Ca(2+)-handling proteins and heart failure: novel molecular targets?

Calcium (Ca(2+)) ions are the currency of heart muscle activity. During excitation-contraction coupling Ca(2+) is rapidly cycled between the cytosol (where it activates the myofilaments) and the sarcoplasmic reticulum (SR), the Ca(2+) store. These fluxes occur by the transient activity of Ca(2+)-pumps and -channels. In the failing human heart, changes in activity and expression profile of Ca(2+)-handling proteins, in particular the SR Ca(2+)-ATPase (SERCA2a), are thought to cause an overall reduction in the amount of SR-Ca(2+) available for contraction. In the steady state, the Ca(2+)-content of the SR is essentially a balance between Ca(2+)-uptake via SERCA2a pump and Ca(2+)-release via the cardiac SR Ca(2+)-release channel complex (Ryanodine receptor, RyR2). This review discusses current pharmacological options available to enhance cardiac SR Ca(2+) content and the implications of this approach as an inotropic therapy in heart failure. Two options are considered: (i) activation of the SERCA2a pump to increase SR Ca(2+)-uptake, and (ii) reduction of SR Ca(2+)-leakage through RyR2. RyR2 forms a macromolecular complex with a number of regulatory proteins that either remain permanently bound or that interact in a time- and/or Ca(2+)-dependant manner. These regulatory proteins can dramatically affect RyR2 function, e.g. over-expression of the accessory protein FK 506-binding protein 12.6 (FKBP12.6) has recently been shown to reduce SR Ca(2+)-leak. Recent attempts to design positive inotropes for chronic administrations have focussed on the use of phosphodiesterase III inhibitors (PDE III inhibitors). These compounds, which increase intracellular cAMP-levels, have failed in clinical trials. Therefore medical researchers are seeking new drugs that act through alternative pathways. Novel cardiac inotropes targeting SR Ca(2+)-cycling proteins may have the potential to fill this gap.