1. The possible role of nicotinamide‐adenine dinucleotide (NAD+) and cyclic adenosine diphosphate ribose (cADPR) as regulators of M‐type K+ currents (IK(M)) has been studied in whole‐cell patch‐clamped NG108‐15 mouse neuroblastoma x rat glioma cells that had been transformed to express m1 muscarinic acetylcholine receptors (mAChRs). 2. Pre‐incubation of NG108‐15 cells for 6‐8 h with streptozotocin (2‐5 mM) reduced NAD+ levels by 40‐50%. Nicotinamide (2‐5 mM) increased NAD+ levels and prevented depletion by streptozotocin. 3. Streptozotocin pretreatment reduced the inhibition of IK(M) produced by 100 microM acetylcholine (ACh) from 51.6 +/‐ 7.0 to 29.1 +/‐ 7.5%. This was prevented by simultaneous pre‐incubation with 2 mM nicotinamide or by adding 2 mM NAD+ to the pipette solution. Neither procedure significantly affected the initial amplitude of IK(M). 4. Inclusion of 2 microM cADPR in the pipette solution induced a slow loss of IK(M) with a time constant of about 20 min. 5. It is concluded that mAChR‐induced inhibition of IK(M) requires intracellular NAD+. This might be needed for the formation of cADPR as a regulator or messenger for IK(M) inhibition.
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