Effects of cyclic AMP and analogues on neurogenic transmission in the rat tail artery

Cell membrane changes of structure and function in protein kinase inhibitor-induced polyploid cells

Exogenous cyclic AMP has been thought to be a chemical without marked pharmacological effect until now, as it is not capable of penetrating the cell membrane in most eucaryotic cells. The present study obtained results consistent with those of most previous studies, showing that exogenous cyclic AMP itself did not interfere with the cell cycle even at the high dose of 100 microM. However, it was found that K252a, a potent inhibitor of protein kinases including protein kinase C, induced DNA re-replication, i.e. DNA synthesis at a elevated DNA ploidy in cells that had not undergone cytokinesis (leading to polyploidization), and that exogenous cyclic AMP markedly potentiated the K252a-induced polyploidization at a very low dose similar to the effective dose of membrane-permeable cyclic AMP analogue dibutyryl cyclic AMP. These findings suggested that the cell membrane changed during the formation of polyploid cells. This supposition was confirmed by scanning electron microscopy to observe structural changes and by determination of cellular attachment to investigate functional changes.

Stimulatory effect of paeoniflorin on the release of noradrenaline from ileal synaptosomes of guinea-pig in-vitro

The effect of paeoniflorin (an active principle of Paeoniae Radix, commonly used in traditional Chinese medicine) on the release of noradrenaline (norepineprhine) from nerve terminals was investigated using guinea-pig isolated ileal synaptosomes. Release was determined as the amount of noradrenaline, quantified by high-performance liquid chromatography-electrochemical detection, from samples incubated with paeoniflorin or vehicle. Paeoniflorin stimulated the release of noradrenaline in a concentration-dependent manner without an effect on the level of lactate dehydrogenase in the bathing medium. Tetrodotoxin abolished the action of paeoniflorin at concentrations sufficient to block sodium channels. The depolarizing effect of paeoniflorin on the membrane potential was also illustrated by a concentration-dependent increase in the fluorescence of bisoxonol. Moreover, the effect of paeoniflorin on bisoxonol fluorescence in ileal synaptosomes seems more potent than that of 4-aminopyridine. That paeoniflorin causes influx of calcium ions via the depolarization of nerve terminals could be considered. The noradrenaline-releasing action of paeoniflorin was abolished by removal of calcium chloride from the bathing medium. This action of paeoniflorin was also attenuated by Rp-cAMP atconcentrations sufficientto inhibitthe action of cyclicAMP. Therefore, paeoniflorin could induce a calcium-dependent and cyclic-AMP-related release of noradrenaline from sympathetic nerve terminals of guinea-pig ileum. Guanethidine inhibited the noradrenaline-releasing action of paeoniflorin in a concentration-dependent manner. The effect of paeoniflorin on the increase of bisoxonol fluorescence was not modified by atropine. Release of noradrenaline by paeoniflorin from noradrenergic nerve terminals was characterized. These findings suggest that paeoniflorin can stimulate tetrodotoxin-sensitive depolarization of membranes to result in a calcium-dependent and cyclic-AMP-related release of noradrenaline from noradrenergic nerve terminals.

Stimulatory effect of trans-cinnamaldehyde on noradrenaline secretion in guinea-pig ileum myenteric nerve terminals

The effect of trans-cinnamaldehyde (CNMA) on the release of noradrenaline (NA) from nerve terminal was investigated using isolated ileal synaptosomes of guinea-pig. Release was determined as the amount of NA, quantified by h.p.l.c.-electrochemical detection, from samples incubated with CNMA minus that in parallel blanks treated with same volume of vehicle. CNMA stimulated the secretion of NA in a concentration-dependent manner from 5 microM to 50 microM, while the value of lactate dehydrogenase in the incubated medium was not influenced by CNMA. However, trans-cinnamic acid, cinnamoyl chloride and cinnamamide failed to produce similar effect. Specific action of CNMA can thus be considered. Guanethidine inhibited the release of NA by CNMA in a concentration- dependent manner. Saxitoxin attenuated the action of CNMA at concentrations sufficient to block sodium channels. The depolarizing effect of CNMA on the membrane potential was also illustrated by a concentration-dependent increase in the fluorescence of bisoxonol, a potential sensitive dye. The NA releasing action of CNMA was deleted by removal of calcium chloride from the bathing medium. This action of CNMA was also attenuated by Rp-cAMP at concentrations sufficient to inhibit the action of cyclic AMP. These findings suggest that CNMA can depolarize the membrane to result in a calcium-dependent and cyclic AMP-related release of NA from noradrenergic terminals.

Effects of NKH477 on renal nerve stimulation-induced responses in anesthetized dogs

We evaluated the effects of an adenylate cyclase activator, N, N-dimetyl-beta-alanine[3R-(3alpha,4alphabeta,5beta+ ++,6beta,6aalpha, 10alpha,10abeta,10balpha)]-5(acetyloxy)-3-eth enyldodecahydro-10, 10b-dihydroxy-3,4a,7,7,10a-pentamethyl-1-oxo-1H-naphtho[2, 1-b]pyran-6-yl ester hydrochloride (NKH477), on neural control of renal functions in anesthetized dogs. Renal nerve stimulation (2 Hz) increased renal norepinephrine efflux and reduced renal blood flow, glomerular filtration rate, urine flow rate, urinary Na(+) excretion and fractional Na(+) excretion. Intrarenal arterial infusion of NKH477 (300 ng/kg/min) suppressed the stimulation-induced reductions in renal blood flow and glomerular filtration rate and attenuated the reductions in urine flow rate and urinary Na(+) excretion but not the changes in renal norepinephrine efflux and fractional Na(+) excretion. Infusion of NKH477 did not affect the urinary responses induced by renal nerve stimulation at a lower frequency (0.5-1 Hz) which had little influence on renal blood flow and glomerular filtration rate. The present results demonstrate that NKH477 inhibits renal vasoconstriction and hypofiltration but not the enhanced tubular Na(+) reabsorption during activation of the renal sympathetic nervous system.

Specific inhibition of cardiac and skeletal muscle sarcoplasmic reticulum Ca2+ pumps by H-89

The isoquinolinesulfonamide H-89, an inhibitor of cyclic AMP-dependent protein kinases (EC 2.7.1.37, cAPrK), inhibited the Ca2+-ATPase activity of cardiac and skeletal muscle sarcoplasmic reticulum (SR) with concentrations giving half-maximal inhibition of 8.1 +/- 1.3 and 7.2 +/- 0.9 micromol/L, respectively. The effect of H-89 on cardiac SR Ca2+-ATPase (EC 3.6.1.38) was the same irrespective of the presence or absence of inhibitors of cAPrK and furthermore, was not affected by a neutralising monoclonal antibody raised against phospholamban. Thus, the action of H-89 in inhibiting SR Ca2+-ATPase would not appear to be mediated by inhibition of cAPrK to reduce the phosphorylation state of phospholamban. In both cardiac and skeletal muscle SR, the inhibition by H-89 was noncompetitive with respect to ATP at a low concentration of ATP (<1 mmol/L) and of a mixed pattern at high concentrations of ATP. H-89 produced a decrease in affinity of the SR Ca2+ pump to Ca2+ with an increase in the Km for Ca from 0.52 +/- 0.01 to 0.94 +/- 0.03 micromol/L (P < 0.05) in cardiac SR and from 0.39 +/- 0.01 to 0.79 +/- 0.02 micromol/L (P < 0.05) in skeletal muscle SR. These results suggest that H-89 inhibits SR Ca2+-ATPase by a direct action on the SR Ca2+ pump to decrease its affinity to Ca2+. Such an action may contribute to the pharmacological effect of H-89.

Protein kinase A inhibitors selectively inhibit the tonic contraction of the guinea pig ureter to high potassium

1. We have investigated the effect of various protein kinase A (PKA) inhibitors on the phasic and tonic components of the response to potassium chloride (KCl) in the guinea pig ureter. All experiments were performed in ureters pretreated with capsaicin (10 microM for 15 min) to prevent the release of sensory neuropeptides and in the presence of 1 microM Bay K 8644 to maximize calcium (Ca) entry via voltage-sensitive channels. The addition of 80 mM hypertonic KCl produced maximal shortening of the ureter with distinct phasic and tonic components, the latter further showing a transient and a sustained component. Nifedipine (30 microM for 120 min) totally abolished all the responses to KCl. 2. The selective PKA inhibitor, H89 (10 microM), abolished the tonic response to KCl in about 30 min with minor inhibitory effect on the phasic contraction. This pattern was unchanged when extending the contact time to 120 min. When added 30 min before the next challenge, H89 (1-30 microM) concentration-dependently inhibited the responses to KCl with a preferential inhibitory effect on the tonic contraction. Another PKA inhibitor, H8, produced similar effects at tenfold higher concentrations (10-300 microM) than H89, consistent with the known potency ratio of these isoquinoline derivatives in inhibiting PKA. 3. The potent and nonselective protein kinase inhibitor, staurosporine (10-100 nM) produced an even depression of the various phases of the response to KCl. The selective protein kinase G inhibitor, KT 5823 (10 microM for 60 min) produced only a slight reduction of the sustained tonic response to KCl. The selective protein kinase C inhibitor GF 109,203X (1-3 microM) and the cAMP analog, Rp-cAMPS (300 microM for 60 min) had no effect on the three components of the response to KCl. 4. In the presence of Bay K 8644, electrical field stimulation (10 Hz for 1 sec, 60 V, pulse width 5 ms) produces direct myogenic phasic contractions (twitches) of the ureter which are suppressed by nifedipine (10-30 microM). H8 (up to 30 microM) and H89 (up to 300 microM) had minor effect on the amplitude of twitches, consistent with their poor inhibitory activity on the phasic responses to KCl. 5. In sucrose gap, superfusion with 80 mM hypertonic KCl produced action potentials followed by a sustained depolarization of the membrane: the two electrical responses underlie the phasic and tonic components of contraction to KCl, respectively. H89 (10 microM for 30 min) did not affect the resting membrane potential nor the KCl-evoked action potentials and sustained depolarization. H89 had no effect on the phasic contraction to KCl but markedly depressed (about 65% inhibition) the tonic contraction. 6. The present findings are consistent with the view that phosphorylation by PKA increases the availability of L-type Ca channels in the ureter smooth muscle. Blockade of PKA dissociates the electromechanical coupling between the sustained membrane depolarization produced by KCl and the corresponding sustained increase in tension. The L-type Ca channel responsible for generating action potentials and phasic contractions to KCl are less sensitive to PKA inhibitors than those responsible for the tonic contraction.

Neuropeptide Y inhibits 3[H]noradrenaline release in the rat vas deferens independently of cAMP levels

The purpose of the present investigation was to ascertain the functional significance of the reduction in cyclic AMP (cAMP) levels in the inhibitory action of neuropeptide Y (NPY) on [3H]noradrenaline ([3H]NA) release, as well as to further characterize the subtype(s) of NPY receptors involved in the peptide's actions in the rat vas deferens. We studied the effects of NPY, carboxyterminal fragments of this peptide and the NPY analog (Leu31,Pro34)-NPY on three functional responses, namely, the release of [3H]NA and the associated muscle contractions evoked by electrical stimulation, and the accumulation of cAMP stimulated by forskolin. NPY, a known inhibitor of the electrically-evoked [3H]NA release and neurogenic contractions is also a potent inhibitor of the forskolin-stimulated cAMP synthesis in the prostatic portion of the rat vas deferens. However, the ability of NPY to inhibit cAMP accumulation is lost upon tissue denervation, suggesting that this is likely to be a prejunctional effect. Elevation of cAMP levels by the use of the cell permeant analog of cAMP, 8-(p-chlorophenylthio)-cAMP (8pCPTcAMP) increases the electrically-evoked release of [3H]NA. However, the inhibition of [3H]NA release by NPY is not prevented by 8pCPTcAMP. Structure-activity relationship studies reveal that NPY and related peptides inhibit the release of [3H]NA, the muscle contractions and the synthesis of cAMP with a similar pharmacological profile. NPY is the most potent inhibitory agent, whereas [Leu31,Pro34]-NPY and NPY13-36, the respective Y1 and Y2 selective agonists, display similar potencies to inhibit the three responses. It is concluded that NPY inhibits neurotransmission in the rat vas deferens through the activation of a peptide receptor different from the known NPY-Y1 or NPY-Y2 receptor subtypes. NPY receptor activation in the vas deferens is negatively coupled to adenylyl cyclase activity. This intracellular signalling pathway is, however, not likely to mediate the peptide effects on the prejunctional regulation of noradrenaline release.

Inhibition of cyclic GMP-dependent protein kinase-mediated effects by (Rp)-8-bromo-PET-cyclic GMPS

1. The modulation of the guanosine 3':5'-cyclic monophosphate (cyclic GMP)- and adenosine 3':5'-cyclic monophosphate (cyclic AMP)-dependent protein kinase activities by the diastereomers of 8-bromo-beta phenyl-1, N2-ethenoguanosine 3':5'-cyclic monophosphorothioate, ((Rp)- and (Sp)-8-bromo-PET-cyclic GMPS) was investigated by use of purified protein kinases. In addition, the effects of (Rp)-8-bromo-PET-cyclic GMPS on protein phosphorylation in intact human platelets and on [3H]-noradrenaline release and neurogenic vasoconstriction in electrical field stimulated rat tail arteries were also studied. 2. Kinetic analysis with purified cyclic GMP-dependent protein kinase (PKG) type I alpha and I beta, which are expressed in the rat tail artery, revealed that (Rp)-8-bromo-PET-cyclic GMPS is a competitive inhibitor with an apparent Ki of 0.03 microM. The activation of purified cyclic AMP-dependent protein kinase (PKA) type II was antagonized with an apparent Ki of 10 microM. 3. In human platelets, (Rp)-8-bromo-PET-cyclic GMPS (0.1 mM) antagonized the activation of the PKG by the selective activator 8-(4-chlorophenylthio)-guanosine 3':5'-cyclic monophosphate (8-pCPT-cyclic GMP; 0.2 mM) without affecting the activation of PKA by (Sp)-5, 6-dichloro-1-beta-D-ribofurano-sylbenzimidazole- 3':5'-cyclic monophosphorothioate ((Sp)-5,6-DCl-cyclic BiMPS; 0.1 mM). 4. (Rp)-8-bromo-PET-cyclic GMPS was not hydrolysed by the cyclic GMP specific phosphodiesterase (PDE) type V from bovine aorta but potently inhibited this PDE. 5. The corresponding sulphur free cyclic nucleotide of the two studied phosphorothioate derivatives, 8-bromo-beta-phenyl-1, N2-ethenoguanosine-3':5'-cyclic monophosphate (8-bromo-PET-cyclic GMP), had no effect on electrically-induced [3H]-noradrenaline release but concentration-dependently decreased the stimulation-induced vasoconstriction. (Rp)-8-bromo-PET-cyclic GMPS (3 microM) shifted the vasoconstriction response to the right without affecting stimulation evoked tritium overflow. 6. The NO donor, 3-morpholinosydnonimine (SIN-1) relaxed rat tail arteries precontracted with phenylephrine (1 microM). The SIN-1 concentration-relaxation curve was shifted in a parallel manner to the right by (Rp)-8-bromo-PET-cyclic GMPS, suggesting that the relaxation was mediated by a cyclic GMP/PKG-dependent mechanism. 7. The [3H]-noradrenaline release-enhancing effect and stimulation-induced decrease in vasoconstriction of forskolin were unaffected by (Rp)-8-bromo-PET-cyclic GMPS. Moreover, the forskolin concentration-relaxation curve was not changed in the presence of the PKG inhibitor, suggesting a high selectivity in intact cells for PKG- over PKA-mediated effects. 8. The results obtained indicate that (Rp)-8-bromo-PET-cyclic GMPS presently is the most potent and selective inhibitor of PKG and is helpful in distinguishing between cyclic GMP and cyclic AMP messenger pathways activation. Therefore, this phosphorothioate stereomer may be a useful tool for studying the role of cyclic GMP in vitro.

Possible neuronal origin of ATP release evoked by forskolin and ouabain from guinea-pig atrial segments

The characteristics of ATP release evoked by forskolin and ouabain from atrial segments of guinea-pig were evaluated under electrical stimulation. Forskolin (1 microM) produced a massive release of ATP together with a positive inotropic response. Both 30 microM W-7 (N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide.HCI), a calmodulin antagonist, and 30 microM vinblastine, a mitotic inhibitor, markedly inhibited the evoked release of ATP without affecting the evoked contraction. However, 100 microM N-ethylmaleimide abolished completely the basal and drug-evoked ATP release and further the evoked contraction. Both the ATP release and contraction evoked by ouabain (3 microM) were similarly affected by W-7, vinblastine and n-ethylmaleimide. The release of ATP, but not the contraction, evoked by forskolin was strongly suppressed by 10 microM okadaic acid, a protein phosphatase inhibitor. The suppression by okadaic acid of the evoked release was thoroughly antagonized in the presence of 0.01 microM PMA (phorbol 12-myristate 13-acetate), but not 10 microM H-7 (1-(5-isoquinolinesulfonyl)-2-methylpiperazine). These results suggest that forskolin, like ouabain, may dominantly cause the neuronal release of ATP from cardiac adrenergic nerves, although the possible participation of release from muscular sources cannot be ignored.

Effects of cyclic GMP and analogues on neurogenic transmission in the rat tail artery

1. The effects of membrane permeable analogues of guanosine 3':5'-cyclic monophosphate (cyclic GMP), and of the NO donor, 3-morpholinosydnonimine-N-ethylcarbamide (SIN-1) were investigated on [3H]-noradrenaline release and neurogenic vasoconstriction in electrical field stimulated rat tail arteries. 2. Two 8-substituted analogues of cyclic GMP (8-bromoguanosine 3':5'-cyclic monophosphate; 8-bromo-cyclic GMP and 8-(4-chlorophenylthio)-guanosine 3':5'-cyclic monophosphate; 8-pCPT-cyclic GMP) concentration-dependently enhanced stimulation-induced [3H]-noradrenaline release. These prejunctional effects were antagonized by the cyclic AMP-dependent protein kinase (PKA) inhibitor N-[2-((3-(4-bromophenyl)-2-propenyl)-amino)-ethyl]-5 isoquinolinesulphonamide dihydrochloride (H-89; 100 nM) but not by the cyclic GMP-dependent protein kinase (PKG) inhibitors, Rp-8-bromoguanosine 3':5'-cyclic monophosphorothioate (Rp-8-bromo-cyclic GMPS; 10 microM) or Rp-8-(4-chlorophenylthio)-guanosine 3':5'-cyclic monophosphorothioate (Rp-8-pCPT-cyclic GMPS; 10 microM). 3. beta-Phenyl-1,N2-ethenoguanosine 3':5'-cyclic monophosphate (PET-cyclic GMP) had no effect on stimulation-induced [3H]-noradrenaline release but concentration-dependently decreased the stimulation-induced vasoconstriction. 4. The two 8-substituted cyclic GMP derivatives, PET-cyclic GMP and SIN-1, both decreased stimulation-induced vasoconstriction. In addition, SIN-1 relaxed rat tail arteries precontracted with phenylephrine (1 microM). The SIN-1 concentration-relaxation curve was shifted in parallel manner to the right by Rp-8-bromo-cyclic GMPS (10 microM) and Rp-8-pCPT-cyclic GMPS (10 microM) with no change in the maximum effect, showing that the relaxation was mediated by a cyclic GMP/PKG-dependent mechanism. 5. It is concluded that PKA activation is involved in the noradrenaline release enhancing effect of the two 8-substituted cyclic GMP analogues, whereas a cyclic GMP/PKG-operated pathway accounts for the inhibitory effects of the cyclic GMP and its analogues on vascular smooth muscle contraction.