Background and purpose: Adding spironolactone to standard therapy in heart failure reduces morbidity and mortality, but the underlying mechanisms are not fully understood. We analysed the effect of canrenone, the major active metabolite of spironolactone, on myocardial contractility and intracellular calcium homeostasis. Experimental approach: Left ventricular papillary muscles and cardiomyocytes were isolated from male Wistar rats. Contractility of papillary muscles was assessed with force transducers, Ca 2+ transients by fluorescence and Ca 2+ fluxes by electrophysiological techniques. Key results: Canrenone (300-600 mmol·L -1 ) reduced developed tension, maximum rate of tension increase and maximum rate of tension decay of papillary muscles. In cardiomyocytes, canrenone (50 mmol·L -1 ) reduced cell shortening and L-type Ca 2+ channel current, whereas steady-state activation and inactivation, and reactivation curves were unchanged. Canrenone also decreased the Ca 2+ content of the sarcoplasmic reticulum, intracellular Ca 2+ transient amplitude and intracellular diastolic Ca 2+ concentration. However, the time course of [Ca 2+ ]i decline during transients evoked by caffeine was not affected by canrenone. Conclusion and implications:Canrenone reduced L-type Ca 2+ channel current, amplitude of intracellular Ca 2+ transients and Ca 2+ content of sarcoplasmic reticulum in cardiomyocytes. These changes are likely to underlie the negative inotropic effect of canrenone. British Journal of Pharmacology

Abstract. The sarcoplasmic reticulum (SR) is the main source of contraction-activating Ca 2+ in the adult mammalian myocardium.

Hypertonic NaCl solutions have been used for small-volume resuscitation from hypovolemic shock. We sought to identify osmolality- and Na(+)-dependent components of the effects of the hyperosmotic NaCl solution (85 mOsm/kg increment) on contraction and cytosolic Ca(2+) concentration ([Ca(2+)](i)) in isolated rat ventricular myocytes. The biphasic change in contraction and Ca(2+) transient amplitude (decrease followed by recovery) was accompanied by qualitatively similar changes in sarcoplasmic reticulum (SR) Ca(2+) content and fractional release and was mimicked by isosmotic, equimolar increase in extracellular [Na(+)] ([Na(+)](o)). Raising osmolality with sucrose, however, augmented systolic [Ca(2+)](i) monotonically without change in SR parameters and markedly decreased contraction amplitude and diastolic cell length. Functional SR inhibition with thapsigargin abolished hyperosmolality effects on [Ca(2+)](i). After 15-min perfusion, both hyperosmotic solutions slowed mechanical relaxation during twitches and [Ca(2+)](i) decline during caffeine-evoked transients, raised diastolic and systolic [Ca(2+)](i), and depressed systolic contractile activity. These effects were greater with sucrose solution, and were not observed after isosmotic [Na(+)](o) increase. We conclude that under the present experimental conditions, transmembrane Na(+) redistribution apparently plays an important role in determining changes in SR Ca(2+) mobilization, which markedly affect contractile response to hyperosmotic NaCl solutions and attenuate the osmotically induced depression of contractile activity.

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