The TRPV4 Ion Channel Alters Intracellular Calcium Transients in Cardiomyocytes of Aged Mice

Abstract

Cardiac excitation-contraction coupling (ECC) occurs via calcium-induced calcium release and is therefore critically dependent on precise regulation of intracellular calcium homeostasis. Cardiomyocytes of the aged heart exhibit alterations in calcium homeostasis which predisposes the heart to calcium overload, contractile dysfunction, and arrhythmia. In this investigation we tested the hypothesis that the transient receptor potential vanilloid member 4 (TRPV4) osmotically-sensitive cation channel is expressed in cardiomyocytes of the aged mouse heart and alters intracellular calcium transients. Western blot analysis of whole heart homogenates of Young (3-6 month) and Aged (24-26 month) C57BL/6 male mice revealed increased expression of TRPV4 protein in hearts of Aged. Enzymatically isolated left-ventricular cardiomyocytes were loaded with the calcium indicator dye fluo-4/AM and electrically stimulated at 0.5 Hz to induce intracellular calcium transients. Pharmacological activation of TRPV4 with the agonist RN1747 (100 nM) produced an increase in calcium transient amplitude in cardiomyocytes of Aged (F/F0:3.3±0.4 RN1747 versus 2.6±0.4 control, P<0.05), but not Young (F/F0:2.8±0.3 RN1747 versus 2.9±0.2 control) mice. Following hypo-osmotic stress (250 mOsm, 40 minutes), cardiomyocytes of Aged exhibited enhanced responsiveness to RN1747 (F/F0:4.3±0.6 RN1747 versus 3.5±0.4 control, P<0.05) which associated with altered transient decay kinetics and cellular calcium overload (7/8 cells); these effects were prevented by the TRPV4 antagonist HC067047 (1 uM; F/F0:2.9±0.3 RN1747+HC versus 3.0±0.2 HC, 0/5 cells). In conclusion, the TRPV4 ion channel is expressed in cardiomyocytes of the aged heart and alters calcium transients during ECC.