Changes in myoplasmic sodium concentration during exposure to lactate in perfused rat heart

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.

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.

The [Na+]i under control conditions was 6.4 mM. When the hearts were exposed to 20 mM lactate at the normal extracellular pH (pHo = 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 (pHo = 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 pHo 7.4 and 6.4.

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 pHo 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.