CHANGES IN MYOPLASMIC SODIUM CONCENTRATION DURING EXPOSURE TO LACTATE IN PERFUSED RAT-HEART

Objective: The free myoplasmic sodium concentration ([Na+](i)) is thought to change during acidosis and ischaemia in the mammalian heart. Although there are methods available for measuring [Na+](i) in isolated cells or small preparations, none is very effective in the isolated heart, which is one of the best models for studying ischaemia. The use of a fluorescent sodium indicator, sodium binding benzofuran isophthalate (SBFI), to measure [Na+](i) in the isolated, perfused rat heart is described. This method was used to estimate the activity of the Na/H exchanger under conditions resembling some aspects of ischaemia. Methods: The acetoxymethyl ester of SBFI was loaded into the perfused rat heart via the perfusate. Fluorescence signals developed on loading and were shown to be [Na+](i) sensitive. An in vivo calibration technique allowed the fluorescence signals to be converted to [Na+](i). Results: The [Na+](i) under control conditions was 6.4 mM. When the hearts were exposed to 20 mM lactate at the normal extracellular pH (pH(o) = 7.4) for 10 min, developed pressure fell abruptly and then slowly recovered [Na+](i) rose slowly by 1.5 mM during lactate exposure and paralleled the recovery of pressure. Exposure to lactate at reduced extracellular pH (pH(o) = 6.4) produced a larger rise in [Na+](i). In the presence of the Na/H exchange inhibitor 5-(N-methyl-N-isobutyl)-amiloride exposure to lactate caused a small fall in [Na+](i) at pH(o) 7.4 and 6.4. Conclusions: The advantages and disadvantages of SBFI as an [Na+](i) indicator in the perfused heart are described. The Na/H exchanger is an important regulator of [Na+](i) during lactate induced intracellular acidosis, and it remains active in the presence of a substantial extracellular acidosis. This shows that low pH(o) does not necessarily inhibit the Na/H exchanger. The use of SBFI to estimate [Na+](i) in the perfused heart is novel and has considerable potential for studies of ischaemia and related conditions.