The transient receptor potential vanilloid 4 ion channel and calcium homeostasis in cardiomyocytes of aged mice

Abstract

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] Aging and cardiovascular disease are critically linked and have now become a growing concern for the healthcare community. Unfortunately, medicine remains uncertain about all of the changes that occur in the heart throughout the aging process. Many pathological observations have been made regarding heart function, but the underlying mechanisms remain elusive. The purpose of this study was to identify the ion channel protein TRPV4 (Transient Receptor Potential Vanilloid 4) and examine how its expression changes in the aging heart. TRPV4 is an ion channel that allows calcium entry into heart muscle cells, known as cardiomyocytes. Calcium is responsible for many processes within cardiomyocytes, but its primary function is to serve as the signal for contraction. If aging hearts express more TRPV4 than young hearts this may result in differences in how they handle cellular calcium. This research used a murine model of aging with Young categorized as 3-6 months old and Aged categorized as 24-26 months old. Primarily this study utilized individual cardiomyocytes from both Young and Aged hearts that were isolated and then used in microscopy to observe dynamic calcium handling processes. Saline solutions with altered osmolarities (particle concentrations) were used to induce stretching of cardiomyocytes, which is known to alter TRPV4 activity. Furthermore, specific antibodies that recognize and bind TRPV4 were used to identify differences in TRPV4 expression patterns. Pharmacologic modulators of TRPV4 activity were utilized to activate or attenuate the effect of TRPV4 on cellular calcium handling. Finally, genetically altered mice which express high levels of TRPV4 were generated to serve as a positive control for TRPV4 activity, independent of advancing age. Results from this study indicate that Aged mice on average express TRPV4 more than their Young counterparts. Furthermore, TRPV4 alters major calcium handling pathways in Aged cardiomyocytes. Alterations were observed in Aged cardiomyocytes as increases in intracellular calcium concentrations upon osmotic-induced stretch. This indicates that when Aged hearts expressing TRPV4 are stretched, TRPV4 responds by opening and mediating calcium entry. Similar results were observed when utilizing the TRPV4 pharmacological agonist. However, if TRPV4 is highly active for an extended period of time, too much calcium enters into cardiomyocytes which may lead to detrimental results such as arrhythmias. Importantly, both the osmotic and agonist induced alterations could be prevented by the TRPV4 pharmacological antagonist. In summary, this research has identified and characterized a potentially detrimental calcium handling process within the aged heart.