Molecular and cellular level of action of digitalis

The pharmacological receptor of cardiac glycosides is the Na+/K(+)-ATPase which consists of a catalytic alpha (M(r) = 112,000) and glycosylated beta (M(r) = 35,000) subunit. The enzyme is responsible for the vectorial transport across the sarcolemma of three Na+ ions outward and two K+ ions inward against their electrochemical gradient. Specific inhibition of the Na+ pump by digitalis induces a positive inotropic effect by increasing the intracellular Na+ concentration which in turn induces an increase in the intracellular Ca2+ concentration by the Na+/Ca2+ exchange and an increase in the Ca2+ pool of the sarcoplasmic reticulum; toxic effects are observed at higher doses of cardiac glycosides leading to spontaneous calcium release from the sarcoplasmic reticulum. Three isoforms of the alpha catalytic subunit have been identified by molecular cloning. They share a high homology in the deduced amino acid sequence with eight transmembrane domains. The ouabain binding domain is located on the extracellular side and ouabain sensitivity depends mainly on the two residues at the border of the first extracellular domain. The isoforms differed by their ouabain sensitivity, are expressed in a tissue-specific and hormonally-regulated manner. Moreover, expression of the isoforms and their ouabain sensitivity vary from species to species with an alpha 1 isoform of very low affinity being the major isoform (80%) in the adult rat heart and an alpha 1 isoform of high affinity representing 50% of total alpha mRNA abundance in the human heart. Therefore the effect of digitalis on the heart depends mainly on the isoform which is expressed and on the regulation of their expression according to age, hormonal influence and pathology.