Calcium handling in rabbit ventricular myocytes: Regional differences and changes after myocardial infarction

BACKGROUND: Calcium (Ca) has a central role in the mechanical and electrical properties of cardiac myocytes. This thesis examines cellular Ca handling in a rabbit infarct model of heart failure (8 weeks after coronary artery ligation), which exhibits left ventricular dysfunction (LVD), pulmonary and hepatic congestion and a propensity to arrhythmias. AIMS: The main aims of this thesis were (i) to determine the properties of Ca handling in single cells from different transmural regions in sham-operated (Sham) and postinfarction animals, and (ii) to identify the underlying mechanisms behind any differences seen. METHODS: Voltage clamp studies were performed at 37 °C on enzymatically-isolated single cells from the sub-endocardium (Endo) and sub-epicardium (Epi), loaded with the Ca indicator Fura-2. Baseline Ca transient characteristics were determined at different stimulation frequencies. Sodium-calcium exchanger (NCX) activity was estimated from the rate constant (RC) of Ca decay during 10 mmol/L caffeine application. Sarco(endo)plasmic reticulum Ca ATPase (SERCA) function was estimated from the RC of Ca decay after brief caffeine application in the presence of 10 mmol/L Ni. SR Ca content was calculated from the integral of the NCX current induced by caffeine application, and further analysis was used to quantify Ca buffering. These measures were each performed at steady state and were corrected for non-NCX Ca removal mechanisms. NCX current density was measured as the Ni-sensitive current in response to a ramp protocol, with major interfering currents blocked and intracellular [Ca] buffered to ~250 nmol/L. Allosteric regulation of NCX was studied by examining the outward NCX current through a range of [Ca], using a voltage step protocol. Finally, integrative analysis was used to determine Ca fluxes during systole and diastole, and separate protocols were used to perturb excitation-contraction (E-C) coupling to give further insights into cellular Ca handling. RESULTS: For both Sham and LVD animals, transmural differences in NCX were found, with higher current densities in Endo cells. In Sham animals, SERCA activity showed the converse, with greater Ca uptake in Epi cells, and this was associated with a higher SR Ca content in this region. In LVD cells, Ca transients were, in general, smaller compared to Sham, and exhibited a slower Ca decay. For both Endo and Epi regions, these findings were associated with reduced SR Ca content in LVD, and lower NCX current density and activity. For Epi cells, SERCA activity was also reduced in LVD. Studies of allosteric regulation of NCX by Ca showed that in LVD cells the exchanger was more sensitive to [Ca]. When Ca fluxes were studied, no difference in Ca entry via L-type Ca chamiels was found in LVD, but the gain of E-C coupling was reduced. This was likely to be due to the changes in SR Ca content. By studying E-C coupling in the presence of low concentrations of caffeine, there was some evidence of regional differences in SR Ca release. CONCLUSIONS; Ca handling in the rabbit heart shows a number of differences between sub-endocardial and sub-epicardial cells. Following myocardial infarction, significant changes occur which may contribute to the impaired contraction and relaxation observed in this model of LVD