Cardiac fibroblasts are electrically non-excitable cells with a resting membrane potential that is sensitive to mechanical stretch (Kiseleva et al. 1998). Increased sensitivity of the membrane potential of atrial fibroblasts to mechanical stretch has been implicated in the pathophysiology of cardiac arrhythmia after myocardial infarction (Kamkin et al. 2002). The cellular mechanisms that underlie the membrane potential changes of cardiac fibroblasts in response to tissue ischaemia are unknown. In this study we analysed the effect of hypoxia/reoxygenation, which are major components of tissue ischaemia/reperfusion, on the electrical function of atrial fibroblasts. Intracellular microelectrode recordings were performed simultaneously with isometric force measurements on spontaneously contracting right atrial tissue preparations from adult rat hearts, which were excised under deep ether anaesthesia. The standard perfusate contained (mmol l^-1): 118 NaCl, 2.7 KCl, 1.2 CaCl₂, 1.2 MgSO₄, 2.2 NaH₂PO₄, 25 NaHCO₃ and 5 glucose. The osmolarity of the solution was 290 ± 3 mosmol; pH was adjusted to 7.4. The perfusate was bubbled either with carbogen (5 % CO₂ and 95 % O₂) to adjust P_O2 to ~80 kPa, or with a 5 % CO₂ and 95 % N₂ mixture to lower the oxygen tension to 3 kPa. Lowering the oxygen tension in the extracellular perfusate increased resting force (RF), dramatically reduced active force (AF) development (Table 1), and decreased the resting membrane potential (E_m) of the cardiac fibroblasts from -23 ± 5 to -16 ± 4 mV (Table 2, n = 35).

The following reoxygenation led to the following decrease in E_m to -5 ± 2 mV. These electrical changes were associated with increased frequency of spontaneous contractions of the right atrial preparations during hypoxia (Table 2). The further tissue reoxygenation increased the resting membrane potential to maximally -60 ± 8 mV and significantly lowered rhythmic contractile activity. The nature of depolarization during hypoxia connected with the compression of fibroblasts that results in opening of mechanosensitive channels. This possibility is shown in isolated cardiac fibroblasts during the compression and stretch. The double increase E_m cannot be explained by the decrease in RF to initial values. These mechanisms may include a rise of cytoplasmic Ca²⁺, which is thought to contribute to impaired contractile function of the myocardium during acute ischaemia/reperfusion injury (Jennings et al. 1985). Increased intracellular Ca²⁺ may activate Ca²⁺-operated K⁺ channels and thereby hyperpolarize the membrane potential of the fibroblasts (Brooks et al. 1995; Kiseleva et al. 1996). Our findings indicate that the membrane potential of cardiac fibroblasts and the contractile function of the atrial myocardium are sensitive to hypoxia. It is suggested that alterations in the electrical function of atrial fibroblasts may contribute to cardiac arrhythmia during ischaemia/reperfusion injury of the heart.