Age-related differences in Na+-dependent Ca2+ accumulation in rabbit hearts exposed to hypoxia and acidification

In this study, we test the hypothesis that in newborn hearts ( as in adults) hypoxia and acidification stimulate increased Na+ uptake, in part via pH-regulatory Na+/H+ exchange. Resulting increases in intracellular Na+ (Na-i) alter the force driving the Na+/Ca2+ exchanger and lead to increased intracellular Ca2+. NMR spectroscopy measured Nai and cytosolic Ca2+ concentration ([Ca2+](i)) and pH (pH(i)) in isolated, Langendorff-perfused 4- to 7-day-old rabbit hearts. After Na+/K+ ATPase inhibition, hypoxic hearts gained Na+, whereas normoxic controls did not [19 +/- 3.4 to 139 +/- 14.6 vs. 22 +/- 1.9 to 22 +/- 2.5 ( SE) meq/kg dry wt, respectively]. In normoxic hearts acidified using the NH4Cl prepulse, pHi fell rapidly and recovered, whereas Nai rose from 31 +/- 18.2 to 117.7 +/- 20.5 meq/kg dry wt. Both protocols caused increases in [Ca](i); however, [Ca](i) increased less in newborn hearts than in adults (P < 0.05). Increases in Nai and [Ca](i) were inhibited by the Na+/H+ exchange inhibitor methylisobutylamiloride (MIA, 40 μM; P < 0.05), as well as by increasing perfusate osmolarity (+30 mosM) immediately before and during hypoxia (P < 0.05). The data support the hypothesis that in newborn hearts, like adults, increases in Nai and [Ca](i) during hypoxia and after normoxic acidification are in large part the result of increased uptake via Na+H+ and Na+/Ca2+ exchange, respectively. However, for similar hypoxia and acidification protocols, this increase in [Ca](i) is less in newborn than adult hearts.