Comparison of corelease of noradrenaline and ATP evoked by hypogastric nerve stimulation and field stimulation in guinea-pig vas deferens

Alkaline Phosphatase Inhibitors Attenuate Renovascular Responses to Norepinephrine

Tissue nonspecific alkaline phosphatase (TNAP) contributes to the production of adenosine by the kidney, and A₁-receptor activation enhances renovascular responses to norepinephrine. Therefore, we hypothesized that TNAP regulates renovascular responsiveness to norepinephrine. In isolated, perfused rat kidneys, the TNAP inhibitor l-p-bromotetramisole (0.1 mmol/L) decreased renal venous levels of 5'-AMP (adenosine precursor) and adenosine by 61% (P<0.0384) and 62% (P=0.0013), respectively, at 1 hour into treatment and caused a 10-fold rightward shift of the concentration-response relationship to exogenous norepinephrine (P<0.0001). Similarly, 2 other TNAP inhibitors, levamisole (1 mmol/L) and 2,5-dimethoxy-N-(quinolin-3-yl)benzenesulfonamide (0.02 mmol/L), also right shifted the concentration-response relationship to norepinephrine. The ability of TNAP inhibition to blunt renovascular responses to norepinephrine was mostly prevented or reversed by restoring A₁-adenosinergic tone with the A₁-receptor agonist 2-chloro-N⁶-cyclopentyladenosine (100 nmol/L). All 3 TNAP inhibitors also attenuated renovascular responses to renal sympathetic nerve stimulation, suggesting that TNAP inhibition attenuates renovascular responses to endogenous norepinephrine. In control propranolol-pretreated rats, acute infusions of norepinephrine (10 μg/kg/min) increased mean arterial blood pressure from 95±5 mm Hg to a peak of 169±4 mm Hg and renovascular resistance from 12±2 mm Hg/mL/min to a peak of 55±12 mm Hg/mL/min; however, in rats also treated with intravenous l-p-bromotetramisole (30 mg/kg), the pressor and renovascular effects of norepinephrine were significantly attenuated (blood pressure: basal and peak, 93±7 and 146±6 mm Hg, respectively; renovascular resistance: basal and peak, 13±2 and 29±5 mm Hg/mL/min, respectively). TNAP inhibitors attenuate renovascular and blood pressure responses to norepinephrine, suggesting that TNAP participates in the regulation of renal function and blood pressure.

Effects of Heptanol on Neurogenic Contractions of Vas Deferens: A Comparative Study of Stimulation Frequency in Guinea Pig and Rat

This study examines the role of gap junctional communication in smooth muscle in relation to the frequency of stimulation and the innervation density of the tissue in the generation of neurogenic contractions. Toward this end the effects of heptanol, a gap junctional blocker, on the neurogenic contractions of guinea pig and rat vas deferens at different frequencies of stimulation (single pulse, 5, 10, 20, 40, 60, and 80 Hz) were studied. In both the prostatic and epididymal halves of these tissues, heptanol abolished the neurogenic contractions at the lower frequencies of stimulation. At higher frequencies, contractions were resistant to heptanol action. The effect of heptanol on the neurogenic contractions was found to decrease with increasing stimulation frequency. The neurogenic contractions of rat vas deferens were more resistant to heptanol than those of guinea pig vas deferens. Our data indicate that gap junctional communication is significant in the generation of neurogenic contractions in both guinea pig and rat vas deferens in a frequency-dependent manner, and we discuss the mechanisms underlying these findings.

Novel dual 'small' vesicle model of ATP- and noradrenaline-mediated sympathetic neuromuscular transmission

The main question asked was if sympathetic nerves in guinea-pig vas deferens release the co-transmitters ATP and noradrenaline from the same, or different vesicles, i.e. in fixed combinations or independently. The extracellularly recorded excitatory junction current (EJC) and the fractional increase in overflow of tritium (delta T) were used to monitor the per pulse secretion of ATP and [3H]NA, respectively, during electrical stimulation with 1-3000 pulses at 0.1-40 Hz. The frequency- and train length-dependence and alpha 2-adrenoceptor-mediated autoinhibition of these parameters, and of the ATP-mediated twitch contraction, were compared first in the presence of cocaine (to block noradrenaline reuptake), then after brief exposure to phenoxybenzamine (PBA, to irreversibly 'destroy' alpha 2-autoreceptors). Parallel variations of EJC/p(ulse) and delta T/p(ulse) under all conditions would support, non-parallel variations argue against exocytosis of ATP and noradrenaline from the same vesicles. The main findings were that facilitation and alpha 2-autoinhibition of EJC/p and delta T/p were remarkably similar during stimulation at 2 Hz but increasingly dissimilar at higher frequencies. delta T/p remained strongly facilitated and tightly controlled by activation of alpha 2-autoreceptors at 10-40 Hz, but both the facilitation and the sensitivity to alpha 2-autoinhibition of EJC/p were inversely related to frequency. At 40 Hz EJCs were 'small', minimally facilitated and totally unaffected by cocaine or PBA, i.e. insensitive to alpha 2-autoinhibition. Nevertheless, activation of alpha 2-receptors during the 40 Hz train strongly restricted the 'post-tetanic augmentation' (PTA) of the first EJC 10 s after the tetanus. Comparison between the frequency dependence of EJCs and the twitch contraction in the presence of cocaine or after PBA treatment indicates that it is the 'summed EJC per second', i.e. the ATP-driven current injection per unit time into smooth muscle, which triggers the twitch. The working hypothesis is proposed that these nerves use two classes of 'small vesicles' (SVs) to store and release either 'big' or 'small' ATP and noradrenaline 'quanta', and that differences in properties (Ca2+ affinity, capacity) of Ca2+ receptors in the SV membranes enable the nerves to selectively secrete 'big quanta' at low frequency and 'small quanta' during trains at high frequency.

P2X2 receptor expression by interstitial cells of Cajal in vas deferens implicated in semen emission

Male reproduction is dependent upon seminal emission mediated by vas deferens contraction. This drives spermatic fluid to the prostatic urethra during ejaculation. We localize interstitial cells of Cajal (ICC), which express P2X2 receptor, subunits of ATP-gated ion channels, to rat, mouse and guinea-pig vas deferens submucosa. Reverse transcription-polymerase chain reaction (RT-PCR) analysis of rat vas deferens resolved two functional splice variant transcripts of the P2X2 receptor subunit. The P2X2 receptor mRNA was localized principally within the lamina propria (submucosal) region of the rat vas deferens using in situ hybridization (ISH) and in situ RT-PCR-ISH. Immunohistochemistry using rat, mouse and guinea-pig vas deferens tissues confirmed expression of P2X2 receptor protein within the lamina propria, particularly within a dense column of small spindle-shaped cells adjacent to the columnar epithelial cells which line the lumen. This immunoreactivity was co-localized with neurone-specific enolase (NSE) and c-Kit protein expression, gene markers for ICC. Mucosal mast cells were distinguished from ICC by toluidine blue staining. Choline acetyltransferase immunoreactivity, a marker for post-ganglionic parasympathetic innervation, occurred on the lateral margin of the lamina propria and extended into the inner longitudinal muscle layer. P2X1 receptor immunolabelling was associated with sympathetic innervation of the smooth muscle in the outer longitudinal and circular muscle layers, but not the inner longitudinal layer. The physiological significance of the vas deferens ICC which express P2X2 receptors remains to be established. Possible roles include regulation of smooth muscle activity or mucosal secretion utilizing local ATP signaling, both of which would affect semen transport.

Response to prejunctional adenosine receptors is dependent on stimulus frequency

PURPOSE

To characterize the adenosine receptor modulation of norepinephrine (NE) release from sympathetic neurons in the isolated rabbit iris/ciliary body.

METHODS

Iris/ciliary bodies were isolated from New Zealand White rabbits and incubated in the presence of 3H-NE. Norepinephrine release was elicited by field stimulation with varied frequencies from 5 to 30 Hz. The effects of adenosinergic and alpha 2 adrenergic compounds on NE release were determined and compared.

RESULTS

At a stimulation frequency of 5 Hz, the addition of the adenosine A1 agonist CHA did not significantly alter 3H-NE release. However, in the presence of the alpha 2 adrenergic antagonist yohimbine, the addition of CHA produced a dose-related reduction in 3H-NE release. The EC50 for this reduction was 14 nM. At a stimulus frequency of 20 Hz, the addition of CHA alone (10(-6) M) produced a significant reduction in 3H-NE release of 41%. The EC50s for the adenosine A1 agonists CHA- and R-PIA-induced suppression of 3H-NE release at 20 Hz were 32 and 24 nM, respectively. The adenosine A2 agonist CV-1808 did not alter 3H-NE release at stimulation frequencies of 5 or 20 Hz. Pretreatment of tissues with the adenosine A1 antagonist CPT or pertussis toxin reversed the suppression of 3H-NE release induced by CHA. Comparison of the inhibitory responses of CHA to the alpha 2 adrenergic agonist UK-14,304 at stimulus frequencies of 5 to 30 Hz demonstrated that this adenosine A1 agonist was effective only in suppressing NE release at frequencies of 20 Hz or greater. In contrast, the alpha 2 adrenergic agonist UK-14,304 was most effective in reducing NE release at 5 Hz.

CONCLUSIONS

These results provide evidence that adenosine agonists inhibit 3H-NE release in the iris/ciliary body via prejunctional adenosine A1 receptors linked to Gi/o-protein. However, the expression of this response was dependent on the frequency of neuronal stimulation. Hence, prejunctional adenosine A1 receptors may act to selectively limit high-frequency neurotransmission.

Opposite modulation of noradrenaline and ATP release in guinea-pig vas deferens through prejunctional beta-adrenoceptors: evidence for the beta 2 subtype

Activation of prejunctional beta-adrenoceptors has been suggested to increase the release of noradrenaline but to decrease the neural release of ATP in the guinea-pig vas deferens. Experiments were carried out to determine the subtype of beta-adrenoceptor involved. In [3H]-noradrenaline-preincubated tissues superfused with medium containing prazosin and suramin, isoprenaline (1-100 nM), salbutamol (0.01-1 microM) and terbutaline (0.1-10 microM) increased the overflow of tritium but reduced the overflow of ATP elicited by electrical stimulation (210 pulses/7 Hz). The effects of isoprenaline were blocked by the beta 2-selective antagonist 1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3- [(1-methylethyl)amino]-2-butanol (ICI 118,551; 100 nM). In prazosin- and suramin-free medium, isoprenaline (100 nM) did not change the overflow of ATP elicited by exogenous noradrenaline (10 microM). Isoprenaline (1-100 nM), salbutamol (0.01-1 microM) and terbutaline (0.1-10 microM) reduced the initial twitch contraction elicited by electrical stimulation (210 pulses/7 Hz) in prazosin- and suramin-free medium as well as the isolated purinergic neurogenic contraction obtained by exposure to prazosin. They increased or tended to increase the secondary sustained concentration elicited by electrical stimulation in prazosin- and suramin-free medium as well as the isolated adrenergic neurogenic contraction obtained in the presence of suramin. The inhibition by isoprenaline of the isolated purinergic contraction was attenuated by ICI 118,551 (100 nM) but not by the beta 1-selective antagonist 1-[2-((3-carbamoyl- 4-hydroxy)phenoxy)ethylamino]-3-[4-(1-methyl-4- trifluoromethyl-2-imidazolyl)phenoxy]-2-propanol (CGP 20712A; 100 nM). The results confirm the opposite beta-adrenoceptor-mediated modulation of noradrenaline and neural ATP release in the guinea-pig vas deferens. They show that the prejunctional beta-adrenoceptor is of the beta 2-subtype.

Inhibition of neural ATP release by atrial natriuretic peptide in guinea-pig vas deferens

Effects of atrial natriuretic peptide (ANP) and 8-bromoguanosine 3':5'-cyclic monophosphate (8-BrcGMP) on contraction, overflow of tritium (after [3H]noradrenaline labelling) and overflow of ATP elicited by electrical field stimulation (210 pulses/7 Hz) were studied in guinea-pig vas deferens. ANP (1-100 nM) slightly increased contractions, did not alter the overflow of tritium but decreased the overflow of ATP by up to 50%. 8-BrcGMP (3-300 mu M) markedly reduced contractions and ATP overflow with no effect on tritium overflow. Contractions were suppressed in the presence of prazosin plus suramin, and evoked overflow of ATP declined to 11%. ANP now gradually increased tritium overflow but again decreased the overflow of ATP. 8-BrcGMP did not change tritium overflow, as before, but increased ATP overflow. The results indicate that ANP inhibits neural release of ATP by a mechanism independent of guanylyl cyclase activation with no major effect on noradrenaline release.

Failure of tyramine to release neuronal ATP as a cotransmitter of noradrenaline in the guinea-pig vas deferens

Contractions, release of noradrenaline and release of ATP elicited by the indirectly acting sympathomimetic amine tyramine and responses elicited by exogenous noradrenaline were studied in the isolated vas deferens of the guinea pig. Release of noradrenaline was assessed as overflow of tritium after preincubation with [3H]-noradrenaline. ATP was measured by means of the luciferin-luciferase technique. In tissues pretreated with pargyline 1 mM, tyramine 300 microM, when added to the superfusion medium for 2 min, elicited contraction and an overflow of tritium (mainly [3H]-noradrenaline) and ATP. Contraction and ATP overflow responses were prevented and tritium overflow was greatly reduced by desipramine 10 microM. Prazosin 0.3 microM abolished contractions and evoked ATP overflow without changing tritium overflow. Blockade of postjunctional P2-purinoceptors by suramin 300 microM caused a marked decrease of tyramine-evoked contractions and a slight reduction of tritium overflow whereas evoked ATP overflow was markedly increased. The effect on contraction was not shared by two other P2-purinoceptor antagonists, namely pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) 32 microM and diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) 32 microM: PPADS increased contractions about fourfold, whilst DIDS had no effect at all. When the vas deferens was superfused for 24 min with medium containing tyramine 300 microM, evoked contractions and tritium overflow continued throughout whereas ATP overflow faded rapidly to basal values. In the presence of prazosin 0.3 microM, tyramine 300 microM again failed to elicit contractions as well as an overflow of ATP. Application of noradrenaline 10 microM instead of tyramine also resulted in prolonged contraction and an overflow of ATP that declined rapidly. It is concluded that all ATP released by tyramine is non-neuronal in origin, secondary to the activation of postjunctional alpha 1-adrenoceptors by released noradrenaline. The non-neural ATP does not seem to play a functional role in smooth muscle contraction and derives from a postjunctional source which is subject to a rapid depletion upon sustained alpha 1-adrenoceptor activation.