Modulation of neural noradrenaline and ATP release by angiotensin II and prostaglandin E2 in guinea-pig vas deferens

Effect of heart failure on catecholamine granule morphology and storage in chromaffin cells

One of the key mechanisms involved in sympathoexcitation in chronic heart failure (HF) is the activation of the adrenal glands. Impact of the elevated catecholamines on the hemodynamic parameters has been previously demonstrated. However, studies linking the structural effects of such overactivation with secretory performance and cell metabolism in the adrenomedullary chromaffin cells in vivo have not been previously reported. In this study, HF was induced in male Sprague-Dawley rats by ligation of the left coronary artery. Five weeks after surgery, cardiac function was assessed by ventricular hemodynamics. HF rats showed increased adrenal weight and adrenal catecholamine levels (norepinephrine, epinephrine and dopamine) compared with sham-operated rats. Rats with HF demonstrated increased small synaptic and dense core vesicle in splanchnic-adrenal synapses indicating trans-synaptic activation of catecholamine biosynthetic enzymes, increased endoplasmic reticulum and Golgi lumen width to meet the demand of increased catecholamine synthesis and release, and more mitochondria with dilated cristae and glycogen to accommodate for the increased energy demand for the increased biogenesis and exocytosis of catecholamines from the adrenal medulla. These findings suggest that increased trans-synaptic activation of the chromaffin cells within the adrenal medulla may lead to increased catecholamines in the circulation which in turn contributes to the enhanced neurohumoral drive, providing a unique mechanistic insight for enhanced catecholamine levels in plasma commonly observed in chronic HF condition.

E-ring 8-isoprostanes inhibit ACh release from parasympathetic nerves innervating guinea-pig trachea through agonism of prostanoid receptors of the EP3-subtype

1. In the present study, we examined the effect of E-ring 8-isoprostanes on cholinergic neurotransmission in guinea-pig trachea and identified the receptor(s) involved. As isoprostanes are isomeric with prostaglandins, PGE(2) and sulprostone (a selective EP(3)-receptor agonist) were examined in parallel. 2. 8-Iso-PGE(1), 8-iso-PGE(2) (0.1 nm-1 microM), sulprostone (1 nm-1 microM) and PGE(2) (1 microM) suppressed EFS-evoked [(3)H]ACh release from guinea-pig trachea in a concentration-dependent manner, producing 39.5, 53.9, 61.2 and 59.9% inhibition, respectively, at 1 microM. It should be noted that an established maximum effective concentration was not determined. 3. Neither SQ 29,548 (1 microm; a TP-receptor antagonist) nor AH 6809 (10 microM; an EP(1)-/EP(2)-/DP-receptor antagonist) reversed the inhibitory effect of these compounds. 4. L-798,106, a novel and highly selective EP(3)-receptor antagonist, produced a parallel shift to the right of the concentration-response curves that described the inhibitory action of sulprostone on EFS-evoked contractile responses in guinea-pig vas deferens (an established EP(3)-receptor-expressing tissue), from which a mean pA(2) of 7.48 was derived. On guinea-pig trachea, L-798,106 also antagonised sulprostone-induced inhibition of EFS-induced twitch responses, with similar potency (mean pA(2)=7.82). 5. The inhibitory effects of 8-iso-PGE(1), 8-iso-PGE(2), sulprostone and PGE(2) on EFS-induced [(3)H]ACh release was blocked by L-798,106 at a concentration (10 microM) that binds only weakly to human recombinant EP(1)-, EP(2)- and EP(4)-receptor subtypes expressed in HEK 293 cells. 6. These data suggest that E-ring 8-isoprostanes, PGE(2) and sulprostone inhibit EFS-evoked [(3)H]ACh release from cholinergic nerves innervating guinea-pig trachea, by interacting with prejunctional prostanoid receptors of the EP(3)-subtype.

ATP release by angiotensin II from segments and cultured smooth muscle cells of guinea-pig taenia coli

Effects of angiotensin II on ATP release were evaluated in segments and cultured smooth muscle cells from the guinea-pig taenia coli. In the segments, angiotensin II (0.3-3 microM) elicited release of ATP which was blocked by losartan and SC-52458, non-peptide angiotensin II type-1 receptor (AT1)-antagonists, but not by PD-123319, an AT2-antagonist. In superfused cultured cells, 10 microM angiotensin II likewise elicited release of ATP. Again the response was blocked by losartan and SC-52458 but not by PD-123319. These findings suggest that angiotensin II releases ATP from the smooth muscles by activation of angiotensin II-, presumably AT1-, receptors.

Inhibition of purinergic transmission by prostaglandin E1 and E2 in the guinea-pig vas deferens: an electrophysiological study

1. The effects of prostaglandin E1 (PGE1) and E2 (PGE2) on postjunctional electrical activity in the guinea-pig vas deferens evoked by sympathetic nerve stimulation were investigated using both intracellular and focal extracellular recording techniques in vitro. 2. Bath application of PGE1 (1-100 nM) or PGE2 (0.1-100 nM) concentration-dependently inhibited the amplitudes of all excitatory junction potentials (e.j.ps) evoked during short trains of stimuli (10 stimuli at 1 Hz). Increasing the duration of nerve stimulation (100 stimuli at 1 Hz) did not overcome this inhibitory effect. At these concentrations PGE1 and PGE2 were without any apparent inhibitory effect on the amplitudes of spontaneous e.j.ps. 3. Local application of PGE1 (10-100 nM) or PGE2 (10-30 nM) markedly reduced the frequency of occurrence of excitatory junction currents (e.j.cs) evoked by trains of 20-100 stimuli at 1 to 4 Hz without changing the amplitudes of spontaneous e.j.cs or the configuration of the nerve terminal impulse. 4. In the presence of PGE1 or PGE2, raising the frequency of stimulation (from 1 to 4 Hz), increased the likelihood of e.j.c. occurrence. 5. The postjunctional electrical activity recorded in the guinea-pig vas deferens is believed to be due to ATP released from the sympathetic nerve endings. Thus the present study demonstrates that both PGE1 and PGE2 powerfully inhibit quantal ATP release in the guinea-pig vas deferens.

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.

A comparison of the effects of exogenous and endogenous prostaglandins on fast and slow contractions of field-stimulated guinea-pig vas deferens

1. This study has compared the effects of exogenous and endogenous prostaglandins on the two phases of contraction of the guinea-pig vas deferens produced by electrical field stimulation. Prostaglandin E2 (PGE2), sulprostone and arachidonic acid dose-dependently and completely inhibited the first (fast) phase of contraction, with IC50s of 2.6 nM, 0.65 nM and 2.2 microM, respectively. 2. Following desensitization of the receptor for adenosine triphosphate (ATP) with alpha, beta-methylene ATP, PGE2, sulprostone and arachidonic acid dose-dependently inhibited the second (slow) phase of contraction of the guinea-pig vas deferens produced by electrical field stimulation, but the inhibition was incomplete (up to only 30%). Indomethacin (2.8 microM) reduced the effect of arachidonic acid. On its own, indomethacin (0.3 to 6.0 microM) had no consistent effect although, on some tissues, a slight potentiation of the contractions was seen. 3. Cicaprost (a PGI2 analogue) at low concentrations (0.5 to 30 nM) potentiated the first phase of contraction but even at high concentrations, had no consistent effect on the second phase of contraction of the guinea-pig vas deferens produced by electrical field stimulation. 4. PGE2, sulprostone and cicaprost potentiated contractions of the guinea-pig vas deferens produced by exogenous ATP. PGE2 and sulprostone also potentiated contractions produced by exogenous noradrenaline, whereas cicaprost had no consistent effect on the response to noradrenaline. 5. These findings indicate that prostaglandins of the E-series inhibit the second phase of contraction as well as the first phase of contraction of the guinea-pig vas deferens produced by electrical field stimulation. However, the extent of the inhibition is much less for the second phase than for the first phase. The reasons for this differential action of PGE are not clear. 6. Cicaprost potentiates the first phase but not the second phase of contraction. Since cicaprost potentiates the contractions produced by exogenous ATP, but not by exogenous noradrenaline, by an action presumably on post-junctional IP receptors, the potentiating action of cicaprost on the first phase of contraction produced by electrical field stimulation would appear to be satisfactorily explained through the action of cicaprost on these post-junctional IP receptors. 7. Exogenous arachidonic acid is apparently converted predominantly to PGE2 by the vas deferens, since the action of arachidonic acid mimicked that of PGE2 and was reduced by indomethacin. However, there was little evidence that sufficient PGE2 is generated during a short period (15 s) of sympathetic nerve stimulation for it to have any significant inhibitory effect on the size of the contractions produced.