The apelin-APJ system is a recently discover hormone system that has several important actions in the cardiovascular system. Apelin causes the heart pump with more force and also causes blood vessels to relax. Studies to date show that people with reduced pumping function of the heart have lower levels of apelin in the bloodstream compared to people with normal heart function. We have previously given apelin to healthy volunteers and people with reduced pumping function and this increases the heart's contraction. However, this has only been assessed over a short time period, around fifteen minutes, and we now want to know if this effect is seen over a longer period.