunits or the alpha-, beta- and the putative gamma-subunits of Na+/K(+)-ATPase were expressed in the yeast Saccharomyces cerevisiae and ouabain-sensitive ATPase, p-nitrophenylphosphatase, and 86Rb uptake activities were measured either in membranes prepared from transformed yeast cells, or in intact yeast cells. Nontransformed yeast cells or yeast cells transformed with the gamma-subunit alone served as controls. Northern analysis and Western blots demonstrated that yeast cells do not contain an endogenous peptide with significant sequence homology to the putative gamma-subunit. Yeast samples containing only Na+/K(+)-ATPase alpha and beta subunits were capable of ouabain-inhibitable enzymatic activity and 86Rb transport. No gamma-subunit-dependent differences in the measured enzymatic activities or transport properties were detected in the different samples. These observations establish that the alpha beta-subunit complex is the minimum structural unit required for all the ouabain-sensitive reactions of Na+/K(+)-ATPase.

Influence of digitalis and diuretics on ouabain binding sites on human erythrocytes

It has been reported that during chronic treatment with digitalis, the number of digitalis binding sites is increased in human erythrocytes [22]. From this finding a tachyphylaxis for cardiac glycosides has been postulated. We reinvestigated this problem in several groups of patients. The number of 3H-ouabain binding sites per erythrocyte in control persons (group I) was 214 +/- 60, n = 43 (means +/- SD). The dissociation constant (KD) was 1.8 +/- 0.5 nM. Thirteen patients (group II) taking cardiac glycosides only, for at least 6 months, had 281 +/- 99 (p less than 0.05) ouabain binding sites per single red cell, KD = 1.8 +/- 0.7 nM. Group III (34 patients) took digitalis for more than 6 months and diuretics for at least 3 months (352 +/- 126 (p less than 0.001), KD = 1.6 +/- 0.6). Twenty-three of these (group IV) were taking a combination with "K+-saving" diuretics (336 +/- 194 (p less than 0.01), KD = 1.6 +/- 0.5) and (group V, 11 patients) a combination with "K+-losing" diuretics (462 +/- 133 (p less than 0.001), KD = 1.4 +/- 0.4). Nine patients (group VI) had a chronic hypokalemia, mainly due to taking furosemide (437 +/- 98 (p less than 0.001), KD = 1.5 +/- 0.4). Four control persons took 50 mg hydrochlorothiazide daily for more than 4 months without measurable K+-losses and without changes in ouabain binding sites. It is concluded from these findings that diuretic treatment with chronic hypokalemia in addition to digitalis is accompanied by a significant increase in ouabain binding sites in human red cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Biochemical regulators in cardiac hypertrophy

In recent years research has shown that muscle is capable of reacting to mechanical stimuli by altering biochemical processes. Myocardium is probably the source of a biochemical factor, or factors which activate myocardial protein synthesis. In experimentally induced cardiac hypertrophy adaptive alterations have been shown to occur not only in the adrenal medulla but also in the adrenal cortex. Finally, detection of cross reactivity between digitalis glycosides and a number of steroid hormones has succeeded. We assume that such cross reactivity indicates the existence of an endogenic factor of steroid character, which is produced in the adrenal gland and functions as an endogenic cardiotonic agent. During experimental cardiac hypertrophy its synthesis is possibly increased. We propose the term "endocardin" or "endocardiotonin" for this agent.