901-91 Diastolic Dysfunction During Demand Ischemia is due to a Reversible Rigor Force and is not a Calcium (Ca2+) Activated Tension

BackgroundExercise-induced angina (demand ischemia (D1)) causes an acute increase in LV diastolic chamber stiffness (↑DCS), via a mechanism that may involve i) diastolic persistence of excessive cytosolic Ca2+(particularly from incomplete resequestration by an energy limited sarcoplasmic reticulum (SR)) with continued cross-bridge cycling, or ii) rigor force from ATP depletion. AimTo determine the nature of the tension underlying ↑DCS in DI.Methods and ResultsA) Diastolic anginal physiology was created in isolated isovolumic (balloon-in-LV) blood perfused rabbit hearts. Tachyardia (T: 7 Hz) imposed during restricted coronary blood flow (30% of baseline) increased DCS (ie isovolumic LVEOP) 7–9mmHg after 15±1 min (n=381, increased lactate production (pre-T vs post-T: 0.12±0.03 vs 0.45±0.03 μM/min/gL Vww, p<0.001) without increasing oxygen extraction. ↑DCS was reversible on reperfusion, similar to patients with angina. B) During the state of ↑DCS, brief intracoronary (ic) infusion of saline (S, n=7) was compared to 50mM ammonium chloride (NH4C1, n=7) and to 14mM Ca2+ (n=6). Function was similar during low-flow ischemia pre-T (LV systolic pressure (LVSP)/LVEDP S=67/18 vs NH4CI 58/17 vs Ca2+ 67/17mmHg, NS) and post-T (S=38/25 vs NH4C1 39/26 vs Ca2+ 43/24mmHg, NS). i) NH4CI exerted a biphasic effect on LVSP over 2.5min (LVSP maximum increase to 70±4, P<0.001, followed by decrease to minimum 52±2mmHg, p<0.001) commensurate with an action as an intracellular alkalinising and then acidifying agent, to alternately increase and decrease myofilament Ca2+sensitivity. However LVEDP remained unaffected suggesting ↑DCS was not Ca2+dependent. ii) Ca2+increased LVSP (S vs Ca2+57±3 vs 97±5 mmHg, p<0.001). decreasedDCS (LVEOP S vs Ca2+25±1 vs 21±1 mmHg, p i 0.001). and increasedrelaxation rate (–dP/dt/P 13±1 vs 10±l/s, p<0.005). Hence Ca2+resequestration capacity during a state of ↑DCS was intact and capable of acceleration, consistent with a functional and responsive SR. C) To further investigate the nature of ↑DCS we applied a method novel to the isolated heart with a quick (0.5s) stretch-release (QSR) of 25% of intraventricular balloon volume in three groups of hearts with ↑DCS i) classic rigor with zero-flow ischemia ii) ic infusion of 5 mM caffeine and 5 mMCa2+(producing Ca2+activated ↑DCS) and iii) in D1. QSR instantly lysed increased diastolic tension during zero-flow ischemia (LVEDP pre-vs post-QSR 27±1 vs 17±1mmHg, p<0.001), and also in D1 (27±2 to 15±1mmHg, p<0.001). By contrast, LVEDP was unaffected by QSR during Ca2+activated ↑DCS (28±1 to 26±1mmHg, NS). Hence, ↑DCS in D1 behaved as a rigor tension with QSR.ConclusionIn D1, neither does ↑ DCS behave as a Ca2+activated tension nor is subcellular Ca2+reuptake disabled. Rather, ↑DCS displays properties of a rigor force which may produce ischemic diastolic dysfunction in D1