A small leak may sink a great ship but what does it do to the heart?

Abstract

While Benjamin Franklin realized the importance of a leak, in much of cell physiology the term ‘leak’ has been used to describe the unmeasurable. In the original formulation for the squid giant axon, the leak included all ionic movements not through voltage gated sodium and potassium channels (Hodgkin & Huxley, 1952). In other words it represented everything which was not characterized in a more sophisticated way. More recent work has shown for a variety of biological membranes that leaks are often made up from the activity of many ion channels. It is now well established that most of the Ca2+ that activates contraction in the heart is provided by release from the sarcoplasmic reticulum (SR). Ca2+ is released through a specialized channel known as the ryanodine receptor (RyR). The RyR has the important property that the probability that it is open (po) is increased by an increase of cytoplasmic Ca2+ concentration ([Ca2+]i). This results in the process of calcium-induced calcium release (CICR) in which the entry of a small amount of calcium into the cell on the L-type Ca2+ current leads to the opening of the RyR and thence release of Ca2+ from the SR. Once release has occurred, the Ca2+ channels close and Ca2+ is taken back into the SR by the SR Ca2+-ATPase (SERCA). Work in recent years has shown, however, that it is too simplistic to think of the RyR as being shut in diastole and open in systole. There is evidence that the SR leaks calcium even in the absence of stimulation and this can be measured directly (Smith et al. 2000). Such a leak has several important consequences. (1) Since leak increases with SR Ca2+ content, it may have a role in protecting against an excessive increase of SR Ca2+ content. (2) It will decrease the speed with which the SR refills during diastole and thereby decrease the SR Ca2+ content leading to a decrease of the amplitude of the systolic Ca2+ transient and thence of contractility. In this way a diastolic Ca2+ leak can be thought of as one of the factors that regulate SR Ca2+ content. Diastolic leak is increased in heart failure thereby decreasing SR Ca2+ content (Shannon et al. 2003). (3) Leak of Ca2+ from the SR during diastole has been implicated in the initiation of waves of CICR that trigger various arrhythmias (see Venetucci et al. 2008 for a recent review). It is therefore important to understand which fluxes make up the diastolic leak of Ca2+ from the SR. Previous work has shown that fluxes through the RyR are not the only contributor to Ca2+ efflux from the SR. In an article in a recent issue of The Journal of Physiology, Zima et al. (2010) now show that the contribution of the RyR to Ca2+ efflux depends critically on the SR Ca2+ content. Below about 330 μm SR free Ca2+, the RyR accounts for less than 50% of the Ca2+ efflux from the SR. Interestingly, not all of the RyR-dependent component of Ca2+ leak is seen as Ca2+ sparks. The fraction of total Ca2+ efflux that can be accounted for by Ca2+ sparks is very low at low SR Ca2+ and even at 900 μm SR Ca2+ it only accounts for 50% of efflux from the SR. Previous work measuring SR Ca2+ content found that at a stimulation rate of 0.1 Hz it was 500 μm, increasing to about 1500 μm at 1 Hz (Guo et al. 2007). At the lower end of this range then most of the Ca2+ leak from the SR will be occurring through the RyR but not in the form of Ca2+ sparks. It is important, however, to note that at a normal rabbit heart rate (∼3–4 Hz) the SR Ca2+ content and spark-mediated Ca2+ leak will be increased. One important take home message from this paper is that it is unwise to take spark frequency as a measure of Ca2+ leak from the SR. The observation that leak depends on SR Ca2+ raises the question of how it is affected by cytoplasmic Ca2+ and whether such effects are physiologically or pathophysiologically important. Finally, it should also be noted that Zima et al used skinned myocytes. It would be useful (albeit technically challenging) to repeat the experiments in intact cells. The observation of increased SR Ca2+ leak in heart failure raises two questions. (1) To what extent does the observed decrease of SR Ca2+ content result from increased SR Ca2+ leak as opposed to a decrease of SERCA activity? Much early work found a decrease in SERCA activity and this is one explanation for decreased SR Ca2+ content (reviewed by Hasenfuss, 1998). Subsequent work, however, demonstrated the importance of increased SR leak and one study (Belevych et al. 2007), indeed, found that SR leak was essentially the only factor responsible for reduced SR Ca2+. One general point is that much previous work has been performed at rates below normal heart rate. Under these conditions, the diastolic period is longer than will be the case at normal heart rate and therefore the effects of elevated Ca2+ leak on diastolic Ca2+ content may well be expected to be greater. It is therefore of paramount importance that future work studies Ca2+ leak and SR content under physiological conditions. We should be grateful to Zima et al for providing the approach to make this possible.