Neuronal and extraneuronal release of ATP and NAD⁺ in smooth muscle

Abstract

SUMMARY Adenosine 5′-triphosphate (ATP) and nicotinamide adenine dinucleotide (NAD+) are key intracellular constituents involved in energy transfer and redox homeostasis in the cell. ATP is also released in the extracellular space and in the past half century it has been assumed to be the purinergic neurotransmitter in many systems including smooth muscle. In some smooth muscles (i.e., the human urinary bladder detrusor muscle) ATP does appear to be primarily released from nerves upon action potential firings, but in other smooth muscles (i.e., the human large intestine) ATP does not mimic the endogenous purine neurotransmitter. It was recently found that NAD+, another ubiquitous intracellular adenine nucleotide, also follows a regulated release in neurosecretory cells, vascular and visceral smooth muscles, and the brain. In some cases NAD+ fulfills pre- and postsynaptic criteria for a neurotransmitter better than ATP. Therefore, the purine hypothesis of neural regulation in smooth muscle is in need of reevaluation. This article will briefly review the current understanding of neuronal and extraneuronal release of purines in smooth muscle with emphasis on the roles of extracellular ATP and NAD+, and, further, will discuss more recent information about the likely involvement of multiple purines in smooth muscle neurotransmission.

The purinergic neurotransmitter revisited: A single substance or multiple players?

“…Therefore, it could be technically demanding to detect purine release from perivascular nerve terminals. Instead, blood vessels represent an excellent model for extraneuronal release of ATP by various mechanisms (see Mutafova-Yambolieva, 2012 and Burnstock & Ralevic, 2014). …”

“…Finally, we now know that multiple substances with adenine and pyrimidine moieties, in addition to ATP, could activate the ionotropic and metabotropic P2 purinergic receptors that mediate postjunctional responses to purine neurotransmitter(s). As mentioned, some of these substances have recently been demonstrated to be released upon nerve stimulation (Smyth et al, 2004; Breen et al, 2006; Mutafova-Yambolieva et al, 2007; Hwang et al, 2011; Durnin et al, 2012b; Durnin et al, 2013) and in some systems such substances appear to mimic the endogenous neurotransmitters better than ATP (Mutafova-Yambolieva et al, 2007; Hwang et al, 2011; Mutafova-Yambolieva, 2012; Durnin et al, 2013). Taken together, such observations bring about the necessity to reevaluate purinergic neurotransmitter identity.…”

“…Amid this entire advance, possible roles of purines in chemical neurotransmission have attracted particular attention as it was found that in many systems there is a component in neural responses that is inhibited by purine receptor antagonists. The nature of the purine neurotransmitter substance, however, is still uncertain, several decades after the purinergic neurotransmission hypothesis was introduced (Burnstock et al, 1970; Burnstock, 1972; Westfall et al, 1982; Chaudhry et al, 1984; Mutafova-Yambolieva et al, 2007; Hwang et al, 2011; Goyal, 2011; Mutafova-Yambolieva, 2012; Gil et al, 2013; Durnin et al, 2013). This might be due in part to technological obstacles and heavy reliance on indirect evidence from measuring radioactive tracers (i.e., tritiated adenosine or adenine), from assaying adenine nucleotide metabolites (i.e., adenosine and inosine) as a measure of released ATP, from common use of chemiluminescence assays that are restricted to ATP (and hence may overlook the release of other purines), and from widely used assessments of post-junctional electrical or mechanical responses to nerve stimulation or exogenous purinergic receptor ligands.…”

β-Nicotinamide adenine dinucleotide acts at prejunctional adenosine A1 receptors to suppress inhibitory musculomotor neurotransmission in guinea pig colon and human jejunum

“…Based on reports that ␤-NAD might be an inhibitory neurotransmitter at motor nerve junctions in smooth muscles of the GI tract and that it is released inside ganglia of the ENS, we investigated if it might have a role as a neurotransmitter at synapses in the ENS or at prejunctional nerve terminals in the musculature (43). We obtained evidence for an action of ␤-NAD at transmitter release sites on ENS inhibitory musculomotor neurons.…”

“…Nevertheless, other purines have been put forward as mediators of GI inhibitory neuromuscular transmission. ␤-Nicotinamide adenine dinucleotide (␤-NAD) and its bioactive metabolite ADPribose (ADPR), rather than ATP, were proposed as the most probable purinergic neurotransmitters (18,43,50). However, the validity of the ␤-NAD-ADPR hypothesis has been questioned (29).…”