Mechanisms Underlying Pulsed Infrared Stimulation of Cardiomyocytes

Abstract

In this study, we investigated the origins of the endogenous cellular mechanisms underlying IR (1862 nm, 3-4 ms/pulse, 9.1 - 11.6 J/cm2 /pulse, Capella, LHM Aculight) stimulation of neonatal rat ventricular and adult rabbit ventricular cardiomyocytes, in vitro. Confocal imaging (FV1000, Olympus; Fluo-4 AM, 4-6 μM, Invitrogen) of neonatal cardiomyocytes revealed IR-induced transient [Ca2+]i responses consisting of a rapid [Ca2+]i buffering component, discernable during periods of elevated [Ca2+]i, followed by consistent, sub-threshold [Ca2+]i rises that resulted in visible cell contractions with each IR pulse. Pharmacological block of the IR-evoked responses in neonatal cardiomyocytes with CGP-37157 (20 μM, N=12 cells) and Ruthenium Red (40 μM, N=13) suggested an integral role of the mitochondrial Ca2+ transporters in the IR-induced [Ca2+]i cycling in neonatal cardiomyocytes. While initial results with adult cardiomyocytes during comparable IR stimulation also revealed visible contractile responses, the corresponding [Ca2+]i transients were surprisingly not detected. To further investigate the response in adult cardiomyocytes, whole cell patch clamp measurements were performed to monitor sarcolemma membrane potential (Vm) changes during IR stimulation. Preliminary data revealed either depolarizing or hyperpolarizing Vm responses in the cells, the nature of which was determined by the relative timing of the IR pulse applications to threshold, electrically-induced cell depolarization. Based on these findings, additional efforts focused on resolving the extent and nature of this sarcolemmal involvement in the IR-evoked responses of both neonatal and adult cardiomyocytes.