Evidence for Na(+)−Ca2+ exchange and Ca(2+)−induced Ca2+ release in a cultured vascular smooth muscle cell line from the rat

Measurements of intracellular calcium (Cai2+) and sodium (Nai+) have been made in single smooth muscle cells from the rat aortic cell line (A10) using the Ca(2+)‐ and Na(+)‐sensitive dyes Fura‐2 and SBFI (sodium‐binding benzofuran isophthalate). The effects of manipulation of intracellular and extracellular Na+ on Cai2+ have been investigated. Reversal of the Na+ gradient in control cells does not result in any measurable increase in Cai2+ 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 Ca(2+)‐handling mechanisms. Reversal of the Na+ gradient in ouabain‐pre‐treated cells results in a transient increase in Cai2+ 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 Ca(2+)‐induced Ca(2+)‐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 Ca(2+)‐induced Ca2+ release by internal Na+ is discussed.