The current concept for the cardiac glycoside receptor

Sequestration of cardenolides inOncopeltus fasciatus: Morphological and physiological adaptations

The morphological and physiological adaptations associated with sequestration of cardenolides by the lygaeidOncopeltus fasciatus are summarized and discussed. Cardenolides are efficiently accumulated inO. fasciatus; however, the insect does not appear to suffer any physiological cost as a result of handling large amounts of these plant toxins. Morphological adaptations of the insect include a modified integument composed of a double layered epidermis with an inner layer (the dorsolateral space) specialized for cardenolide storage. Special weak areas of the cuticle are found on both the thorax and abdomen, which rupture when the insect is squeezed, resulting in the cardenolide-rich contents of the inner epidermal layer being released onto the body surface in the form of discrete spherical droplets. Physiological adaptations include selective sequestration of food plant cardenolides, concentration of cardenolides in the dorsolateral space, passive uptake of cardenolides at the gut and dorsolateral space requiring little energy output, reabsorption of secreted cardenolides by the Malpighian tubules, high in vivo tolerance to cardenolides, and the presence of cardenolide-resistant Na,K-ATPases.

Signaling mechanisms that link salt retention to hypertension: endogenous ouabain, the Na(+) pump, the Na(+)/Ca(2+) exchanger and TRPC proteins

Salt retention as a result of chronic, excessive dietary salt intake, is widely accepted as one of the most common causes of hypertension. In a small minority of cases, enhanced Na(+) reabsorption by the kidney can be traced to specific genetic defects of salt transport, or pathological conditions of the kidney, adrenal cortex, or pituitary. Far more frequently, however, salt retention may be the result of minor renal injury or small genetic variation in renal salt transport mechanisms. How salt retention actually leads to the increase in peripheral vascular resistance (the hallmark of hypertension) and the elevation of blood pressure remains an enigma. Here we review the evidence that endogenous ouabain (an adrenocortical hormone), arterial smooth muscle α2 Na(+) pumps, type-1 Na/Ca exchangers, and receptor- and store-operated Ca(2+) channels play key roles in the pathway that links salt to hypertension. We discuss cardenolide structure-function relationships in an effort to understand why prolonged administration of ouabain, but not digoxin, induces hypertension, and why digoxin is actually anti-hypertensive. Finally, we summarize recent observations which indicate that ouabain upregulates arterial myocyte Ca(2+) signaling mechanisms that promote vasoconstriction, while simultaneously downregulating endothelial vasodilator mechanisms. In sum, the reports reviewed here provide novel insight into the molecular mechanisms by which salt retention leads to hypertension.

Halenaquinol, a natural cardioactive pentacyclic hydroquinone, interacts with sulfhydryls on rat brain Na(+),K(+)-ATPase

Halenaquinol inhibited the partial reactions of ATP hydrolysis by rat brain cortex Na(+),K(+)-ATPase, such as [3H]ATP binding to the enzyme, Na(+)-dependent front-door phosphorylation from [gamma-(33)P]ATP, and also Na(+)- and K(+)-dependent E(1)<-->E(2) conformational transitions of the enzyme. Halenaquinol abolished the positive cooperativity between the Na(+)- and K(+)-binding sites on the enzyme. ATP and sulfhydryl-containing reagents (cysteine and dithiothreitol) protected the Na(+),K(+)-ATPase against inhibition. Halenaquinol can react with additional vital groups in the enzyme after blockage of certain sulfhydryl groups with 5,5'-dithio-bis-nitrobenzoic acid. Halenaquinol inhibited [3H]ouabain binding to Na(+),K(+)-ATPase under phosphorylating and non-phosphorylating conditions. Binding of fluorescein 5'-isothiocyanate to Na(+),K(+)-ATPase and intensity of fluorescence of enzyme tryptophanyl residues were decreased by halenaquinol. We suggest that interaction of halenaquinol with the essential sulfhydryls in/or near the ATP-binding site of Na(+),K(+)-ATPase resulted in a change of protein conformation and subsequent alteration of overall and partial enzymatic reactions.

Inhibition of membrane transport ATPases by halenaquinol, a natural cardioactive pentacyclic hydroquinone from the sponge Petrosia seriata

Halenaquinol, a natural cardioactive pentacyclic hydroquinone from the sponge Petrosia seriata, was found to be a powerful inhibitor of the rat brainstem and of the rat brain cortex Na+, K(+)-ATPases and the rabbit muscle sarcoplasmic reticulum Ca(2+)-ATPase with I50 values of 7.0 x 10(-7), 1.3 x 10(-6) and 2.5 x 10(-6) M, respectively. Halenaquinol also inhibited K(+)-phosphatase activity of the rat brain cortex Na+, K(+)-ATPase with an I50 value of 3 x 10(-6) M. Ouabain-insensitive Mg(2+)-ATPase activity of the microsomal fraction of the rat brain cortex was weakly inhibited by halenaquinol. Inhibition was irreversible, dose- and time-dependent. Naphthohydroquinone fragment in structures of halenaquinol, related natural and model compounds was very important for an inhibiting effect.

Stimulatory effects of starfish sapogenins (Asterias amurensis and Lethasterias nanimensis chelifera) on molluscan heart (Spisula sachalinensis)

Sapogenins from the starfish Asterias amurensis and Lethasterias nanimensis chelifera, 5 alpha-pregn-9(11)-ene-3 beta,6 alpha-diol-20-one, 5 alpha-cholest-9(11)-ene-3 beta,6 alpha-diol-23-one, 5 alpha-cholesta-9(11),24(25)-diene-3 beta,6 alpha-diol-23-one, (20E)-5 alpha-cholesta-9(11),20(22)-diene-3 beta,6 alpha-diol-23-one and 24 zeta-methyl-5 alpha-cholesta-9(11),20(22)-diene-3 beta,6 alpha-diol-23-one, stimulated the contractile force of the heart of the mollusk Spisula sachalinensis at concentration of 5 x 10(-5) M. Ouabain, a specific inhibitor of Na+,K(+)-ATPase, at concentration of 5 x 10(-5) M had no effect on this physiological model. Starfish sapogenins of the cholestane series moderately inhibited rat brain cortex Na+,K(+)-ATPase and decreased Ca2+ influx into Ehrlich carcinoma cells. In contrast, pregnane asterogenin asterone did not inhibit Na+,K(+)-ATPase and increased the influx of Ca2+ into cells. These effects were not the result of cell membrane damage, because none of the compounds tested have hemolytic activity.