NMR studies of intracellular sodium ions in mammalian cardiac myocytes.

The unambiguous measurement of intracellular sodium ion [Na+]i by the noninvasive NMR technique offers a new opportunity to monitor precisely the maintenance and fluctuations of [Na+]i levels in intact cells and tissues. The anionic frequency shift reagent, dysprosium (III) tripolyphosphate, which does not permeate intact cells, when added to suspensions of intact adult rat cardiac myocytes, alters the NMR frequency of extracellular sodium ions, [Na+]o, leaving that of intracellular ions, [Na+]i, unaffected. Using 23Na NMR in conjunction with this shift reagent, we have determined NMR-visible intracellular Na+ ion concentration in a suspension of isolated cardiac myocytes under standard conditions with insulin and Ca2+ in the extracellular medium to be 8.8 +/- 1.2 mmol/liter of cells (n = 4). This value is comparable to that measured by intracellular ion-selective microelectrodes in heart tissue. Cardiac myocytes incubated for several hours in insulin-deficient, Ca2+-containing medium prior to NMR measurement exhibited a somewhat lower [Na+]i value of 6.9 +/- 0.5 mmol/liter of cells (n = 3). Reversible Na+ loading of the cells by manipulation of extracellular calcium levels is readily measured by the NMR technique. Incubation of myocytes in a Ca2+-free, insulin-containing medium causes a 3-fold increase in [Na+]i to a level of 22.8 +/- 2.6 mmol/liter of cells (n = 10). In contrast to cells with insulin, insulin-deficient myocytes exhibit a markedly lower level of [Na+]i of only 14.6 +/- 2.0 mmol/liter of cells (n = 4) in Ca2+-free medium. These observations suggest that insulin may stimulate a pathway for Na+ influx in heart cells.