Pituitary adenylate cyclase-activating polypeptide activates KATP current in rat atrial myocytes

Because the electrophysiological effects of pituitary adenylate cyclase-activating polypeptide (PACAP) on the heart are little known, we studied the regulation of the atrial ATP-sensitive K+ (K-ATP) current by PACAP on primary cultured neonatal rat atrial myocytes. PACAP-38 stimulates cAMP production with EC50 = 0.28 nmol/l (r = 0.92, P < 0.02). PACAP-38 and PACAP-27 (10 nmol/l) have similar maximal effects, whereas 100 nmol/l vasoactive intestinal polypeptide (VIP) is 2.7 times less effective (P < 0.05). RT-PCR shows the presence of cloned PACAP receptors PAC(1) (greater than or equal to2 isoforms), VPAC(1), and VPAC(2). PACAP-38 dose dependently activates the whole cell atrial KATP current with EC50 = 1-3 nmol/l (n = 44). Maximal effects occur at 10 nmol/l (91 +/- 15 pA/pF, n = 18). Diazoxide further increases the PACAP-activated current by 78% (P < 0.05; n = 6). H-89 (500 nmol/l), a protein kinase A (PKA) inhibitor, reduces the PACAP-activated K-ATP current to 17.8 +/- 9.6% (n = 5) of the maximal diazoxide-induced current and totally inhibits the cAMP-induced K-ATP current. A protein kinase C (PKC) inhibitor peptide (50 mol/l) in the pipette reduces the PACAP-38-induced K-ATP current to 33 +/- 17 pA/pF (P < 0.05, n = 6) without significantly affecting the currents induced by cAMP or VIP. The results suggest that: 1) PAC1, VPAC1, and VPAC2 are present in atrial myocytes; and 2) PACAP-38 activates the atrial KATP channels through both PKA and PKC pathways.