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.
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