Modulation of the local SR Ca2+ release by intracellular Mg2+ in cardiac myocytes

In cardiac muscle, Ca(2+)-induced Ca(2+) release (CICR) from the sarcoplasmic reticulum (SR) defines the amplitude and time course of the Ca(2+) transient. The global elevation of the intracellular Ca(2+) concentration arises from the spatial and temporal summation of elementary Ca(2+) release events, Ca(2+) sparks. Ca(2+) sparks represent the concerted opening of a group of ryanodine receptors (RYRs), which are under the control of several modulatory proteins and diffusible cytoplasmic factors (e.g., Ca(2+), Mg(2+), and ATP). Here, we examined by which mechanism the free intracellular Mg(2+) ([Mg(2+)](free)) affects various Ca(2+) spark parameters in permeabilized mouse ventricular myocytes, such as spark frequency, duration, rise time, and full width, at half magnitude and half maximal duration. Varying the levels of free ATP and Mg(2+) in specifically designed solutions allowed us to separate the inhibition of RYRs by Mg(2+) from the possible activation by ATP and Mg(2+)-ATP via the adenine binding site of the channel. Changes in [Mg(2+)](free) generally led to biphasic alterations of the Ca(2+) spark frequency. For example, lowering [Mg(2+)](free) resulted in an abrupt increase of spark frequency, which slowly recovered toward the initial level, presumably as a result of SR Ca(2+) depletion. Fitting the Ca(2+) spark inhibition by [Mg(2+)](free) with a Hill equation revealed a K(i) of 0.1 mM. In conclusion, our results support the notion that local Ca(2+) release and Ca(2+) sparks are modulated by Mg(2+) in the intracellular environment. This seems to occur predominantly by hindering Ca(2+)-dependent activation of the RYRs through competitive Mg(2+) occupancy of the high-affinity activation site of the channels. These findings help to characterize CICR in cardiac muscle under normal and pathological conditions, where the levels of Mg(2+) and ATP can change.