Abstract
Although ATP is the physiological substrate for cardiac contraction, cardiac contractility is significantly enhanced in vitro when only 10% of ATP substrate is replaced with 2′‐deoxy‐ATP (dATP). To determine the functional effects of increased intracellular [dATP] ([dATP]~i~) within living cardiac cells, we used hypertonic loading with varying exogenous dATP/ATP ratios, but constant total nucleotide concentration, to elevate [dATP]~i~ in contractile monolayers of embryonic chick cardiomyocytes. The increase in [dATP]~i~ was estimated from dilution of dye added in parallel with dATP. Cell viability, average contractile amplitude, rates of contraction/relaxation, spontaneous beat frequency, and Ca^2+^ transient amplitude and kinetics were examined. At total [dATP]~i~ above ∼70 µM, spontaneous contractions ceased, and above ∼100 µM [dATP]~i~, membrane blebbing was also observed, consistent with apoptosis. Interestingly, [dATP]~i~ of ∼60 µM (∼40% increase over basal [dATP]~i~ levels) enhanced both amplitude of contraction and the rates of contraction and relaxation without affecting beat frequency. With total [dATP]~i~ of ∼60 µM or less, we found no significant change in Ca^2+^ transients. These data indicate that there is an “optimal” concentration of exogenously loaded [dATP]~i~ that under controlled conditions can enhance contractility in living cardiomyocytes without affecting beat frequency or Ca^2+^ transients. J. Cell. Biochem. 104: 2217–2227, 2008. © 2008 Wiley‐Liss, Inc.