Actions of vasoactive intestinal peptide and neuropeptide Y on the pacemaker current in canine Purkinje fibers.

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Abstract
We have investigated the actions of vasoactive intestinal peptide (VIP) and neuropeptide Y (NPY) on the pacemaker current (I(f)) in canine Purkinje fibers. On voltage pulses to the middle of the I(f) activation range, VIP reversibly increases I(f), whereas NPY reversibly decreases I(f). A three-pulse voltage protocol suggests that VIP shifts I(f) activation in the positive direction and that NPY shifts I(f) activation in the negative direction on the voltage axis without changing maximal I(f) conductance. These effects of VIP and NPY on I(f) are exerted through their specific peptide receptors, since the effects are blocked by VIP and NPY receptor antagonists. VIP and NPY are colocalized in cardiac parasympathetic and sympathetic nerve endings, respectively, and can be released preferentially on high and long-lasting nerve stimulation. Given this colocalization and frequency-dependent release, these results suggest a role for these neuropeptides in controlling cardiac I(f) and consequently heart rate.