The role of dihydropyridine-sensitive voltage-gated calcium channels in potassium-mediated neuronal survival

Abstract

The survival of isolated neurons from chick embryo ciliary, sympathetic, and dorsal root ganglia is greatly enhanced by concentrations of extracellular potassium that significantly depolarize the neurons (ED 50 = 20–25mM). The survival-promoting effect of elevated potassium on each of these 3 types of neurons appears to be the result of the opening of voltage-gated calcium channels. The dihydropyridine, Bay K 8644, which increases calcium influx through L-type voltage-gated calcium channels in neurons, strongly potentiated the survival-promoting action of elevated potassium (ED 50 = 10.8 ± 7.0mM). In contrast, chemically closely related dihydropyridines, PN200-110 (ED 50 = 0.33 ± 0.15nM) and nitrendipine (ED 50 = 1.3 ± 0.3nM), which block calcium influx through the same voltage-gated channels, completely inhibited potassium-mediated neuronal survival. Chemically different agents that also block calcium influx through voltage-gated channels also inhibited potassium-mediated neuronal survival: the phenylalkylamine verapamil (ED 50 = 0.78 ± 0.38 μM), the benzothiazepine diltiazem (ED 50 = 1.7 μM), and the inorganic ion cadmium (ED 50 = 5.8 μM). These calcium-channel blockers are not simply toxic to neurons, since they did not inhibit neuronal survival mediated by the neurotrophic proteins, nerve growth factor, basic fibroblast growth factor, or ciliary neurotrophic factor, also suggesting that voltage-gated calcium channels are not involved in the action of these factors. These results suggest that neuronal survival in elevated potassium in ciliary, sympathetic, and dorsal root ganglion neurons is the result of calcium influx through dihydropyridine-sensitive, L-type voltage-gated calcium channels. These findings are discussed in relation to the neuronal toxicity of excitatory amino acids which is also thought to occur through increased calcium influx.