Differential Activation of Guinea Pig Intrinsic Cardiac Neurons by the PAC1 Agonists Maxadilan and Pituitary Adenylate Cyclase-Activating Polypeptide 27 (PACAP27)

Abstract

Pituitary adenylate cyclase-activating polypeptide (PACAP) evokes tachycardia followed by a larger cholinergic bradycardia in isolated guinea pig hearts. We used the selective PAC1 receptor agonist maxadilan and vasoactive intestinal polypeptide (VIP) to test the hypothesis that PACAP27-evoked tachycardia and bradycardia are mediated by VPAC and PAC1 receptors, respectively. Chronotropic actions of these peptides were evaluated in isolated perfused hearts. Direct neuronal actions were determined by intracellular voltage recordings from cholinergic neurons in atrial ganglion whole mounts. Administration of 1 nmol of PACAP27 to isolated hearts evoked typical biphasic rate responses, whereas 1 nmol of maxadilan caused only a minor rate decrease. Desensitization with VIP eliminated the positive chronotropic effect of PACAP27 selectively. Local application of PACAP27 to cardiac neurons frequently evoked slow depolarization and caused prolonged increase of neuronal excitability. Maxadilan rarely affected membrane potential but consistently increased excitability. VIP had no effect on excitability and evoked depolarization in only a few neurons. Because maxadilan increased neuronal excitability but did not trigger action potentials as PACAP often does, we evaluated the interaction of maxadilan with substance P (SP) in isolated hearts. SP depolarizes cardiac neurons more consistently than PACAP, often triggers neuronal action potentials, and causes bradycardia but does not increase neuronal excitability. Maxadilan had a persistent effect to augment negative chronotropic responses to SP. These findings support our hypothesis that PACAP evokes tachycardia and bradycardia through VPAC and PAC1 receptors, respectively. They also suggest that maxadilan and PACAP27 differ in activating PAC1 receptors on cardiac neurons and/or stimulating downstream signaling mechanisms.Pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP) are neuropeptides that belong to the same peptide family, have a wide distribution in the autonomic nervous system, and exert prominent pharmacological effects on cardiovascular function (Vaudry et al., 2000;Laburthe and Couvineau, 2002). Endogenous PACAP occurs as a full-length peptide composed of 38 amino acids (PACAP38) and as a truncated peptide with 27 amino acids (PACAP27). Both forms of PACAP have substantial sequence homology with VIP, and as a consequence, there is some overlap of the pharmacological profiles of these neuropeptides. Shared pharmacological actions of PACAP and VIP can be attributed, in large part, to both peptides having comparable high affinities for the VPAC1 and VPAC2 subtypes of the VIP receptor. In contrast, PACAP receptors (i.e., PAC1) have a very low affinity for VIP but exhibit a high affinity for PACAP27 and PACAP38. Structural analogs of VIP have been developed as selective agonists for VPAC1 and VPAC2 receptors, whereas the only selective PAC1 receptor agonist is a peptide named maxadilan, which was isolated from the...

Backup Mechanisms Maintain PACAP/VIP-Induced Arterial Relaxations in Pituitary Adenylate Cyclase-Activating Polypeptide-Deficient Mice

“…In addition, PACAP regulates PAC1R expression [43,44], and absence of PACAP protein could be the reason for the differences in mRNA expression and protein levels between the arteries of WT and KO mice (and also for the different responses to PACAP1-38, but not to maxadilan). Hoover et al [45] showed that both maxadilan and PACAP1-27 activate PAC1R located on cardiac neurons; however, they differ in downstream signaling, indicating their importance. This suggests that PAC1R could be modified with its potency preserved.…”

Pituitary Adenylate Cyclase-Activating Polypeptide and Its Receptors: 20 Years after the Discovery

“…For instance, intravenous injection of PACAP in cat, sheep, and human provokes an increase in heart rate and enhances the contractile ventricular force (Minkes et al, 1992a;Sawangjaroen et al, 1992;Sawangjaroen and Curlewis, 1994;Birk et al, 2007). PACAP also caused bradycardia in isolated perfused guinea pig heart through both PAC1-R and VPAC-R (Chang et al, 2005;Hoover et al, 2009). In the anesthetized dog and on the isolated canine heart, PACAP evokes a transient positive followed by negative chronotropic and inotropic responses (Hirose et al, 1997b;Hirose et al, 1998).…”

Nickel suppresses the PACAP-induced increase in guinea pig cardiac neuron excitability

“…The PACAP peptide amino acid sequence is highly conserved, and the action of PACAP is tissue specific dependent on the activation of the different isoforms of the seven transmembrane G proteincoupled PACAP-selective PAC 1 receptor (Adcyap1r1) and/or the vasoactive intestinal peptide (VPAC) receptors (2,4,20,45,54). PACAP/PAC 1 receptor signaling modulates synaptic transmission and plasticity via pre-and postsynaptic mechanisms, and these effects have been shown to be critically important in central stress responses, peripheral sensory and autonomic function, and maintenance of physiological homeostasis (9,18,19,21,25,31,32,40,41,47,49,54).We previously identified colocalization of PACAP with acetylcholine in cholinergic parasympathetic preganglionic terminals innervating guinea pig cardiac neurons (5, 6) and demonstrated that both endogenously released and exogenously applied PACAP significantly increases cardiac neuron excitability through PAC 1 receptor activation (5,22,35,49). Cardiac neurons are more readily accessible than CNS neurons for experimental manipulations and accordingly, we have used these cells to further our understanding of cellular PAC 1 receptor signaling mechanisms.…”