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

Regulation of smooth muscle contractility by the epithelium in rat vas deferens: role of ATP-induced release of PGE2

Recent studies suggest that the epithelium might modulate the contractility of smooth muscle. However, the mechanisms underlying this regulation are unknown. The present study investigated the regulation of smooth muscle contraction by the epithelium in rat vas deferens and the possible factor(s) involved. Exogenously applied ATP inhibited electrical field stimulation (EFS)-evoked smooth muscle contraction in an epithelium-dependent manner. As the effects of ATP on smooth muscle contractility were abrogated by inhibitors of prostaglandin synthesis, but not by those of nitric oxide synthesis, prostaglandins might mediate the effects of ATP. Consistent with this idea, PGE(2) inhibited EFS-evoked smooth muscle contraction independent of the epithelium, while ATP and UTP induced the release of PGE(2) from cultured rat vas deferens epithelial cells, but not smooth muscle cells. The ATP-induced PGE(2) release from vas deferens epithelial cells was abolished by U73122, an inhibitor of phospholipase C (PLC) and BAPTA AM, a Ca(2+) chelator. ATP also transiently increased [Ca(2+)](i) in vas deferens epithelial cells. This effect of ATP on [Ca(2+)](i) was independent of extracellular Ca(2+), but abolished by the P2 receptor antagonist RB2 and U73122. In membrane potential measurements using a voltage-sensitive dye, PGE(2), but not ATP, hyperpolarized vas deferens smooth muscle cells and this effect of PGE(2) was blocked by MDL12330A, an adenylate cyclase inhibitor, and the chromanol 293B, a blocker of cAMP-dependent K(+) channels. Taken together, our results suggest that ATP inhibition of vas deferens smooth muscle contraction is epithelium dependent. The data also suggest that ATP activates P2Y receptor-coupled Ca(2+) mobilization leading to the release of PGE(2) from epithelial cells, which in turn activates cAMP-dependent K(+) channels in smooth muscle cells leading to the hyperpolarization of membrane voltage and the inhibition of vas deferens contraction. Thus, the present findings suggest a novel regulatory mechanism by which the epithelium regulates the contractility of smooth muscle.

Novel cyclooxygenase-catalyzed bioactive prostaglandin F2alpha from physiology to new principles in inflammation

Prostaglandin F2alpha (PGF2alpha), a foremost stable vasoactive cyclooxygenase (COX)-catalyzed prostaglandin, regulates a number of key physiological functions such as luteolysis, ovarian function, luteal maintenance of pregnancy, and parturition as a constitutive part of ongoing reproductive processes of the body. It has recently been implicated in the regulation of intricate pathophysiological processes, such as acute and chronic inflammation, cardiovascular and rheumatic diseases. Since the discovery of a second isoform of COXs, it has been shown that PGF2alpha can be formed in vivo from arachidonic acid through both isoforms of COXs, namely cyclooxygenase-1 (COX-1) and cyclooxygenase-2 (COX-2). Being synthesized in various parts of the body, it metabolizes instantly to a number of rather inactive metabolites mainly in the lungs, liver, kidney, and efficiently excretes into the urine. 15-Keto-dihydro-PGF2alpha, a major stable metabolite of PGF2alpha that reflects in vivo PGF2alpha biosynthesis, is found in larger quantities than its parent compound in the circulation and urine in basal physiological conditions, with short-lived pulses during luteolysis, induced termination of pregnancy and parturition, and is increased in tissues and various body fluids during acute, sub-chronic, and severe chronic inflammation. Further, the close relationship of PGF2alpha with a number of risk factors for atherosclerosis indicates its major role in inflammation pathology. This review addresses multiple aspects of PGF2alpha in addition to its emerging role in physiology to inflammation.

Bradykinin and electrical stimulation increase prostaglandin production in the rat vas deferens

The epididymal portion of the rat vas deferens produced prostaglandins (PG) E(2), F(2alpha)and 6-keto F(1alpha). Electrical stimulation (ES, 0.1 Hz, 1 ms) increased such production by 100%, and similar results were obtained in the presence of 1.0 microM bradykinin (Bk). When both stimuli were applied simultaneously, the increases in PG production were 1100% for PGE(2), 800% for PGF(2alpha)and 400% for PG6-keto F(1alpha). Prazosin abolished the effect of ES on PG production. A selective Bk B(2)-receptor antagonist abolished the increase in PG production induced by Bk, both in non-stimulated and in ES tissues. Bk (1.0 microM) elicited contractile responses in non-stimulated as well as in ES tissues, responses that were not modified in the presence of 10 microM indomethacin. In conclusion, the effects of Bk on prostaglandin production appears to depend on the activation of B(2) receptors, while the increase in prostaglandin release induced by ES, and the effects observed with both stimuli simultaneously, should be mediated by the release of noradrenaline and the subsequent activation of alpha(1) adrenoceptors.

Endogenous prostaglandin formation in the field-stimulated guinea-pig vas deferens: comparison of the inhibitory effects of indomethacin and NS-398

Prostaglandin (PG) E2 inhibited both phases of contraction produced by electrical field stimulation of the guinea-pig vas deferens. PGF2alpha and PGD2 were without effect on this preparation. Carbacyclin (a PGI2) analogue inhibited the first phase of contraction at higher concentrations, whereas U46619 (a thromboxane mimetic) potentiated both phases of contraction. As exogenous arachidonic acid inhibits both phases of contraction of the electrically field-stimulated guinea-pig vas deferens, it is likely that the arachidonic acid is converted to PGE2 in the vas deferens. Indomethacin, a non-specific inhibitor of prostaglandin H synthase (PGHS), attenuated the inhibitory actions of exogenous arachidonic acid when examined on the first phase of contraction. NS-398, a relatively specific inhibitor of PGHS-2, also prevented the inhibitory action of exogenous arachidonic acid. However, NS-398 was much less effective than indomethacin in this respect even though NS-398 and indomethacin inhibit PGHS-2 with similar potencies. Consequently, the findings suggest that exogenous arachidonic acid is converted to PGE2 in the guinea-pig vas deferens by the actions of PGHS-1 and, to a lesser extent, by PGHS-2.

Neurogenic and non-neurogenic responses in the urinary bladder of hibernating hamster

1. Purinergic and cholinergic components of parasympathetic neurotransmission and contractile responses to exogenous alpha,beta-methylene ATP, acetylcholine, substance K, substance P, calcitonin gene-related peptide, vasoactive intestinal polypeptide and capsaicin have been investigated in the urinary bladder of hibernating hamsters (4 weeks), cold exposed (4 weeks) and age-matched controls. 2. Electrical field stimulation (EFS) evoked increased frequency-dependent contractions in the detrusor strips from hibernating hamsters compared with those obtained from cold-exposed and age-matched animals. Tetrodotoxin (10(-6) M) completely blocked the frequency-dependent contractions in all groups. 3. The purinergic component of the parasympathetic neurotransmission was not affected in hibernating and cold-exposed animals while the cholinergic component was increased with respect to age-matched animals. The neurogenic response to EFS, still present after incubation with atropine (10(-6) M) and suramin (10(-4) M), was attenuated by indomethacin (10(-6) M) and blocked by tetrodotoxin (10(-6) M). 4. Exogenous administration of alpha,beta-methylene ATP elicited a significantly reduced contraction in strips from hibernating and cold-exposed hamsters relative to age-matched animals. The contractile response to exogenous acetylcholine was greater in the detrusors from hibernating hamsters than in cold-exposed and age-matched animals. Substance K elicited reduced contractions in preparations from hibernating animals compared with cold-exposed and control animals. Calcitonin gene-related peptide, vasoactive intestinal polypeptide, substance P and capsaicin did not elicit any relaxant or contractile response either at resting tone or in carbachol (5 x 10(-7) M)-precontracted tissues. 5. In summary, our findings indicate that 4 weeks of hibernation can significantly increase neurogenic responses in the hamster urinary bladder. This appears to be due to an increase in postjunctional responses to acetylcholine. In contrast, there was a decrease of the postjunctional responses to the parasympathetic cotransmitter ATP and also to the sensory-motor neurotransmitter substance K.

The mechanisms of enhancement and inhibition of field stimulation responses of guinea-pig vas deferens by prostacyclin analogues

1. In the guinea-pig isolated vas deferens preparation bathed in Tyrode's solution, the prostacyclin analogues, cicaprost, TEI-9063, iloprost, taprostene and benzodioxane-prostacyclin, enhanced twitch responses to submaximal electrical field stimulation (20%-EFS). The high potency of cicaprost (EC150 = 1.3 nM) and the relative potencies of the analogues (equi-effective molar ratios = 1.0, 0.85, 1.6, 17 and 82, respectively) suggest the involvement of a prostacyclin (IP-) receptor. 2. Maximum enhancement induced by cicaprost in 2.5 mM K+ Krebs-Henseleit solution was similar to that in Tyrode solution (2.7 mM K+), but was progressively reduced as the K+ concentration was increased to 3.9, 5.9 and 11.9 mM. There was also a greater tendency for the other prostacyclin analogues to inhibit EFS responses in 5.9 mM standard K+ Krebs-Henseleit solution; this may be attributed to their agonist actions on presynaptic EP3-receptors resulting in inhibition of transmitter release. 3. The EFS enhancing action of cicaprost was not affected by the alpha1-adrenoceptor antagonist prazosin (100 and 1000 nM). Cicaprost (20 and 200 nM) did not affect contractile responses of the vas deferens to either ATP (5 microM) or alpha,beta-methylene ATP (1 microM) in the presence of tetrodotoxin (TTX, 100 nM). In addition, enhancement by cicaprost of responses to higher concentrations of ATP (30 and 300 microM) in the absence of TTX, as shown previously by others, was not seen. Prostaglandin E2 (PGE2, 10 nM) and another prostacyclin analogue TEI-3356 (20 nM) enhanced purinoceptor agonist responses. Unexpectedly, TTX (0.1 and 1 microM) partially inhibited contractions elicited by 10-1000 microM ATP; contractions elicited by 1-3 microM ATP were unaffected. Further studies are required to establish whether a pre- or post-synaptic mechanism is involved. 4. In a separate series of experiments, cicaprost (5-250 nM), TEI-9063 (3-300 nM), 4-aminopyridine (10-100 microM) and tetraethylammonium (100-1000 microM) enhanced both 20%-EFS responses and the accompanying overflow of noradrenaline to a similar extent. In further experiments with the EP1-receptor antagonist AH 6809, TEI-3356 (1.0-100 nM) and the EP3-receptor agonist, sulprostone (0.1-1.0 nM) inhibited both maximal EFS responses and noradrenaline overflow, thus confirming previous reports of the high activity of TEI-3356 at the EP3-receptor. Cicaprost had no significant effect on noradrenaline overflow at 10 and 100 nM, but produced a modest inhibition at 640 nM. 5. In conclusion, our studies show that prostacyclin analogues (particularly TEI-3356) can inhibit EFS responses of the guinea-pig vas deferens by acting as agonists at presynaptic EP3-receptors. Prostacyclin analogues (particularly cicaprost and TEI-9063) can also enhance EFS responses through activation of IP-receptors. The mechanism of the enhancement has not been rigorously established but from our results we favour a presynaptic action to increase transmitter release.