Sustained high O-2 use for Ca2+ handling in rat ventricular slices under decreased free shortening after ryanodine

We hypothesized that O-2 wasting of Ca2+ handling in the excitation-contrac tion coupling in ryanodine-treated failing hearts might derive from an incr eased external Ca2+ extrusion via Na+/Ca2+ exchanger and futile Ca2+ cyclin g via sarcoplasmic reticulum (SR) Ca2+-ATPase. We tested this hypothesis by mechanoenergetic studies using rat left ventricular slices. After the slic es were treated with ryanodine (0.1 muM), 1-Hz free shortening significantl y decreased by 78-85%, whereas the observed O-2 consumption ((V) over dot ( O2)) required for total Ca2+ handling, increased from 0.79 to 1.13 ml O-2. min(-1). 100 g LV-1 (155.6% of control). We reconfirmed that cyclopiazonic acid (10 muM), a blocker of SR Ca2+-ATPase, decreased (V) over dot (O2) by 75-80% in normal slices. However, 100 muM of cyclopiazonic acid was needed to inhibit the (V) over dot (O2) by 80% after ryanodine treatment. Blockade of a sarcolemmal Na+/Ca2+ exchanger by KB-R7943 (10 muM) significantly dec reased (V) over dot (O2) by 45% after ryanodine treatment without significa nt effects on normal slices. Our results indicated that the (V) over dot (O 2) increase following ryanodine treatment was derived from a net change of an increased external Ca2+ extrusion via Na+/Ca2+ exchanger and futile Ca2 cycling via SR Ca2+-ATPase.