CA2-FLOW ISCHEMIA IN RAT HEARTS( TRANSIENT DECLINE AND MYOCARDIAL RELAXATION ARE SLOWED DURING LOW)

The mechanisms that impair myocardial relaxation during ischemia are b elieved to involve abnormalities of calcium handling. However, there i s little direct evidence to support this hypothesis. Therefore, we sou ght to determine whether the time constant of cytosolic calcium ([Ca2](c)) decline (tau(Ca)) was increased during low flow ischemia, and if there was a relationship between the time constant of left ventricula r pressure decline (tau(P)) and tau(Ca). Isolated perfused hearts were studied using indo-1 fluorescence ratio as an index of [Ca2+](c).tau( P) was used as an index of myocardial relaxation. The time constant of decline of the indo-1 ratio increased from 74+/-5 ms to 95+/4, 144+/- 10, and to 204+/-16 ms when coronary flow was reduced was reduced to 5 0, 20, and 10% of control, respectively. Indo-1 transients were calibr ated to calculate tau(Ca). tau(Ca) increased from 67+/-6 ms to 108+/-9 and 158+/-19 ms when coronary flow was reduced to 20 and 10% of contr ol, respectively. There was a linear relationship between tau(Ca) and tau(P) (r = 0.82). These data support the hypothesis that during low f low ischemia, impaired myocardial relaxation may be caused by slowing of [Ca2+](c) decline.