PAI-1 also has a diurnal variation with a peak plasma level occurring between 8 and 9 AM that may help explain why the incidence of acute MI is highest in the morning and why thrombolysis is least effective at that time. PAI-1's diurnal variation is been shown to be directly regulated by central and peripheral circadian pacemakers in vitro, and in vivo. Our group has observed that the diurnal variation of plasma PAI-1 levels is blunted and delayed in blind individuals who's circadian mechanisms are free running (not controlled by a central circadian pacemaker) when compared to those whose circadian rhythms are entrained (controlled by a central circadian pacemaker) (unpublished data), suggesting an additional system may modulate diurnal variation of PAI-1. As PRA and aldosterone levels peak earlier than PAI-1 levels, they may be partially responsible. Indeed, continuous infusion of candesartan eliminated diurnal variation of aortic PAI-1 message expression in Wistar-Kyoto and spontaneously hypertensive rats, while hydralazine did not.