Na +-dependent alkaline earth metal uptake in cardiac sarcolemmal vesicles

Abstract

The ability of alkaline earth metals (M 2+) to substitute for Ca 2+ in Na +-Ca 2+ exchange was examined in sarcolemmal vesicles isolated from the canine heart. 85Sr 2+ and 133Ba 2+, in addition to 45Ca 2+, were used to determine the characteristics of Na +-M 2+ exchange. The Na i +-dependent M 2+ uptake was measured as a function of time, with t ranging from 0.5 to 360 s, [ Na +] i = 140 mM and [M 2+] o = 40 μM . This function was linear for Ca 2+ and Sr 2+ uptake for approx. 6 s and for Ba 2+ for about 60 s. Plateau levels were achieved within 120 s for Ca 2+ and Sr 2+ but Ba 2+ took considerably longer. The K m values for Na +-M 2+ exchange, derived from Eadie-Hofstee plots, were 30, 58, and 73 μM for Ca 2+, Sr 2+ and Ba 2+, respectively. The Na i +-dependent uptake of all three ions was stimulated in the presence of 0.36 μM valinomycin. Na +-Ca 2+ exchange was also measured in the presence of either 20 μM Sr 2+ or 100 μM Ba 2+. Both of these ions behaved (at these concentrations) as competitive inhibitors of Na +-Ca 2+ exchange with the K I being 32 μM for Sr 2+ and 92 μM for Ba 2+. Passive efflux was determined by first allowing Na +-M 2+ exchange to continue to plateau values and then diluting the loaded vesicles in the presence of EGTA. The rate constants for the passive efflux were 8.4, 6.3 and 4.4 min −1 for Ca 2+, Sr 2+ and Ba 2+, respectively.