The effects of modulating Ca2+‐induced Ca2+ release (CICR) in single cardiac myocytes were investigated using low concentrations of caffeine (< 500 μm) in reduced external Ca2+ (0.5 mm). Caffeine produced a transient potentiation of systolic [Ca2+]i (to 800 % of control) which decayed back to control levels. Caffeine decreased the steady‐state sarcoplasmic reticulum (SR) Ca2+ content. As the concentration of caffeine was increased, both the potentiation of the systolic Ca2+ transient and the decrease in SR Ca2+ content were increased. At higher concentrations, the potentiating effect decayed more rapidly but the rate of recovery on removal of caffeine was unaffected. A simple model in which caffeine produces a fixed increase in the fraction of SR Ca2+ which is released could account qualitatively but not quantitatively for the above results. The changes in total [Ca2+] during systole were obtained using measurements of the intracellular Ca2+ buffering power. Caffeine initially increased the fractional release of SR Ca2+. This was followed by a decrease to a level greater than that under control conditions. The fraction of systolic Ca2+ which was pumped out of the cell increased abruptly upon caffeine application but then recovered back to control levels. The increase in fractional loss is due to the fact that, as the cytoplasmic buffers become saturated, a given increase in systolic total[Ca2+] produces a larger increase in free [Ca2+] and thence of Ca2+ efflux. These results confirm that modulation of the ryanodine receptor has no maintained effect on systolic Ca2+ and show the interdependence of SR Ca2+ content, cytoplasmic Ca2+ buffering and sarcolemmal Ca2+ fluxes. Such analysis is important for understanding the cellular basis of inotropic interventions in cardiac muscle.

1. The aim of these experiments was to compare the time course of changes in intracellular Ca2+ concentration ([Ca2+]i) measured in the bulk cytoplasm with those estimated to occur near the sarcolemma. Sarcolemmal Na(+)-Ca2+ exchange current and [Ca2+]i were measured in single, voltage-clamped ventricular myocytes. 2. Spontaneous Ca2+ release from the sarcoplasmic reticulum (SR) resulted in a transient inward current. This current developed and decayed more quickly than the accompanying changes in [Ca2+]i (measured with indo-1) resulting in a hysteresis between [Ca2+]i and current. A similar hysteresis was also observed if [Ca2+]i was elevated with caffeine and was removed if the current was low pass filtered with a time constant of 132 ms. 3. Digital video imaging (using fluo-3 or calcium green-1 to measure [Ca2+]i) allowed measurement of [Ca2+]i at all points in the cell during the wave of spontaneous Ca2+ release. The hysteresis between [Ca2+]i and current remained, even after allowing for the spatial and temporal properties of this wave. 4. The hysteresis can be accounted for if there is a barrier to diffusion of Ca2+ ions separating the bulk cytoplasm from the space under the sarcolemma (into which Ca2+ is released from the sarcoplasmic reticulum). The calculated subsarcolemmal [Ca2+] rises and falls more quickly (and reaches a higher peak) than does the bulk [Ca2+]. The delay introduced by this barrier is equivalent to a time constant of 133 ms. 5. The subsarcolemmal space described in this paper may be equivalent to the 'fuzzy space' previously suggested to be important in controlling SR Ca2+ release.

1. Caffeine was applied locally to one region of a resting cell via an extracellular pipette while simultaneously imaging the concentrations of intracellular calcium ([Ca2+]i) and intracellular caffeine ([caffeine]i). 2. Local application of caffeine produced a rise of [caffeine]i which was confined to the region of the cell near the pipette. There was also a local increase of [Ca2+]i which then, in most resting cells, propagated along the cell as a linear Ca2+ wave. The initial magnitude of the rise of [Ca2+]i was greater than that of the electrically stimulated Ca2+ transient. 3. As the wave of increase of [Ca2+]i propagated along the cell it decreased in both amplitude and velocity in cells that had not been treated to elevate the cellular Ca2+ load. 4. In some cells the caffeine response did not propagate significantly. In these cases an increase of the cellular Ca2+ load enabled caffeine‐induced Ca2+ wave propagation along the entire cell length without significant decay in amplitude and velocity. 5. Previous work has shown that an electrically evoked local systolic Ca2+ transient does not propagate. The fact that the caffeine‐evoked response does propagate and the correlation between decay of amplitude and velocity suggest that the transient has to be a certain size before it can propagate. It is suggested that one of the factors which favour propagation of waves under conditions of elevated sarcoplasmic reticulum Ca2+ content is the increased release of Ca2+.

Genetic cardiac diseases are major causes of morbidity and mortality. Although animal models have been created to provide some useful insights into the pathogenesis of genetic cardiac diseases, the significant species differences and the lack of genetic information for complex genetic diseases markedly attenuate the application values of such data. Generation of induced pluripotent stem cells (iPSCs) from patient-specific specimens and subsequent derivation of cardiomyocytes offer novel avenues to study the mechanisms underlying cardiac diseases, to identify new causative genes, and to provide insights into the disease aetiology. In recent years, the list of human iPSC-based models for genetic cardiac diseases has been expanding rapidly, although there are still remaining concerns on the level of functionality of iPSC-derived cardiomyocytes and their ability to be used for modeling complex cardiac diseases in adults. This review focuses on the development of cardiomyocyte induction from pluripotent stem cells, the recent progress in heart disease modeling using iPSC-derived cardiomyocytes, and the challenges associated with understanding complex genetic diseases. To address these issues, we examine the similarity between iPSC-derived cardiomyocytes and their ex vivo counterparts and how this relates to the method used to differentiate the pluripotent stem cells into a cardiomyocyte phenotype. We progress to examine categories of congenital cardiac abnormalities that are suitable for iPSC-based disease modeling.

This review discusses the control of the amplitude of the cardiac systolic Ca transient. The Ca transient arises largely from release from the sarcoplasmic reticulum (SR). Release is triggered by calcium-induced calcium release (CICR) whereby the entry of a small amount of Ca on the L-type Ca current, "the trigger", results in the release of much more Ca from the SR. There are three potential control points: (1) the Ca content of the SR; (2) the properties of the SR Ca release channel or ryanodine receptor (RyR); (3) the amplitude of the L-type Ca current. The data reviewed show that the Ca content of the SR has pronounced effects on systolic [Ca2+]i and, reciprocally, the amount of Ca released from the SR affects sarcolemmal Ca fluxes thereby "autoregulating" SR content. Modulation of the ryanodine receptor has no steady-state effect due to compensating changes of SR Ca content. An increase of the L-type Ca current results in an abrupt increase of systolic [Ca2+]i with little change of SR content. This is because of a coordinated increase of both the trigger and loading function of the Ca current. These results emphasise the importance of considering all aspects of Ca handling in the context of SR Ca release and thus the regulation of the systolic Ca transient and contraction in cardiac muscle.