Key pointsr In the majority of species, including humans, increased heart rate increases cardiac contractility.This change is known as the force-frequency response (FFR). The majority of mammals have a positive force-frequency relationship (FFR). In rat the FFR is controversial.r We derive a species-and temperature-specific data-driven model of the rat ventricular myocyte. r As a measure of the FFR, we test the effects of changes in frequency and extracellular calcium on the calcium-frequency response (CFR) in our model and three altered models.r The results show a biphasic peak calcium-frequency response, due to biphasic behaviour of the ryanodine receptor and the combined effect of the rapid calmodulin buffer and the frequency-dependent increase in diastolic calcium.r Alterations to the model reveal that inclusion of Ca 2+ /calmodulin-dependent protein kinase II (CAMKII)-mediated L-type channel and transient outward K + current activity enhances the positive magnitude calcium-frequency response, and the absence of CAMKII-mediated increase in activity of the sarco/endoplasmic reticulum Ca 2+ -ATPase induces a negative magnitude calcium-frequency response.Abstract An increase in heart rate affects the strength of cardiac contraction by altering the Ca 2+ transient as a response to physiological demands. This is described by the force-frequency response (FFR), a change in developed force with pacing frequency. The majority of mammals, including humans, have a positive FFR, and cardiac contraction strength increases with heart rate. However, the rat and mouse are exceptions, with the majority of studies reporting a negative FFR, while others report either a biphasic or a positive FFR. Understanding the differences in the FFR between humans and rats is fundamental to interpreting rat-based experimental findings in the context of human physiology. We have developed a novel model of rat ventricular electrophysiology and calcium dynamics, derived predominantly from experimental data recorded under physiological conditions. As a measure of FFR, we tested the effects of changes in stimulation frequency and extracellular calcium concentration on the simulated Ca 2+ transient characteristics and showed a biphasic peak calcium-frequency relationship, consistent with recent observations of a shift from negative to positive FFR when approaching the rat physiological frequency range. We tested the hypotheses that (1) and (3) Na + /K + pump dynamics play a significant role in the rat FFR. The results reveal a major role for CAMKII modulation of SERCA in the peak Ca 2+ -frequency response, driven most significantly by the cytosolic calcium buffering system and changes in diastolic Ca 2+ .

Key pointsr At the cellular level cardiac hypertrophy causes remodelling, leading to changes in ionic channel, pump and exchanger densities and kinetics.r Previous studies have focused on quantifying changes in channels, pumps and exchangers without quantitatively linking these changes with emergent cellular scale functionality.r Two biophysical cardiac cell models were created, parameterized and validated and are able to simulate electrophysiology and calcium dynamics in myocytes from control sham operated rats and aortic-banded rats exhibiting diastolic dysfunction. r Results show that the ability to dynamically change systolic Ca 2+ , through changes in expression of key Ca 2+ modelling protein densities, is drastically reduced following the aortic banding procedure; however the cells are able to compensate Ca 2+ homeostasis in an efficient way to minimize systolic dysfunction.Abstract Elevated left ventricular afterload leads to myocardial hypertrophy, diastolic dysfunction, cellular remodelling and compromised calcium dynamics. At the cellular scale this remodelling of the ionic channels, pumps and exchangers gives rise to changes in the Ca 2+ transient. However, the relative roles of the underlying subcellular processes and the positive or negative impact of each remodelling mechanism are not fully understood. Biophysical cardiac cell models were created to simulate electrophysiology and calcium dynamics in myocytes from control rats (SHAM) and aortic-banded rats exhibiting diastolic dysfunction. The model parameters and framework were validated and the fitted parameters demonstrated to be unique for explaining our experimental data. The contribution of each ionic pathway to the calcium kinetics was calculated, identifying the L-type Ca 2+ channel (LCC) and the sarco/endoplasmic reticulum Ca 2+ -ATPase (SERCA) as the principal regulators of systolic and diastolic Ca 2+ , respectively. In the aortic banding model, the sensitivity of systolic Ca 2+ to LCC density and diastolic Ca 2+ to SERCA density decreased by 16-fold and increased by 23%, respectively, relative to the SHAM model. The energy cost of ionic homeostasis is maintained across the two models. The models predict that changes in ionic pathway densities in compensated aortic banding rats maintain Ca 2+ function and efficiency. The ability to dynamically alter systolic function is significantly diminished, while the capacity to maintain diastolic Ca 2+ is moderately increased.

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