Endogenous cardiac glycosides

Digitalis use in children: an uncertain future

The therapeutic indications for digoxin in the treatment of children with large left-to-right shunts continue to be reassessed. New insights into the alterations in cardiac function imposed by this hemodynamic burden have shown preserved systolic performance. Pharmacological interventions that improve cardiac output by afterload reduction or other modalities have proven useful and potentially have low risk for serious toxicity. Early and successful surgical treatment of most conditions causing pulmonary overcirculation has shortened the duration of medical management and prevented many of the untoward complications of this pathology. As new agents are forthcoming to treat various cardiac conditions, use-by-tradition of cardiac glycosides has appropriately diminished. While the understanding of complicated molecular aspects of cation transport have been enhanced by the unique actions of cardiac glycosides, their clinical utility has decreased. This report summarizes studies on the use of cardiac glycosides in the treatment of large left-to-right shunts in children.

The alpha4 isoform of the Na,K-ATPase is expressed in the germ cells of the testes

In addition to the three isoforms of the catalytic subunit of the Na, K-ATPase originally identified (alpha1, alpha2, and alpha3), a fourth alpha polypeptide (alpha4) has recently been found in mammalian cells. This novel alpha-subunit of the Na,K-ATPase is selectively expressed in male gonadal tissues. In the testes, alpha4 is functionally active and comprises approximately half of the Na, K-ATPase activity of the organ. At present, the pattern of expression of the alpha4 polypeptide within the cells of the male gonad is unknown. By in situ hybridization, immunocytochemistry, and the ouabain inhibition profile of Na,K-ATPase activity, we show that the alpha4-subunit is expressed in the germ cells of rat testes. The highest amounts of the isoform are found in spermatozoa, where it constitutes two thirds of the Na,K-ATPase activity of the gametes. The other Na pump present in the cells is the ubiquitously expressed alpha1 polypeptide. The characteristic localization of alpha4 in the gonad is further supported by the drastic reduction of the polypeptide in mice that are infertile as a consequence of arrest in maturation of the germ cells. In addition, GC-1spg cells, a murine cell line derived from testis spermatogonia, also contain the Na, K-ATPase alpha4 polypeptide. However, the level of expression of the isoform in these cells is much lower than in the spermatozoa, a fact that may depend on the limited ability of the GC-1spg cells to differentiate in vitro. The particular expression of the Na,K-ATPase alpha4 isoform we encounter and the specific enzymatic properties of the polypeptide suggests its importance for ionic homeostasis of the germ cells of the testes.

Digitalis

Cardiac glycosides have played a prominent role in the therapy of congestive heart failure since William Withering codified their use in his late 18th century monograph on the efficacy of the leaves of the common foxglove plant (Digitalis purpurea). Despite their widespread acceptance into medical practice in the ensuing 200 years, both the efficacy and the safety of this class of drugs continue to be a topic of debate. Moreover, despite the fact that the molecular target for the cardiac glycosides, the alpha-subunit of sarcolemmal Na+K+-ATPase (or sodium pump) found on most eukaryotic cell membranes, has been known for several decades, it remains controversial whether the sympatholytic or positive inotropic effects of these agents is the mechanism most relevant to relief of heart failure symptoms in humans with systolic ventricular dysfunction. Herein, we review the molecular and clinical pharmacology of this venerable class of drugs, as well as the manifestations of digitalis toxicity and their treatment. We also review in some detail recent clinical trials designed to examine the efficacy of these drugs in heart failure, with a focus on the Digoxin Investigation Group data set. Although, in our opinion, the data on balance warrant the continued use of these drugs for the treatment of symptoms of heart failure in patients already receiving contemporary multidrug therapy for this disease, the use of digitalis preparations will inevitably decline with the maturation of newer pharmacotherapies.

Isozymes of the Na-K-ATPase: heterogeneity in structure, diversity in function

The Na-K-ATPase is characterized by a complex molecular heterogeneity that results from the expression and differential association of multiple isoforms of both its alpha- and beta-subunits. At present, as many as four different alpha-polypeptides (alpha1, alpha2, alpha3, and alpha4) and three distinct beta-isoforms (beta1, beta2, and beta3) have been identified in mammalian cells. The stringent constraints on the structure of the Na pump isozymes during evolution and their tissue-specific and developmental pattern of expression suggests that the different Na-K-ATPases have evolved distinct properties to respond to cellular requirements. This review focuses on the functional properties, regulation, and possible physiological relevance of the Na pump isozymes. The coexistence of multiple alpha- and beta-isoforms in most cells has hindered the understanding of the roles of the individual polypeptides. The use of heterologous expression systems has helped circumvent this problem. The kinetic characteristics of different Na-K-ATPase isozymes to the activating cations (Na+ and K+), the substrate ATP, and the inhibitors Ca2+ and ouabain demonstrate that each isoform has distinct properties. In addition, intracellular messengers differentially regulate the activity of the individual Na-K-ATPase isozymes. Thus the regulation of specific Na pump isozymes gives cells the ability to precisely coordinate Na-K-ATPase activity to their physiological requirements.