MYOCARDIAL VO2 OF MECHANICALLY UNLOADED CONTRACTION OF RAT VENTRICULAR SLICES MEASURED BY A NEW APPROACH

We instituted a new approach of measuring mechanically unloaded myocar dial oxygen consumption (VO2) by using rat left ventricular (LV) slice s in an air-tight chamber filled with oxygenated Tyrode solution. Myoc ardial slices (300 mu m in thickness) freely shortened without externa l load by electrical field stimulation (St). Vo(2) without St (n = 6) was 1.69 +/- 0.41 mi O-2 . min(-1) . 100 g LV(-1). Vo(2) with St (n = 6) increased to 2.28 +/- 0.36 mi O-2 . min(-1) . 100 g LV(-1). Vo(2) i n Ca2+-free Tyrode solution irrespective of St was nearly equal to Vo( 2) without St in normal Tyrode solution, indicating that all these Vo( 2) correspond to basal metabolic Vo(2). The increment in Vo(2) by St ( Delta Vo(2)) increased up to twice normal with the extracellular Ca2concentration up to 4 times normal. Inhibition of cross-bridge cycling by 2,3-butanedione monoxime (5 and 10 mM) did not decrease Delta Vo(2 ). These results suggest that Delta Vo(2) consists of Vo(2) primarily for excitation-contraction coupling but not for cross-bridge cycling.