Mechanisms for pituitary adenylate cyclase-activating polypeptide-induced increase in excitability in guinea-pig and mouse adrenal medullary cells

Pituitary adenylate cyclase-activating polypeptide (PACAP) acts on adrenal medullary (AM) cells as a neurotransmitter of the sympathetic preganglionic nerve. In guinea-pig AM cells, PACAP induces little catecholamine secretion, but enhances secretion evoked by stimulants, whereas in other animals, such as mouse, PACAP itself induces depolarization and/or catecholamine secretion. The present studies aim to explore the physiological implication of these species differences in PACAP actions, the ion channel mechanism for PACAP-induced depolarization, and the mechanism for facilitation of muscarinic receptor-mediated cation currents in mouse and guinea-pig AM cells. The perforated patch clamp technique was used to record the whole-cell current in isolated AM cells. The amplitudes of 3 nM PACAP-induced inward currents were significantly larger in mouse AM cells than guinea-pig, whereas 1 μM muscarine-induced currents were larger in guinea-pig AM cells than mouse. Exposure to PACAP consistently resulted in enhancement of muscarine-induced currents in guinea-pig AM cells and facilitation of cell membrane insertion of heteromeric TRPC1-TRPC4 channels in response to muscarine in PC12 cells. The PACAP-induced current was inhibited by 30 μM 9-phenanthrol, a specific TRPM4 channel inhibitor, and abolished by replacement of external Na+ with N-methyl D-glucamine. TRPM4-like immunoreactivity was located at the cell periphery in AM cells. The present results indicate that PACAP and muscarinic receptors are major metabotropic receptors mediating generation of depolarizing inward currents in mouse and guinea-pig AM cells, respectively. We conclude that PACAP activates TRPM4-like channels and enhance the muscarinic current through facilitating the membrane insertion of TRPC1-TRPC4 channels in AM cells.