ROLE OF CALCIUM CHANNELS AND PROTEIN-KINASE-C FOR RELEASE OF NOREPINEPHRINE AND NEUROPEPTIDE-Y

The role of calcium for the release of norepinephrine (NE, determined by highpressure liquid chromatography) and neuropeptide Y (NPY, determined by radioimmunoassay) was investigated in guinea pig perfused hearts with intact sympathetic innervation. In the presence of extracellular calcium (1.85 mM), electrical stimulation of the left stellate ganglion (12 Hz, 1 min) induced a closely related release of NE and NPY with the molar ratio of approximately 400-600 (NE) to 1 (NPY). The stimulation-evoked overflow of both transmitters was dependent from the extracellular calcium concentration and was almost completely suppressed by calcium-free perfusion. The corelease of both transmitters was not affected by the L-type calcium channel blocker felodipine (1-10-mu-M). However, the overflow of NE and NPY was markedly attenuated by the unselective calcium antagonist flunarizine (1-10-mu-M) and completely prevented by the neuronal (N-type) calcium channel blockers omega-conotoxin (1-100 nM) and cadmium chloride (10-100-mu-M), indicating a key role for N-type calcium channels in the exocytotic release of transmitters from cardiac sympathetic nerve fibers. Possibly due to unspecific actions, such as interference with sodium channels or uptake1-blocking properties, the phenylalkylamines verapamil (0.01-10-mu-M) and gallopamil (1-10-mu-M) reduced NPY overflow with only a minor effect on NE overflow. The stimulation-induced transmitter release was increased up to twofold by activation of protein kinase C (phorbol 12-myristate 13-acetate, 3 nM-3-mu-M) and completely suppressed by inhibition of protein kinase C (polymyxin B, 100-mu-M). The data indicate that the stimulation-evoked exocytotic corelease of NE and NPY from guinea pig perfused hearts depends on the presence of extracellular calcium, calcium influx through N-type calcium channels, and activation of protein kinase C.