Ca dependence of Na influx during treatment of rabbit aorta with NE and high K solutions.

Cellular influx of 24Na was measured in isolated rabbit aorta during stimulation with 10 microM norepinephrine (NE) or depolarization with 80 mM K solution, using a pulse-labeling, cold-wash technique. NE caused a two- to threefold increase in Na influx; a smaller but significant increase was also observed in depolarized tissues. Basal and NE-induced fluxes at 1 min were significantly increased by a 20-min preincubation in a Ca-free solution containing 2 mM EGTA; elevation of [Mg] in this solution reduced these effects. The high K-induced influx was prevented by a combination of low Ca (30 microns) and elevated Mg (10 mM). The Ca agonist, BAY-K 8644, increased 24Na influx. The Ca antagonist, diltiazem, inhibited the depolarization-stimulated 24Na influx in a concentration-dependent manner, but was less effective in blocking the response to NE. Extension of the preincubation in NE plus Ca-free medium from 30 s to 15 min decreased the influx response and contraction. After exposure to NE in Ca-free solution, 24Na influx remained elevated 10 min after washing out NE in the continued absence of Ca. A second exposure to NE at that time did not increase influx. We propose that a component of 24Na influx during excitation depends directly on a rise in intracellular [Ca]. The role of an indirect effect of [Ca] on metabolic H+ production with subsequent stimulation of the Na+-H+ exchange may also be a factor.