EVIDENCE FOR NA+-CA-2+ EXCHANGE AND CA-2+-INDUCED CA-2+ RELEASE IN A CULTURED VASCULAR SMOOTH-MUSCLE CELL-LINE FROM THE RAT

Measurements of intracellular calcium (Ca(i)2+) and sodium (Na(i)+) have been made in single smooth muscle cells from the rat aortic cell line (A1O) using the Ca2+- and Na+-sensitive dyes Fura-2 and SBFI (sodium-binding benzofuran isophthalate). The effects of manipulation of intracellular and extracellular Na+ on Ca(i)2+ have been investigated. Reversal of the Na+ gradient in control cells does not result in any measurable increase in Ca(i)2+ or change in the rate of recovery of the cells from agonist stimulation, suggesting that there is little functional Na+-Ca2+ exchange. In ouabain-pre-treated cells however, the recovery from agonist stimulation is significantly slowed, suggesting that in the presence of an elevated intracellular Na+ concentration there is an alteration in the Ca2+-handling mechanisms. Reversal of the Na+ gradient in ouabain-pre-treated cells results in a transient increase in Ca(i)2+ followed by a slow secondary rise. The transient component of this rise is absent on a second activation of the cell or by prior mobilization of the intracellular stores of Ca2+ by agonist. Data presented in this paper suggest the possibility that the transient component is due to a Ca2+-induced Ca2+-release mechanism triggered by an initial influx of Ca2+. The mechanism underlying this influx is not known but may involve the Na+-Ca2+ exchanger operating in reverse. The possible modulation of the Na+-Ca2+ exchanger and Ca2+-induced Ca2+ release by internal Na+ is discussed.