CARDIAC SYMPATHETIC-STIMULATION INCREASES CARDIAC CONTRACTILITY BUT DECREASES CONTRACTILE EFFICIENCY IN CANINE HEARTS IN-VIVO

The effect of cardiac sympathetic stimulation on cardiac contractile e fficiency was studied in dogs. In 19 anesthetized and open-chest dogs, left ventricular (LV) pressure, LV volume, coronary blood flow and co ronary venous oxygen saturation were measured simultaneously. The LV e nd-systolic pressure volume relations (ESPVR) and the relation between myocardial oxygen consumption ((V) over dot O2)-pressure volume area (PVA) were obtained during a transient occlusion of the inferior vena cava before and after sympathetic stimulation (9V, 6 Hz, 40 sec) both with and without 50 mg/kg of 2,3-butanedione monoxime (BDM). Without B DM, sympathetic stimulation increased the slope of ESPVR by 62% (P<0.0 5), the slope of the (V) over dot O2-PVA line by 19% (p<0.05) and the y-axis intercept of the (V) over dot O2-PVA by 65% (p<0.05). With BDM, the increase in the slope of the (V) over dot O2-PVA line became insi gnificant although other responses were similarly preserved. These dat a imply that cardiac sympathetic stimulation decreases cardiac contrac tile efficiency through mechanisms by which norepinephrine-induced P-a drenergic activation enhances myosin ATPase-operating ATP hydrolysis i n crossbridge formation.