An increase in cytosolic free calcium (Cai) has been shown to occur early during ischemia in perfused rat, ferret, and rabbit hearts. It has been proposed that this increase in Cai may occur as a result of exchange of Nai for Cao, which occurs as a result of an increase in Nai arising from exchange of Nao for H+i. The latter exchange is stimulated by the intracellular acidification that occurs during ischemia. To test this hypothesis, we examined Cai, Nai, ATP, and pHi during ischemia in rats in the presence and absence of 1 mM amiloride, a Na-H exchange inhibitor. Cai was measured using 19F nuclear magnetic resonance (NMR) of 1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetra-acetic acid (5F-BAPTA)-loaded rat hearts. Nai was measured using 23Na NMR, and the shift reagent 1,4,7,10-tetraazacyclododecane-N,N',N",N"'-tetramethylenephosph onate (Tm[DOTP]-5) was used to separate Nai and Nao. ATP and pH were determined from 31P NMR measurements. During 20 minutes of ischemia, amiloride did not significantly alter the ATP decline but did significantly attenuate the rise in Nai and Cai. After 20 minutes of ischemia, time-averaged Cai was 1.0 +/- 0.2 microM (mean +/- SEM) in amiloride-treated hearts compared with 2.3 +/- 0.9 microM in nontreated hearts. After 20 minutes of ischemia, Nai in the untreated heart was threefold greater than control, whereas in the amiloride-treated heart, Nai was not significantly different from control. These data are consistent with the involvement of Na-Ca exchange in the rise in Cai during ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)