The effect of calmodulin, trifluoperazine and other psychoactive drugs on the activity of the Mn2+‐dependent adenylyl cyclase (AC) in testicular germ cells

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Abstract
This paper is concerned with the effects of calmodulin and different psychoactive drugs on the activity of the rat germ cell Mn2+‐dependent adenylyl cyclase (Mn‐AC) of various degrees of purity. Trifluoperazine inhibited the enzyme activity in a dose‐dependent manner (ID50∼50–70 μM) irrespective of the degree of purification. Calmodulin, on the other hand, elicited little or no activation of the Mn‐AC except for a marginal stimulation (20–30%) of the most purified enzyme preparation. The dose of calmodulin required to achieve this effect was extremely high (ED50∼ 3 μg ml). The trifluoperazineinhibition of a partly purified Mn‐AC was not substantially counteracted by the presence of calmodulin (20 μg/ml). We have also, by kinetic analyses, examined the mode by which trifluoperazine inhibits the Mn‐AC activity. Increasing concentrations of the drug decreased the maximal velocity (Vmax) (Ki∼ 50 μM), but did not influence the apparent affinity (Kma) for MnATP2‐. In contrast, the Kma for Mn2+ was decreased, and double reciprocal plots (1/V vs 1/Mn2+ free) were parallel and compatible with uncompetetive inhibition (Ki∼ 55μM). Other psychoactive agents (pimozide, trifluoperazine, medazepam, promethazine, chlordiazepoxide, haloperidol and chlorpromazine‐sulfoxide), all known to inhibit calmodulin‐activated cAMP phosphodiesterase with varying potencies (ID50s), inhibited the germ cell Mn‐AC with ID50s which parallelled those for cAMP phosphodiesterase inactivation. An exception was haloperidol (a potent phosphodiesterase inhibitor) which exerted a very low effect on the Mn‐AC activity (ID50∼ 2500 μM). The present results indicate that calmodulin or a calmodulin‐like protein may be associated with the Mn‐AC. Another possibility is that trifluoperazine and other psychoactive drugs may bind directly to the Mn‐AC molecule and prevent Mn2+‐activation.