Catecholamine secretion in a rat pheochromocytoma cell line: two pathways for calcium entry.

1. The pathways for Ca entry during stimulus-secretion coupling were studied by measuring carbamylcholine and KCl activated dopamine (DA) release from PC12, a clonal cell line originated from a rat pheochromocytoma. Various conditions were used to establish the existence of two independent pathways for Ca entry, i.e. a voltage dependent Ca channel and the acetylcholine (ACh) receptor linked channel. 2. DA release from PC12 was stimulated by activation of nicotinic ACh receptors with carbamylcholine. Release was dependent on external Ca, but not Na, and was insensitive to tetrodotoxin. 3. High concentrations of external KCl (15--56 mM) also stimulated the release of dopamine from PC12. This release required external Ca, but not Na, was insensitive to tetrodotoxin and was diminished by high concentrations of Ca. 4. Ni, Co and, to a lesser extent, Mg inhibited both the carbamylcholine and KCl stimulated release of DA in normal and Na-free media. 5. In the absence of Ca and Na, Mn supported DA release stimulated by either carbamylcholine or KCl. 6. The divalent cation ionophore A23187 stimulated DA release when Ca or Mn, but not Co, Ni or Mg, was the only divalent cation present in the medium. 7. Conclusions can be summarized as follows, (a) KCl stimulates DA release by activation of voltage dependent Ca channels. (b) Carbamylcholine probably stimulates DA release by Ca influx through both the ACh channel and voltage dependent Ca channels. (c) Mn ion is a suitable substitute for Ca ion as regards permeation of the Ca and the ACh channels and activation of the secretory process. (d) Na flux through voltage dependent Na channels is not necessary for stimulation of release by either KCl or carbamylcholine.