Figure 1.

α1‐ARs regulate contractility through phosphorylation of cTnI at Thr144 in adult cardiac myocytes. A, Sarcomere dynamics and (B) Ca²⁺ transients were measured in WT and α1ABKO cardiac myocytes before and 5 minutes after phenylephrine (PE) treatment (10 μmol/L of PE, 2 μmol/L of timolol in all conditions). Single‐twitch contractions or calcium transients are shown at 5 minutes after the addition of PE. Averaged data for percent (%) sarcomere shortening and fold change in calcium (ΔCa²⁺) at 5 minutes after PE are presented as mean±SEM from 33 WT cardiac myocytes from 9 different cultures and 14 α1ABKO cardiac myocytes from 5 different cultures. C, cTnI phosphorylation at Thr144 and Ser23,24 and (D) PLB phosphorylation at Ser16 and Thr17 measured by Western blot from WT and α1ABKO cardiac myocytes treated with PE (20 μmol/L, 15 minutes). Quantitation of cTnI phosphorylation at Thr144 is presented as mean±SEM from 6 different cultures. All data were analyzed by 2‐way ANOVA with repeated measures and Bonferroni's post‐test. Percent sarcomere shortening, P=0.0123; ΔCa²⁺, P=NS; cTnI phosphorylation at Thr144, P=0.0384. Significant comparisons identified by Bonferroni's post‐test are indicated as P<0.05. ANOVA indicates analysis of variance; cTnI, cardiac troponin I; PLB, phospholamban; WT, wild type; α1ABKO, α1A‐ and α1B‐AR double knockout mice; α1‐ARs, α1‐adrenergic receptors.