1. Treatment of monolayers of chick embryo hepatocytes with the calcium channel blocking drugs nifedipine and verapamil resulted in a decrease in the activity of uroporphyrinogen decarboxylase, an increase in the activity of δ-aminolaevulinate synthase and accumulation of porphyrins with uroporphyrin and heptacarboxylic porphyrin predominating.

2. Diltiazem, another calcium channel blocking drug, did not affect uroporphyrinogen decarboxylase activity and had a slight effect only on the accumulation of porphyrins.

3. Experiments with nifedipine and verapamil in the presence of various concentrations of calcium indicate that the porphyrogenic effect is apparently not related to blocking of calcium channels.

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