Cytosolic free magnesium concentration ([Mg²⁺]_i) in rat cardiac myocytes is kept at approximately 0.8 mM, a level well below electrochemical equilibrium, probably by the operation of a Na⁺/Mg²⁺ antiport (Handy et al. 1996; Tashiro & Konishi, 2000). In order to study the Mg²⁺ transporters we raised [Mg²⁺]_i above the resting level and observed the recovery process. We found that [Mg²⁺]_i can be increased substantially above the resting level by superfusing myocytes with a Ca²⁺– and Na⁺-free medium, with a high Mg²⁺ content (Handy et al. 1996). Others, however, were not able to achieve such effective loading (Tashiro & Konishi, 2000). A major difference between these studies is temperature: we worked at 37°C whereas they worked at 25°C. Here we investigate the effects of temperature on both Mg²⁺ loading and efflux in isolated rat ventricular myocytes.

Hearts were collected from anaesthetised (Na⁺ pentobarbitone, 120 mg kg^-1, I.P.) adult Sprague-Dawley rats (250-400 g), and myocytes isolated and loaded at room temperature with the Mg²⁺-sensitive dye, mag-fura-2 as the AM ester (Handy et al. 1996). [Mg²⁺]_i was measured by microfluorimetry using dual excitation at 340 and 380 nm. Myocytes were exposed to sequential Mg²⁺ loading (superfusion with a Na⁺– and Ca²⁺-free medium containing 30 mM Mg²⁺) and recovery (superfusion with normal Tyrode containing 140 mM Na⁺, 1 mM Ca²⁺ and 1 mM Mg²⁺) protocols at 37 and 25 °C (Fig. 1). All media contained 6 mM K⁺ and 10 mM Hepes, pH 7.4. Where necessary Na⁺ was replaced by N-methyl-D-glucamine. Whereas rapid, reproducible Mg²⁺ loading was observed at 37°C (uptake rate: 0.33 ± 0.06 mM min^-1 (mean ± S.E.M., n = 8)), uptake at 25 °C, measured in the same cell, was significantly slower (0.02 ± 0.01 mM min ^-1, P < 0.001, paired t test), and in many cases could not be detected.

The effect of temperature on Mg²⁺ efflux, was investigated by loading myocytes with Mg²⁺ at 37 °C and then following the recovery of [Mg²⁺]_i when they were superfused with normal Tyrode solution at 25 °C. Efflux was much faster at 37 °C (efflux rate constant: 0.26 ± 0.03 min^-1, n = 6) than at 25 °C (0.10 ± 0.02 min^-1, n = 6, P < 0.001, unpaired t test).

In conclusion, Mg²⁺ loading is highly temperature dependent and is not routinely observed at 25°C. Mg²⁺ efflux, probably mediated by Na⁺/Mg²⁺ antiport, has a Q₁₀ of approximately 2.2.

This work was supported by the British Heart Foundation.