Stimulation of hypothalamic histidine decarboxylase by calcium-calmodulin and protein kinase (cAMP-dependent) inhibitor

Abstract

Stimulatory effects of Ca²⁺−CaM^* and PKI on partially purified hypothalamic HD (10 fold purification) have been shown under conditions involving inhibition of the enzyme by cAMP-induced phosphorylation and under control conditions. A 1∶1 (v/v) mixture of 0.1 mM CaCl₂ and 10 units of CaM from human red blood cells reversed the inhibition of HD induced by cAMP-dependent protein phosphorylation activity to the control level. Verapamil (0.01 mM) could partially block the former effect without affecting the control level of enzyme activity. 0.01 mM TPA did not further increase the effect of Ca²⁺−CaM on HD, in the presence of 0.01 mM ATP, indicating that this stimulation does not require the action of Ca²⁺-dependent protein kinase. The control level of HD is not influenced by 0.1 mM CaCl₂ or 0.02 mM EGTA but is raised by CaM in the presence of CaCl₂ (0.1 mM). A highly purified protein kinase (cAMP-dependent) inhibitor (PKI) from bovine heart and a crude inhibitor from rat cerebellum could also reverse the inhibitory effect of cAMP-dependent protein kinase under phosphorylating conditions and enhanced HD activity above control levels. PKI and Ca²⁺−CaM, added together, produced single, not additive effects.

We conclude that cAMP-induced phosphorylation is probable the main regulatory mechanism of histamine formation and this could be influenced by both Ca²⁺−CaM and PKI. Inhibition of cAMP-dependent protein kinase as well as stimulation of phosphoprotein phosphatase and Ca²⁺−CaM-dependent phosphodiesterase might be involved in the above actions.