We employ a coupled electromechanical mathematical model to better understand the biophysical basis of the force-frequency response (FFR) in rat ventricular myocytes under voltage clamp conditions. The model extends our previous work on calcium signaling in the cardiac dyad, and the regulation of Ca2+-concentration in the myoplasm. The present work is focused on achieving a better understanding of mechanisms involved in the rat FFR. Specifically, we examine the role of calmodulin (CaM) in modulating the key control variables Ca2+/calmodulin-dependent protein kinase-II (CaMKII), calcineurin (CaN), and cyclic adenosine monophosphate (cAMP), as they mediate a rate-dependent effect on various intracellular targets controlling the FFR.Our electrochemical model consists of an electrical-equivalent model for the cell membrane; dyadic, myoplasmic and sarcoplasmic reticulum (SR) fluid-compartments; and a modified model of the contractile system by Rice et al. We incorporate frequency-dependent CaM-mediated and spatially heterogenous interaction of the proteins CaMKII and CaN with their principal targets (dihydropyridine (DHPR) and ryanodine (RyR) receptors, and the SERCA pump). Also included are the rate-dependent effects of phospholamban (PLB) on SERCA pump; cAMP on the DHP-sensitive Ca2+ channel; and the enhancement in SERCA pump activity via phosphorylation of PLB.Investigators using multicellular rat ventricular preparations have recorded both positive and negative peak FFRs. Under specific conditions, our VC model can generate either a positive or a negative FFR, while providing mechanistic understanding of its genesis. In addition, the model provides quantitative insight into rate-dependence of CICR by investigating the frequency-dependence of each contributing factor. Since several aforementioned multicellular studies were conducted at different temperatures, we also investigated the temperature-dependence of FFR.Our modeling study suggests that cAMP-mediated stimulation and rate-dependent CaMKII-mediated up-regulation of the trigger current ICa,L are key mechanisms underlying the inconsistency in FFR observations in multicellular rat ventricular tissue.