Abstract

This study aimed to determine the mechanism(s) by which 1,4-dihydropyridine Ca²⁺ channel blockers (DHPs) enhance the binding of neurotensin (NT) to prostate cancer PC3 cells and inhibit NT-induced inositol phosphate formation. Earlier work indicated that these effects, which involved the G protein-coupled NT receptor NTR1, were indirect and required cellular metabolism or architecture. At the micromolar concentrations used, DHPs can block voltage-sensitive and store-operated Ca²⁺ channels, K⁺ channels, and Na⁺ channels, and can inhibit lipid peroxidation. By varying [Ca²⁺] and testing the effects of stimulators and inhibitors of Ca²⁺ influx and internal Ca²⁺ release, we determined that although DHPs may have inhibited inositol phosphate formation partly by blocking Ca²⁺ influx, the effect on NT binding was Ca²⁺-independent. By varying [K⁺] and [Na⁺], we showed that these ions did not contribute to either effect. For a series of DHPs, the activity order for effects on NTR1 function followed that for antioxidant ability. Antioxidant polyphenols (luteolin and resveratrol) mimicked the effects of DHPs and showed structural similarity to DHPs. Antioxidants with equal redox ability, but without structural similarity to DHPs (such as α-tocopherol, riboflavin, and N-acetyl-cysteine) were without effect. A flavoprotein oxidase inhibitor (diphenylene iodonium) and a hydroxy radical scavenger (butylated hydroxy anisole) also displayed the effects of DHPs. In conclusion, DHPs indirectly alter NTR1 function in live cells by a mechanism that depends on the drug's ability to donate hydrogen but does not simply involve sulfhydryl reduction.