Vas deferens neuro-effector junction: from kymographic tracings to structural biology principles

Extracellular cAMP elicits contraction of rat vas deferens: Involvement of ecto-5'-nucleotidase and adenosine A1 receptors

AIMS

It is well established that intracellular cAMP contributes to the relaxation of vas deferens smooth muscle. In many tissues, intracellular cAMP is actively transported to the extracellular space, where it exerts regulatory functions, via its metabolite adenosine. These actions take place through the cAMP conversion to adenosine by ectoenzymes, a process called "extracellular cAMP-adenosine pathway". Herein, we investigated whether, in addition to ATP, extracellular cAMP might be an alternative source of adenosine, influencing the contraction of vas deferens smooth muscle.

MAIN METHODS

The effects of cAMP, 8-Br-cAMP and adenosine were analyzed in the isometric contractions of rat vas deferens. cAMP efflux was analyzed by measuring extracellular cAMP levels after exposure of vas deferens segments to isoproterenol and forskolin in the presence or absence of MK-571, an inhibitor of MRP/ABCC transporters.

KEY FINDINGS

While 8-Br-cAMP, a cell-permeable cAMP analog, induced relaxation of KCl-precontracted vas deferens, the non-permeant cAMP increased the KCl-induced contractile response, which was mimicked by adenosine, but prevented by inhibitors of ecto-5'-nucleotidase or A1 receptors. Our results also showed that isoproterenol and forskolin increases cAMP efflux via an MRP/ABCC transporter-dependent mechanism, since it is inhibited by MK-571.

SIGNIFICANCE

Our data show that activation of β-adrenoceptors and adenylyl cyclase increases cAMP efflux from vas deferens tissue, which modulates the vas deferens contractile response via activation of adenosine A1 receptors. Assuming that inhibition of vas deferens contractility has been proposed as a strategy for male contraception, the extracellular cAMP-adenosine pathway emerges as a potential pharmacological target that should be considered in studies of male fertility.

Interaction of endocannabinoid system and cyclooxygenase metabolites with fatty acid amide hydrolase and cyclooxygenase enzyme activities on contractile responses in rat vas deferens tissue

The endocannabinoid system and prostaglandins are important modulators in the genitourinary system. This study aimed to investigate the possible interactions between the endocannabinoid system and the cyclooxygenase (COX) pathway on rat vas deferens. For this purpose, the concentration responses of the endocannabinoid anandamide, prostaglandin F2α analog latanoprost, and prostaglandin E1 analog misoprostol on the electrical field stimulation (EFS)-induced contractile responses were obtained. The concentration responses to anandamide were obtained again in the presence of nonselective COX inhibitor flurbiprofen and prostaglandin analogs, while the concentration responses of latanoprost and misoprostol were obtained in the presence of cannabinoid receptor antagonists and fatty acid amide hydrolase (FAAH) enzyme inhibitor URB597. FAAH, COX-1, and COX-2 enzyme levels in vas deferens tissue samples were also determined. The cumulative addition of anandamide was not different from the vehicle; however, the EFS-induced contractile responses were significantly increased with the incubation of latanoprost or flurbiprofen in the prostatic portion. Flurbiprofen and misoprostol decreased FAAH enzyme levels in both portions of the vas deferens, while latanoprost induced the inhibition in the prostatic portion. The cumulative administration of latanoprost and misoprostol significantly enhanced the contractile responses in the prostatic portion. This effect of latanoprost was significantly antagonized by URB597 and AM251. The enhancing effect of misoprostol was antagonized by anandamide, URB597, AM251, and AM630. Anandamide, AM251, AM630, and URB597 decreased enzyme levels of COX-1 and COX-2 in both portions of the vas deferens. These results demonstrate an intricate crosstalk between endocannabinoids and prostaglandins in modulation of the vas deferens contractility.

The pivotal role of neuronal nitric oxide synthase in the release of 6-nitrodopamine from mouse isolated vas deferens

6-Nitrodopamine (6-ND) is released from rat and human vas deferens and is considered a major mediator of both tissues contractility. The contractions induced by 6-ND are selectively blocked by both tricyclic antidepressants and α1-adrenoceptor antagonists. Endothelial nitric oxide synthase (eNOS) is the major isoform responsible for 6-ND release in mouse isolated heart, however the origin of 6-ND in the vas deferens is unknown. Here it was investigated by LC-MS/MS the basal release of 6-ND from isolated vas deferens obtained from control, eNOS-/-, nNOS-/-, and iNOS-/- mice. In addition, it was evaluated in vitro vas deferens contractility following electric field stimulation (EFS). Basal release of 6-ND was significantly reduced in nNOS-/- mice compared to control mice, but not decreased when the vas deferens were obtained from either eNOS-/- or iNOS-/- mice. Pre-incubation of the vas deferens with tetrodotoxin (1 μM) significantly reduced the basal release of 6-ND from control, eNOS-/-, and iNOS-/- mice but had no effect on the basal release of 6-ND from nNOS-/- mice. EFS-induced frequency-dependent contractions of the vas deferens, which were significantly reduced when the tissues obtained from control, eNOS-/- and iNOS-/- mice, were pre-incubated with l-NAME, but unaltered when the vas deferens was obtained from nNOS-/- mice. In addition, the EFS-induced contractions were significantly smaller when the vas deferens were obtained from nNOS-/- mice. The results clearly demonstrate that nNOS is the main NO isoform responsible for 6-ND release in mouse vas deferens and reinforces the concept of 6-ND as a major modulator of vas deferens contractility.

Nanomolar clodronate induces adenosine accumulation in the perfused rat mesenteric bed and mesentery-derived endothelial cells

The vesicular nucleotide transporter (VNUT) is critical for sympathetic co-transmission and purinergic transmission maintenance. To examine this proposal, we assessed whether the bisphosphonate clodronate, claimed as a potent in vitro VNUT blocker, modified spontaneous and/or the electrically evoked overflow of ATP/metabolites and NA from mesentery sympathetic perivascular nerve terminals. Additionally, in primary endothelial cell cultures derived from this tissue, we also evaluated whether clodronate interfered with ATP/metabolite cell outflow and metabolism of N⁶-etheno adenosine 5'-triphosphate (eATP), N⁶-etheno adenosine (eADO), and adenosine deaminase enzyme activity. Rat mesenteries were perfused in the absence or presence of .01-1,000 nM clodronate, 1-1,000 nM Evans blue (EB), and 1-10 µM DIDS; tissue perfusates were collected to determine ATP/metabolites and NA before, during, and after perivascular electrical nerve terminal depolarization. An amount of 1-1,000 nM clodronate did not modify the time course of ATP or NA overflow elicited by nerve terminal depolarization, and only 10 nM clodronate significantly augmented perfusate adenosine. Electrical nerve terminal stimulation increased tissue perfusion pressure that was significantly reduced only by 10 nM clodronate [90.0 ± 18.6 (n = 8) to 35.0 ± 10.4 (n = 7), p = .0277]. As controls, EB, DIDS, or reserpine treatment reduced the overflow of ATP/metabolites and NA in a concentration-dependent manner elicited by nerve terminal depolarization. Moreover, mechanical stimulation of primary endothelial cell cultures from the rat mesentery added with 10 or 100 nM clodronate increased adenosine in the cell media. eATP was metabolized by endothelial cells to the same extent with and without 1-1,000 nM clodronate, suggesting the bisphosphonate did not interfere with nucleotide ectoenzyme metabolism. In contrast, extracellular eADO remained intact, indicating that this nucleoside is neither metabolized nor transported intracellularly. Furthermore, only 10 nM clodronate inhibited (15.5%) adenosine metabolism to inosine in endothelial cells as well as in a commercial crude adenosine deaminase enzyme preparation (12.7%), and both effects proved the significance (p < .05). Altogether, present data allow inferring that clodronate inhibits adenosine deaminase activity in isolated endothelial cells as in a crude extract preparation, a finding that may account for adenosine accumulation following clodronate mesentery perfusion.

Contraction of Rat Cauda Epididymis Smooth Muscle to α1-Adrenoceptor Activation Is Mediated by α1A-Adrenoceptors

The cauda epididymis (CE), the site of sperm storage until the ejaculation, is densely innervated by the sympathetic nervous system. Contraction of CE smooth muscle via α₁-adrenoceptors (α₁-ARs) plays a key role during the seminal emission phase of ejaculation and α₁-AR antagonism has been suggested as a nonhormonal and reversible male contraceptive target. Since the α₁-AR subtype mediating contraction of rat CE is not known, this study investigates the expression and role of α₁-AR subtypes on the proximal and distal rat CE duct contraction to norepinephrine in vitro. Alpha_1a, α_1b, and α_1d transcripts were detected by real-time quantitative polymerase chain reaction in proximal and distal CE segments and α_1a and α_1d were shown to predominate over α_1b The inhibition of [³H]prazosin specific binding to intact CE segments from proximal and distal CE by RS 100329 and 5-methylurapidil (α_1A-selective) and BMY 7378 (α_1D-selective) showed that α_1A- and α_1D-ARs are expressed at similar densities. Norepinephrine-induced contractions of CE were competitively antagonized with high affinity by RS 100329 (pK_B ≈ 9.50) and 5-methylurapidil (pK_B ≈ 9.0) and with low affinity by BMY 7378 (pK_B ≈ 7.0) and the α_1B-selective L-765,314 (pA₂ < 7.0), suggesting contractions are mediated by α_1A-ARs. The clinically used α_1A/D-ARs antagonist tamsulosin potently (pA₂ ≈ 10.0) inhibited the norepinephrine-induced CE contractions. Altogether, our results show that α_1A- and α_1D-ARs are expressed in the CE duct and α_1A-AR is the main subtype mediating contraction to norepinephrine. Our results highlight the importance of α_1A-AR in the peripheral control of ejaculation and strengthen the α_1A-AR as a target for a nonhormonal approach to male contraception.

Pharmacological dissection of the cellular mechanisms associated to the spontaneous and the mechanically stimulated ATP release by mesentery endothelial cells: roles of thrombin and TRPV

Endothelial cells participate in extracellular ATP release elicited by mechanosensors. To characterize the dynamic interactions between mechanical and chemical factors that modulate ATP secretion by the endothelium, we assessed and compared the mechanisms participating in the spontaneous (basal) and mechanically stimulated secretion using primary cultures of rat mesentery endothelial cells. ATP/metabolites were determined in the cell media prior to (basal) and after cell media displacement or a picospritzer buffer puff used as mechanical stimuli. Mechanical stimulation increased extracellular ATP that peaked within 1 min, and decayed to basal values in 10 min. Interruption of the vesicular transport route consistently blocked the spontaneous ATP secretion. Cells maintained in media lacking external Ca²⁺ elicited a spontaneous rise of extracellular ATP and adenosine, but failed to elicit a further extracellular ATP secretion following mechanical stimulation. 2-APB, a TRPV agonist, increased the spontaneous ATP secretion, but reduced the mechanical stimulation-induced nucleotide release. Pannexin1 or connexin blockers and gadolinium, a Piezo1 blocker, reduced the mechanically induced ATP release without altering spontaneous nucleotide levels. Moreover, thrombin or related agonists increased extracellular ATP secretion elicited by mechanical stimulation, without modifying spontaneous release. In sum, present results allow inferring that the spontaneous, extracellular nucleotide secretion is essentially mediated by ATP containing vesicles, while the mechanically induced secretion occurs essentially by connexin or pannexin1 hemichannel ATP transport, a finding fully supported by results from Panx1^-/- rodents. Only the latter component is modulated by thrombin and related receptor agonists, highlighting a novel endothelium-smooth muscle signaling role of this anticoagulant.

Impact of ectonucleotidases in autonomic nervous functions

Adenine and uracil nucleotides play key functions in the autonomic nervous system (ANS). For instance, ATP acts as a neurotransmitter, co-transmitter and neuromodulator in the ANS. The purinergic system encompasses (1) receptors that respond to extracellular purines, which are designated as P1 and P2 purinoceptors, (2) purine release and uptake, and (3) a cascade of enzymes that regulate the concentration of purines near the cell surface. Ectonucleotidases and adenosine deaminase (ADA) are enzymes responsible for the hydrolysis of ATP (and other nucleotides such as ADP, UTP, UDP, AMP) and adenosine, respectively. Accordingly, these enzymes are expected to play an important role in the control of neuro-effector transmission in tissues innervated by both the sympathetic and parasympathetic divisions of the ANS. Indeed, ectonucleotidases have the ability to either terminate P2 receptor responses initiated by nucleoside triphosphates (ATP and UTP), and/or to favor the activation of ADP (e.g. P2Y1,12,13) and UDP (e.g. P2Y6) and/or adenosine (P1) specific receptors. In addition, ectonucleotidases can also importantly protect some P2 receptors from desensitization (e.g. P2X1, P2Y1). In this review, we present the (putative) roles of ectonucleotidases and ADA in the ANS with a focus on their regulatory activity at neuro-effector junctions in the following tissues: heart, vas deferens, urinary bladder, salivary glands, blood vessels and the intestine. We also present their implication in nociceptive transmission.

ATP and related purines stimulate motility, spatial congregation, and coalescence in red algal spores

Adenosine 5'-triphosphate (ATP) is a versatile extracellular signal along the tree of life, whereas cAMP plays a major role in vertebrates as an intracellular messenger for hormones, transmitters, tastants, and odorants. Since red algal spore coalescence may be considered analogous to the congregation process of social amoeba, which is stimulated by cAMP, we ascertained whether exogenous applications of ATP, cAMP, adenine, or adenosine modified spore survival and motility, spore settlement and coalescence. Concentration-response studies were performed with carpospores of Mazzaella laminarioides (Gigartinales), incubated with and without added purines. Stirring of algal blades released ADP/ATP to the cell media in a time-dependent manner. 10-300 μM ATP significantly increased spore survival; however, 1,500 μM ATP, cAMP or adenine induced 100% mortality within less than 24 h; the exception was adenosine, which up to 3,000 μM, did not alter spore survival. ATP exposure elicited spore movement with speeds of 2.2-2.5 μm · s(-1) . 14 d after 1,000 μM ATP addition, spore abundance in the central zone of the plaques was increased 2.7-fold as compared with parallel controls. Likewise, 1-10 μM cAMP or 30-100 μM adenine also increased central zone spore abundance, albeit these purines were less efficacious than ATP; adenosine up to 3,000 μM did not influence settlement. Moreover, 1,000 μM ATP markedly accelerated coalescence, the other purines caused a variable effect. We conclude that exogenous cAMP, adenine, but particularly ATP, markedly influence red algal spore physiology; effects are compatible with the expression of one or more membrane purinoceptor(s), discarding adenosine receptor participation.