Heterogeneous changes in action potential and intracellular Ca2+ in left ventricular myocyte sub-types from rabbits...

Myocardial cellular electrophysiology and intracellular Ca2+ regulation are altered in heart failure. The extent of these changes may vary within the layers of the ventricular wall. To examine this, cell size, action potential and intracellular Ca2+ transient characteristics (Fura-2) were measured in single cardiac myocytes from sub-epicardial, mid-myocardial, and sub-endocardial regions of the left ventricle of rabbits with heart failure.

Myocytes were isolated from animals with heart failure induced by chronic coronary artery ligation and from sham operated controls. Trans-membrane potential was measured using high resistance microelectrodes electrodes (30 M omega; 2 M KC1). Fura-2 was loaded into cells by incubation with the AM form. Subsequent fluorescence measurements were used to measure intracellular Ca2+ concentration at a range of stimulus frequencies.

Resting cell length was significantly greater in the heart failure group; approximately 115% of control values in sub-epicardial and mid-myocardial cells, and approximately 108% in sub-endocardial cells. Using criteria described by previous studies on other mammalian hearts, functional M cells were identified by a higher maximum rate of depolarisation and longer action potential duration at 90% repolarisation (APD90) compared to the two other myocyte sub-types. In the heart failure group, APD90 and Ca2+ transient duration (CaD50) were prolonged in sub-epicardial and M cells but shortened in sub-endocardial myocytes. These changes were significant at lower stimulus frequencies, but the relative effect diminished at higher frequencies (3 Hz). Peak systolic [Ca2+] was reduced in sub-epicardial and M cells but increased in sub-endocardial cells in the heart failure group compared to controls. At higher stimulus frequencies, end diastolic Ca2+ levels were lower in sub-epicardial cells but higher in sub-endocardial myocytes of the heart failure group compared with controls. In general, changes were greater in heart failure animals with more severe in vivo ventricular dysfunction (ejection fraction < or = 44%).

Heart failure was associated with an increased cell size throughout the left ventricle, but the form of the changes in electrophysiology and Ca2+ transient were dependent on the myocyte sub-type. In particular sub-endocardial cells displayed markedly different changes compared to the other myocyte sub-types.