Endogenous cardiotonic steroids and cardiovascular disease,where to next?

Ever since British Physician William Withering first described the use of foxglove extract for treatment of patients with congestive heart failure in 1785, cardiotonic steroids have been used clinically to treat heart failure and more recently atrial fibrillation. Due to their ability to bind and inhibit the ubiquitous transport enzyme sodium potassium pump, thus regulating intracellular Na⁺ concentration in every living cell, they are also an essential tool for research into the sodium potassium pump structure and function. Exogenous CTS have been clearly demonstrated to affect cardiovascular system through modulation of vagal tone, cardiac contraction (via ionic changes) and altered natriuresis. Reports of a number of endogenous CTS, since the 1980s, have intensified research into their physiologic and pathophysiologic roles and opened up novel therapeutic targets. Substantive evidence pointing to the role of endogenous ouabain and marinobufagenin, the two most prominent CTS, in development of cardiovascular disease has accumulated. Nevertheless, their presence, structure, biosynthesis pathways and even mechanism of action remain unclear or controversial. In this review the current state-of-the-art, the controversies and the remaining questions surrounding the role of endogenous cardiotonic steroids in health and disease are discussed.     

Evaluation of the rate of basal oxygen consumption in the isolated frog skin and toad bladder

In the study of active transport it is important to distinguish between oxygen consumption sustaining transepithelial transport and that responsible for other tissue functions (basal metabolism). Since amiloride blocks transepithelial active sodium transport and the associated oxygen consumption in the frog skin and toad bladder, we and others have employed this agent to evaluate the rate of basal metabolism. This technique has recently been criticized in a report that amiloride (and ouabain) increased oxygen consumption when no sodium was available for transport. We have been unable to corroborate these observations.With magnesium-Ringer as external bathing solutions, amiloride and ouabain failed to stimulate oxygen consumption. With sodium-Ringer as external bathing solution amiloride reduced oxygen consumption about 30%, to a level indistinguishable from that found on external substitution of magnesium-Ringer for sodium-Ringer. We conclude that the use of amiloride permits evaluation of the rate of basal metabolism with acceptable accuracy; a possible slight depressant effect of ouabain on basal metabolism remains to be investigated.