Calcium Potentiates Cyclic AMP Stimulation of Pineal Arylalkylamine N‐Acetyltransferase

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Abstract
Pineal arylalkylamine N‐acetyltransferase (N‐acetyltransferase) controls large daily changes in melatonin production. It is generally thought that the activity of this enzyme is controlled by norepinephrine acting exclusively via elevation of cyclic AMP. However, norepinephrine also elevates pineal intracellular Ca2+ concentration ([Ca2+]i), and it is not known whether Ca2+ is involved in regulating N‐acetyltransferase activity other than through its established role in cyclic AMP production. In this study, the issue of whether Ca2+ enhances the effects of cyclic AMP on N‐acetyltransferase activity was investigated. The effects of cyclic AMP protagonists (isobutylmethylxanthine, N6, 2′‐O‐dibutyryladenosine 3′,5′‐cyclic monophosphate, 8‐bromoadenosine 3′,5′‐cyclic monophosphate, and adenosine 3′,5′‐cyclic monophosphothioate, Sp‐diastereomer) were examined in combination with [Ca2+]i protagonists (A23187, ionomycin, and phenylephrine). All [Ca2+]i protagonists potentiated the effects of cyclic AMP protagonists. For example, ionomycin potentiated the effects of low concentrations of 8‐bromoadenosine 3′,5′‐cyclic monophosphate, and A23187 potentiated the effects of isobutylmethylxanthine without altering cyclic AMP accumulation. These findings indicate that Ca2+ and cyclic AMP probably act physiologically in a coordinated manner to stimulate N‐acetyltransferase activity; these second messengers could act directly at one or more sites or through indirect actions mediated by kinases.