Attenuation of cardiac contractility in Na,K-ATPase α1 isoform-deficient hearts under reduced calcium conditions

We have previously reported that genetic reduction of the Na,K-ATPase α1 isoform (α1+/–) results in a hypocontractile cardiac phenotype. This observation was surprising and unexpected. In order to determine if calcium overload contributes to the depressed phenotype, cardiac performance was examined by perfusing the hearts with buffer containing 2 or 1.5 mM calcium. At 2 mM calcium, +dP/dt for the α1+/– hearts (1374 ± 180) was significantly less than that of wild-type (2656 ± 75, P < 0.05). At 1.5 mM calcium, a larger decrease in +dP/dt occurred (vs. 2 mM calcium) for the α1+/– hearts (517 ± 92) compared to wild-type (2238 ± 157). At 2 mM calcium, –dP/dt was 50% lower in α1+/– hearts (–1903 ± 141) than wild-type (–982 ± 143). At 1.5 mM calcium relaxation was further reduced in α1+/– compared to wild-type (–443 ± 56 vs. – 1691 ± 109). We also tested whether the compensatory upregulation of the Na,K-ATPase α2 isoform in the α1+/– hearts contributes to the hypocontractile phenotype. At 8 × 10–6 M ouabain, that would completely inhibit the α2 isoform, a 30% increase in contractility was obtained in α1+/– hearts compared to no ouabain treatment, while a 63% faster time-to-peak (TTP) and 67% faster half-time-to-relaxation (RT1/2) were observed in α1+/– hearts treated with ouabain. These results suggest that upregulation of the α2 isoform may play a role in slower TTP and RT1/2 in the α1+/– hearts. Furthermore, lowering extracellular calcium in the perfusate did not alleviate the depressed contractile phenotype in the α1+/– hearts and resulted in further depressed cardiac contractility suggesting that these hearts are not calcium overloaded.