Ouabain potentiation and Ca release from sarcoplasmic reticulum in cardiac and skeletal muscle cells

Abstract

In this article, we describe a possible mechanism of ouabain potentiation in heart based on the following findings in cardiac and skeletal muscles of various species. (1) In heart ventricle muscles of frog and guinea pig, the ouabain potentiation is produced without an effect on Ca influx. In both frog and cat heart ventricle muscles, ouabain potentiates the rapid cooling contracture with or without caffeine in a Ca-deprived medium. It follows, therefore, that the ouabain potentiation is produced by an "intracellular" mechanism. (2) In crab single muscle fibers, contractile responses such as twitch, potassium-induced contracture, caffeine-induced contracture, and water-induced contracture are remarkably potentiated if ouabain is present within the fibers by microinjection, whereas the situation is reversed if the drug is given extracellularly. (3) The ouabain potentiated the Ca release from fragmented sarcoplasmic reticulum (FSR) isolated from cat, guinea pig, and frog heart and from skeletal muscles as a result of the procedures used, such as changing the ionic environment. (4) In frog, cat, and guinea pig heart ventricle muscles, a reduction of contractility as a result of pretreatment with urea--Ringer's was completely cancelled by ouabain almost without influencing the membrane depolarization. Based on these findings and others, the deduction was made that the positive inotropic effect of cardiac glycosides on the heart is brought about by potentiation of contraction - Ca release from the intracellular store sites, namely the sarcoplasmic reticulum.

Activation of cardiac ryanodine receptors by cardiac glycosides

“…This result was interpreted as suggesting that hydrophilic agents required active transmembrane transport to an intracellular site of action whereas lipophillic agents were capable of passive diffusion to that site. These interesting results recalled earlier suggestions that glycosides might have an intracellular site of action, possibly involving the sarcoplasmic reticulum (5,8).…”

“…Nunez-Duran et al (26) found evidence suggesting that hydrophilic glycosides must be transported across the sarcolemma before positive inotropy can occur. These recent studies are consistent with older reports (5,8), demonstrating a direct interaction of glycosides with SR proteins.…”

Effect of Cardiac Glycosides on Action Potential Characteristics and Contractility in Cat Ventricular Myocytes: Role of Calcium Overload

“…Information provided by our investigation indicates that subtle differences in structure (including a simple saturation of the lactone ring) cause significant differences in physiological activity. If intracellular sites are indeed important in determining differences in action between cardiac glycoside agents (Fujino and Fujino, 1982;Isenberg, 1984;Sagawa et al, 2002), including the threshold for Ca 2ϩ overload, one could speculate that the relative ability of a given glycoside to cross the sarcolemma (either passively or actively; Nunez-Duran et al, 1988) and affinity for specific intracellular sites would be important additional determinants of action. In addition, if the ryanodine receptor is an important determinant of this intracellular mechanism as postulated by some investigators (Isenberg, 1984;Rardon and Wasserstrom, 1990;McGarry and Williams, 1993;Sagawa et al, 2002), then differences in binding affinity to this receptor between cardiac glycosides might contribute to the observed differences in action.…”

“…Support for alternative mechanisms underlying inotropic and/or toxic effects of cardiac glycosides includes findings that cardiac glycoside analogs differ significantly in their effects on intracellular [Na ϩ ], action and resting membrane potentials (Wasserstrom et al, 1991), and toxic to therapeutic ratios in both isolated preparations (Karagueuzian and Katzung, 1981) and whole animals (Mendez et.al., 1974). In addition, studies directed at defining alternative cardiac glycoside mechanisms have revealed intracellular sites of action independent of the sodium pump, including the sarcoplasmic reticulum (Fujino and Fujino, 1982;Isenberg, 1984) and its calcium release channel (Rardon and Wasserstrom, 1990;McGarry and Williams, 1993;Sagawa et al, 2002).…”

Ouabain augments calcium‐dependent potassium conductance in visceral primary afferent neurones of the rabbit.

“…Indeed, the effect of the calcium ionophore or intracellular calcium injection was augmented by ouabain in solutions in which calcium entry was almost negligible. It has been reported that ouabain stimulates calcium release from sarcoplasmic reticulum independently of calcium influx across the cell membrane in cardiac and skeletal muscles (Nayler, 1973;Fujino & Fujino, 1982;Finet, Godfraind & Noel, 1983). If there are calcium storage sites in neurones like in sarcoplasmic reticulum in muscles, calcium may be released from them by ouabain and a subsequent increase in intracellular calcium concentration would lead to activation of 9K(Ca).…”