Lithium preferentially inhibits adenylyl cyclase V and VII isoforms

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Abstract
Lithium ions' inhibition of adenylyl cyclase (AC) has not been previously studied for the newly discovered AC isoforms. COS7 cells were transfected with each of the nine membrane-bound AC isoforms cDNAs with or without D1- or D2-dopamine receptor cDNA. AC activity was measured as [3H]cAMP accumulation in cells pre-incubated with [3H]adenine followed by incubation with phosphodiesterase inhibitors together with either the D1 agonist SKF-82958 alone, or forskolin, in the presence or absence of the D2 agonist quinpirole. At 1 mm or 2 mm lithium inhibited only AC-V activity when the enzyme was stimulated by forskolin, a direct activator of AC. Lithium inhibited AC-V (by 50%), AC-VII (by 40%) and AC-II (by 25%) when stimulated via the D1 receptors, but did not affect the Ca2+-activated isoforms when stimulated by the Ca2+ ionophore A23187. Quinpirole inhibits AC via the Gi protein. Lithium did not affect quinpirole-inhibited FSK-activated AC-V activity nor did it affect superactivated AC-V or AC-I following the removal of quinpirole. The data suggest interference of lithium with transduction pathways mediated via AC-V or AC-VII; only the active conformation of these AC isoforms is inhibited by lithium; the inhibitory effect of lithium is abolished when the enzyme is superactivated. The marked inhibition of AC-V and AC-VII by lithium suggests that these two isoforms may be involved in mediating the mood-stabilizing effect of lithium.