Abnormal myocardial contraction in α_1A- and α _1B-adrenoceptor double-knockout mice

We used double-knockout mice (ABKO) lacking both predominant myocardial α₁-adrenergic receptor (AR) subtypes (α_1A and α_1B) to determine if α₁-ARs are required for normal myocardial contraction. Langendorff-perfused ABKO hearts had higher developed pressure than wild type (WT) hearts (123 ± 3 mmHg n = 22 vs. 103 ± 3 mmHg, n = 38, P < 0.001). Acutely inhibiting α ₁-ARs in WT hearts with prazosin did not increase pressure, suggesting that the increased pressure of ABKO hearts was mediated by long-term trophic effects on contraction rather than direct regulatory effects of α₁-AR removal. Similar to perfused hearts, ABKO ventricular trabeculae had higher submaximal force at 2 mM extracellular [Ca²⁺] than WT (11.4 ± 1.7 vs. 6.9 ± 0.6 mN/mm², n = 8, P < 0.05); however, the peaks of fura-2 Ca²⁺ transients were not different (0.79 ± 0.11 vs. 0.75 ± 0.16 μM, n = 10-12, P > 0.05), suggesting ABKO myocardium had increased myofilament Ca ²⁺-sensitivity. This conclusion was supported by measuring the Ca²⁺-force relationship using tetanization. Increased myofilament Ca²⁺-sensitivity was not explained by intracellular pH, which did not differ between ABKO and WT (7.41 ± 0.01 vs. 7.39 ± 0.02, n = 4-6, P > 0.05; from BCECF fluorescence). However, ABKO displayed impaired troponin I phosphorylation, which may have played a role. In contrast to increased submaximal force, ABKO trabeculae had lower maximal force than WT at high extracellular [Ca²⁺] (29.6 ± 1.9 vs. 37.6 ± 1.4 mN/mm², n = 7, P < 0.01). However, peak cytosolic [Ca ²⁺] was not different (1.13 ± 0.15 vs. 1.19 ± 0.04 μM, n = 6-7, P > 0.05), suggesting ABKO myocardium had impaired myofilament function. Finally, ABKO myocardium had decreased responsiveness to β-AR stimulation. We conclude: α₁-ARs are required for normal myocardial contraction; α₁-ARs mediate long-term trophic effects on contraction; loss of α₁-AR function causes some of the functional abnormalities that are also found in heart failure.