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 Na_i 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, pH_i fell rapidly and recovered, whereas Na_i 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 Na_i and [Ca]_i were inhibited by the Na⁺/H⁺ exchange inhibitormethylisobutylamiloride (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 Na_i 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.