Dissociation Between Force and Calcium after a Step Change in Frequency

Abstract

The force frequency response (FFR) is one of the major physiological regulators which govern alterations in contractile function in the body in which an increase in frequency of pacing leads to an increase in force production and a decrease in rate of contractile kinetics. Typically studies on FFR have been conducted at steady state where the changes in contractile function have been elicited by changes in myofilament phosphorylation. Here we aim to determine the role of alterations in calcium homeostasis and myofilament sensitization during the transition from one steady state of contractile function to another.Trabeculae, excised from the right ventricular free wall of New Zealand white rabbits were loaded with Rhod-AM calcium indicator dye and stimulated to contract from 1 to 4 Hz. Data was analyzed through customized Labview software.We hypothesize that the transition from one steady state to another occurs in two phases; the rapid/early phase is due to increases in calcium transient amplitude and diastolic calcium levels, and the gradual/latent phase is due to alterations in phosphorylation state of myofilament proteins such as troponin I (TnI) and myosin light chain 2 (MLC2). During our experiments we are able to measure calcium and developed force simultaneously. Despite previous studies which show alterations in myofilament calcium sensitivity occurring at steady state, we are able to show that during the immediate phase calcium and force change with each other, while at the latent phase there is an increase in diastolic calcium level that is not paralleled by the diastolic force. This suggests a myofilament desensitization during this latent phase that is not seen in the early transitory period as originally thought.