Comparison of cross-bridge cycling kinetics in neonatal vs. adult rat ventricular muscle

The developmental shift in contractile protein isoform expression in the ro dent heart likely affects actin-myosin cross-bridge interactions. We compar ed the Ca2+ sensitivity for force generation and cross-bridge cycling kinet ics in neonatal (postnatal days 0-3) and adult (day 84) rats. The force-pCa relationship was determined in Triton-X skinned muscle bundles activated a t pCa 9.0 to 4.0. In strips maximally activated at pCa 4.0, the following p arameters of cross-bridge cycling were measured: (1) rate of force redevelo pment following rapid shortening and restretching (k(tr)); and (2) isometri c stiffness at maximal activation and in rigor. The fraction of attached cr oss-bridges (alpha(fs)) and apparent rate constants for cross-bridge attach ment (f(app)) and detachment (g(app)) were derived assuming a two-state mod el for cross-bridge cycling. Compared to the adult, the force-pCa curve for neonatal cardiac muscle was significantly shifted to the left. Neonatal ca rdiac muscle also displayed significantly smaller alpha(fs), slower k(tr) a nd f(app); however, g(app) was not significantly different between age grou ps. These data indicate that weaker force production in neonatal cardiac mu scle involves, at least in part, less efficient cross-bridge cycling kineti cs.