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ABSTRACT
Substitution of 2'-deoxy ATP (dATP) for ATP as substrate for actomyosin results in significant enhancement of in vitro parameters of cardiac contraction. To determine the minimal ratio of dATP/ATP (constant total NTP) that significantly enhances cardiac contractility and obtain greater understanding of how dATP substitution results in contractile enhancement, we varied dATP/ATP ratio in porcine cardiac muscle preparations. At maximum Ca^sup 2+^ (pCa 4.5), isometric force increased linearly with dATP/ATP ratio, but at submaximal Ca^sup 2+^ (pCa 5.5) this relationship was nonlinear, with the nonlinearity evident at 2-20% dATP; force increased significantly with only 10% of substrate as dATP. The rate of tension redevelopment (k^sub TR^) increased with dATP at all Ca^sup 2+^ levels. k^sub TR^ increased linearly with dATP/ATP ratio at pCa 4.5 and 5.5. Unregulated actin-activated Mg-NTPase rates and actin sliding speed linearly increased with the dATP/ATP ratio (p < 0.01 at 10% dATP). Together these data suggest cardiac contractility is enhanced when only 10% of the contractile substrate is dATP. Our results imply that relatively small (but supraphysiological) levels of dATP increase the number of strongly attached, force-producing actomyosin cross-bridges, resulting in an increase in overall contractility through both thin filament activation and kinetic shortening of the actomyosin cross-bridge cycle.
INTRODUCTION
Cardiac contractility can be enhanced by a variety of mechanisms, including those that directly affect actomyosin interactions. One such positive inotropic effector that acts primarily via actomyosin is replacement of ATP by 2'-deoxy ATP (dATP) as substrate for cardiac muscle contraction ( 1 ). With both α-myosin heavy chain (α-MHC. fast isoform) from untreated rats and β-myosin heavy chain (β-MHC, slow isofonn) from propylthiouracil (PTU) treated rats, replacement of ATP with dATP as the substrate for cardiac contraction resulted in significant increases of contractile parameters (1). The effects of this substrate substitution on mechanical properties were evaluated with isometric force (F^sub iso^). rate of tension redevelopment (k^sub TR^), unloaded shortening velocity (V^sub us^) at maximum activating [Ca^sup 2+^], as well as unregulated NTP hydrolytic activity (Mg-NTPase) and in vitro motility (IVM) actin sliding speed; all parameters were significantly enhanced in rat cardiac muscle, and these effects are larger than in skeletal muscle (2-4). From these results it is evident that dATP as substrate increases the number of strongly attached,...