Extracellular matrix of collagen modulates intracellular calcium handling and electrophysiological characteristics of HL-1 cardiomyocytes with activation of angiotensin II type 1 receptor

Background: Myocardial fibrosis plays a critical role in heart failure, resulting in cardiac structural and electrical remodeling which can induce atrial arrhythmias. Collagen is the major element of fibrosis. However, it is not clear whether collagen can directly regulate the calcium homeostasis and the electrophysiologic characteristics of cardiomyocytes. The aim of this study was to determine the effects of collagen on calcium homeostasis and the electrical properties of atrial cardiomyocytes. Methods and Results: HL-1 cardiomyocytes were cultured with and without collagen type I (1 or 10 μg/mL) or losartan (10 μmol/L). Whole-cell clamp, indo-1 fluorescence, and Western blotting were used to evaluate the action potential (AP) and ionic currents, intracellular calcium homeostasis, and calcium regulatory proteins. Compared with the control samples, there was no significant difference in collagen (1 μg/mL)-treated HL-1 cardiomyocytes. However, collagen (10 μg/mL)-treated HL-1 cardiomyocytes exhibited larger intracellular calcium ([Ca²⁺]_i) transients by 113% and a larger sarcoplasmic reticulum calcium content by 86%. Collagen (10 μg/mL)-treated HL-1 cardiomyocytes had higher expression of sarcoplasmic reticulum ATPase (SERCA2a) and Thr17-phosphorylated phospholamban but similar protein expressions of the Na⁺/Ca²⁺ exchanger and ryanodine receptor. Collagen (10 μg/mL)-treated HL-1 cardiomyocytes (n = 11) had larger AP amplitude (104 ± 5 vs 83 ± 7 mV; P <.05), and shorter 90% of AP duration (25 ± 2 vs 33 ± 2 ms, P <.05) than control cells (n = 11). Moreover, collagen (10 μg/mL)-treated HL-1 cells had larger I_to and I_Ksus values than control cells. The administration of losartan (10 μmol/L) attenuated collagen-induced changes in [Ca²⁺]_i transients, [Ca²⁺]_i stores, AP morphology, ionic currents, SERCA2a, and Thr17-phosphorylated phospholamban expressions. Conclusions: This study demonstrates that collagen can directly modulate the calcium dynamics and electrical activities of atrial cardiomyocytes, which are associated with the renin-angiotensin system. These findings suggest a critical role of collagen in electrical remodeling during fibrosis.