Activation of the hyperpolarization-activated current (if) in sino-atrial node myocytes of the rabbit by vasoactive intestinal peptide

Vasoactive intestinal peptide (VIP) is a putative neurotransmitter found in extrinsic and intrinsic nerves of the heart. VIP can be released by vagal stimulation but, contrary to ACh, causes positive chronotropic effects as a result of binding to cardiac receptors which stimulate adenylate cyclase, and thus has been implicated in vagal tachycardias. Since the rate of diastolic depolarization of sinoatrial (SA) node myocytes depends on the hyperpolarization-activated current (i_f), which is directly activated by cytoplasmic cAMP, we studied the action of VIP on i f in myocytes isolated from the SA node of the rabbit. VIP (0.65 M) reversibly increased i_f at –65 mV but had no effect at –115 mV suggesting that its primary effect was to shift the activation curve to more positive voltages. Hyperpolarizing ramp and voltage compensation protocols indicated that VIP shifts the activation curve of i_f by approximately 5–6 mV in the positive direction with no change in maximal conductance. This shift may be the mechanism by which VIP produces its positive chronotropic effect and supports a negative feedback role for this peptide during elevated vagal activity.