Lead Enters Bovine Adrenal Medullary Cells Through Calcium Channels

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Abstract
Agents that stimulate secretion also accelerate the rate of Pb uptake into adrenal medullary cells. For example, when cells are suspended in a medium containing 5 μM Pb2+, depolarization by 77 mM K increases the rate of Pb uptake from 12 ± 1 to 47 ± 5 μmol/(L cells × min). K-induced Pb uptake has an apparent Km for Pb2+ of 2.6 μM, and is antagonized by Ca2+ with a K0.5 of 1.4 mM. The Ca channel blocker D-600 inhibits Pb entry with a K0.5 of 0.4 μM. Pb uptake is also stimulated by the Ca channel agonist BAY K 8644. These observations suggest that Pb passes through Ca channels. The permeability of the channels to Pb appears to be at least 10 times the permeability to Ca.