β-nicotinamide adenine dinucleotide is an enteric inhibitory neurotransmitter in human and nonhuman primate colons.
OBJECTIVE
An important component of enteric inhibitory neurotransmission is mediated by a purine neurotransmitter, such as adenosine 5'-triphosphate (ATP), binding to P2Y1 receptors and activating small conductance K(+) channels. In murine colon β-nicotinamide adenine dinucleotide (β-NAD) is released with ATP and mimics the pharmacology of inhibitory neurotransmission better than ATP. Here β-NAD and ATP were compared as possible inhibitory neurotransmitters in human and monkey colons.


METHODS
A small-volume superfusion assay and high-pressure liquid chromatography with fluorescence detection were used to evaluate spontaneous and nerve-evoked overflow of β-NAD, ATP, and metabolites. Postjunctional responses to nerve stimulation, β-NAD and ATP were compared using intracellular membrane potential and force measurements. Effects of β-NAD on smooth muscle cells (SMCs) were recorded by patch clamp. P2Y receptor transcripts were assayed by reverse transcription polymerase chain reaction.


RESULTS
In contrast to ATP, overflow of β-NAD evoked by electrical field stimulation correlated with stimulation frequency and was diminished by the neurotoxins, tetrodotoxin, and ω-conotoxin GVIA. Inhibitory junction potentials and responses to exogenous β-NAD, but not ATP, were blocked by P2Y receptor antagonists suramin, pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate (PPADS), 2'-deoxy-N6-methyladenosine 3',5'-bisphosphate (MRS 2179), and (1R,2S,4S,5S)-4-[2-Iodo-6-(methylamino)-9H-purin-9-yl]-2-(phosphonooxy)bicyclo[3.1.0]hexane-1-methanol dihydrogen phosphate ester tetraammonium salt (MRS 2500). β-NAD activated nonselective cation currents in SMCs, but failed to activate outward currents.


CONCLUSIONS
β-NAD meets the criteria for a neurotransmitter better than ATP in human and monkey colons and therefore may contribute to neural regulation of colonic motility. SMCs are unlikely targets for inhibitory purine neurotransmitters because dominant responses of SMCs were activation of net inward, rather than outward, current.
Hwang
Sung Jin
SJ
Department of Physiology and Cell Biology, University of Nevada School of Medicine, Reno, Nevada 89557-0352, USA.
Durnin
Leonie
L
Dwyer
Laura
L
Rhee
Poong-Lyul
PL
Ward
Sean M
SM
Koh
Sang Don
SD
Sanders
Kenton M
KM
Mutafova-Yambolieva
Violeta N
VN
D000255:Adenosine Triphosphate / Q000032:analysis / Q000037:antagonists & inhibitors / Q000494:pharmacology / Q000502:physiology; D000328:Adult; D000368:Aged; D000818:Animals; D003106:Colon / Q000187:drug effects / Q000294:innervation*; D004558:Electric Stimulation; D017615:Enteric Nervous System / Q000187:drug effects / Q000502:physiology*; D006801:Humans; D007473:Ion Channels / Q000187:drug effects / Q000502:physiology; D008251:Macaca; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D008875:Middle Aged; D009130:Muscle, Smooth / Q000187:drug effects / Q000294:innervation / Q000502:physiology; D009243:NAD / Q000494:pharmacology / Q000502:physiology*; D009498:Neurotoxins / Q000494:pharmacology; D058921:Purinergic P2Y Receptor Antagonists / Q000494:pharmacology; D058470:Receptors, Purinergic P2Y / Q000032:analysis; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology*; D013779:Tetrodotoxin / Q000494:pharmacology; D020866:omega-Conotoxin GVIA / Q000494:pharmacology
D007473:Ion Channels; D009498:Neurotoxins; D058921:Purinergic P2Y Receptor Antagonists; D058470:Receptors, Purinergic P2Y; D009243:NAD; D013779:Tetrodotoxin; D000255:Adenosine Triphosphate; D020866:omega-Conotoxin GVIA