The negative inotropic action of canrenone is mediated by L-type calcium current blockade and reduced intracellular calcium transients.

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 micromol L(-1)) reduced developed tension, maximum rate of tension increase and maximum rate of tension decay of papillary muscles. In cardiomyocytes, canrenone (50 micromol 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.

Atrial natriuretic peptide modulates the effect of angiotensin II on the concentration of free calcium in the cytosol of Mandin-Darby canine kidney cells

The effect of angiotensin II (ANG II) and atrial natriuretic peptide (ANP) on intracellular free calcium concentration [ Ca²+]i was investigated in Mandin-Darby canine kidney (MDCK) cells in culture. Changes in [Ca²+]i were monitored fluorometrically with the Ca²+ -sensitive probel fura -2/AM at 37ºC using Perkin-Elmer LS-5 spectrofluorimeter (excitation 340/380 nm,slite 3 nm; emission 520 nm, slit 10 nm). MDCK cells exhibited a mean baseline [Ca²+]i of 98 ñ 10 nM. the addition of increasing concentrations of SNG II (1 pM to 1 µM) to the cell suspension led to a progressive increase in [Ca²+]i to 2-3 times basal levels. In contrast, addition of 1 µM ANP to the cell suspension led to a very rapid 60 percent decrease in [Ca²+]i. The addition of 1 pM to 1 µM ANG II immediately after 1 µM ANP caused an increase in [Ca²+]i which never exceded the basal level in the absence of ANP. The data indicate that ANG II increases cell [Ca²+]i as expected, and provide the new observation that ANP reduces [Ca²+]i in these cells. Further more, ANP reduces the increase in [Ca²+]i elicited by ANG II, thus modulating the effect of ANG II on [Ca²+]i