Hexachlorocyclohexane inhibits calmodulin-dependent Ca²⁺-ATPase activity in rice shoot membranes

Abstract

Inclusion of hexachlorocyclohexane (HCH), a chlorinated insecticide, or calmodulin antagonists during the germination of paddy retarded the growth of the plant. Retardation of plant growth was accompanied by in vivo inhibition of Ca²⁺-ATPase activity by HCH and calmodulin antagonists. Bromophos, a phosphatic insecticide, had no significant effect either on growth or on the enzyme activity. HCH inhibited Ca²⁺-ATPase activity in vitro (IC₅₀=20 μM) and in vivo (IC₅₀=1.7 mM) in a dose-dependent manner. Likewise chloropromazine inhibited the enzyme activity in vivo with an IC₅₀=50 to 80μM. Growth retardation and enzyme inhibition caused by HCH was reversed when the insecticide was withdrawn from the germinating medium. Similarly, enzyme inhibition due either to calmodulin antagonists or to HCH was overcome by the addition of excess calmodulin to the assay system. Inclusion of crude calmodulin from rice shoots increased Ca²⁺-ATPase activity by 112% suggesting the calmodulin-dependent regulation of the enzyme activity. These results, apart from demonstrating the role of Ca²⁺ and calmodulin in the regulation of plant growth and of Ca²⁺-ATPase activity, also suggest a possible mechanism of phytotoxic action of HCH in vivo.