The objective of our study was to investigate how Mg2+ enters mammalian cardiac cells. During this work, we found evidence for a previously undescribed route for Mg2+ entry, and now provide a preliminary account of its properties. Changes in Mg2+ influx into rat ventricular myocytes were deduced from changes in intracellular ionized Mg2+ concentration ([fMg2+]i) measured from the fluorescence of mag‐fura‐2 loaded into isolated cells. Superfusion of myocytes at 37°C with Ca2+‐free solutions with both reduced [Na+] and raised [Mg2+] caused myocytes to load with Mg2+. Uptake was seen with solutions containing 5 mm Mg2+ and 95 mm Na+, and increased linearly with increasing extracellular [Mg2+] or decreasing extracellular [Na+]. It was very sensitive to temperature (Q10 > 9, 25‐‐37°C), was observed even in myocytes with very low Na+ contents, and stopped abruptly when external [Na+] was returned to normal. Uptake was greatly reduced by imipramine or KB‐R7943 if these were added when [fMg2+]i was close to the physiological level, but was unaffected if they were applied when [fMg2+]i was above 2 mm. Uptake was also reduced by depolarizing the membrane potential by increasing extracellular [K+] or voltage clamp to 0 mV. We suggest that initial Mg2+ uptake may involve several transporters, including reversed Na+–Mg2+ antiport and, depending on the exact conditions, reversed Na+–Ca2+ antiport. The ensuing rise of [fMg2+]i, in conjunction with reduced [Na+], may then activate a new Mg2+ transporter that is highly sensitive to temperature, is insensitive to imipramine or KB‐R7943, but is inactivated by depolarization.

Our objectives were to investigate regulation of intracellular ionised Mg2+ concentration ([fMg2+]i) in cardiac muscle and cardiac Na+/Mg2+ antiport stoichiometry. [fMg2+]i was measured at 37°C in isolated rat ventricular myocytes with mag-fura-2. Superfusion of myocytes with Na+ and Ca2+ free solutions containing 30 mM Mg2+ for 15 min more than doubled [fMg2+]i from its basal level (0.75 mM). Re-addition of Na+ caused [fMg2+]i to fall exponentially with time to basal level, the rate increasing linearly with [Na+]. Log(recovery rate) increased linearly with log([Na+]), the slope of 1.06 (95% confidence limits, 0.94–1.17) suggesting one Na+ ion is exchanged for each Mg2+. [fMg2+]i recovery was complete even if the membrane potential was depolarised to 0 mV or if superfusate [Mg2+] was increased to 3 mM. Recovery was rapid in normal Tyrode (0.3 min−1) with a Q10 of 2.2. It was completely inhibited by 200 μM imipramine but was unaffected by 20 μM KB-R7943 or 1 μM SEA0400, suggesting the Na+ /Ca2+ antiporter is not involved. Membrane depolarisation by increasing superfusate [K+] to 70 mM, or voltage clamp to 0 mV, increased recovery rate in Na+ containing solutions more than threefold. We conclude [fMg2+]i recovery is by Mg2+ efflux on a 1 Na+:1 Mg2+ antiport.

1. The intracellular Na activity of sheep heart Purkinje fibres has been measured using recessed‐tip Na+‐sensitive glass micro‐electrodes.2. The internal Na activity was 7·2 ± 2·0 m M (mean ± S.D., n = 32) at the normal external Na concentration, [Na]o, in these experiments of 140 m M (equivalent to an external Na activity of 105 m M). The equilibrium potential for Na across the fibre membrane was therefore approximately + 70 mV.3. When the [K]o was altered the internal Na activity changed, reaching a new level within about 20 min. Increasing the [K]o from 4 to 25 m M decreased the internal Na by approximately 30%, while decreasing the [K]o from 4 to 1 m M increased internal Na by 20%.4. The removal of external K produced an easily reversible increase in the internal Na with an initial rate equivalent to a concentration change of 0·24 ± 0·07 m‐mole/min (mean ± S.D., n = 8).5. Ouabain produced increases in the internal Na activity that were only very slowly reversible. The threshold concentration for producing an increase was approximately 10−7 M.6. When [Na]o was reduced the internal Na activity fell rapidly with a single exponential time course (time constant 3·3 ± 0·8 min, mean ± S.D., n = 16) to a new, relatively stable level. The recovery of internal Na on return to the normal [Na]o did not have a simple time course. It was normally complete within 10‐30 min.7. The relationship of the stabilized level of the internal Na activity to the [Na]o was approximately linear over the range 140‐14 m M‐[Na]o. When [Na]o was reduced from 140 to 14 m M the internal Na activity fell by 72 ± 5% (mean ± S.D., n = 21).8. When the [Na]o was reduced, the decrease in the internal Na activity was partially inhibited by Mn or by removal external Ca.9. When the [Ca]o was altered over the range 0·2‐16 m M the internal Na activity was reduced by approximately 50% for a tenfold increase in the [Ca]o.10. The relationship between internal Na and contractility is discussed.

This chapter discusses the regulation of the intracellular sodium and pH in mammalian cardiac tissue studied. The performance of the mammalian heart during its excitation-contraction cycle is greatly influenced by the movements of ions across the cell membrane and between intracellular compartments. The key ion for regulating the strength of the heart beat is Ca2+. Other ions such as Na+, H+, and K+ have also been found to affect cardiac performance. The maintenance of extra intracellular and intracellular ionic homeostasis in cardiac tissue is important in providing the conditions for a normal cyclic heart contraction. The chapter describes an experiemnt in which Na+ and pH-sensitive microelectrodes were used to study the regulation of intracellular Na+ and of intracellular pH in quiescent sheep heart Purkinje fibers. These types of electrodes allow continuous monitoring of the intracellular Na+ and intracellular pH simultaneously. The ion-sensitive microelectrodes, namely, Na+ and pH, were prepared. The response times of the ion-sensitive microelectrodes used in the experiments were much faster than the changes in the intracellular ion activities measured. The time courses of these intracellular ion changes measured were, therefore, not limited by the response time of the electrodes used. It appears that the Na gradient across the cell membrane does affect the regulation of the intracellular pH under certain conditions.

Removal of external potassium (Ko), or addition of the cardioactive steroid strophanthidin (10−5 M) resulted in a reversible rise in aNai.