role of nitric oxide in the relaxation elicited by sildenafil in penile resistance arteries

Role of Nitric Oxide in the RelaxationElicited by Sildenafil in Penile Resistance ArteriesDolores Prieto,* Luis Rivera, Paz Recio, José Luis Ruiz Rubio,Medardo Hernández and Albino García-SacristánFrom the Departamento de Fisiología, Facultad de Farmacia, Universidad Complutense de Madrid and Servicio de Urología, HospitalUniversitario Ramón y Cajal, Madrid, Spain

Purpose: We investigated the role of the vascular endothelium and the L-arginine/nitric oxide pathway in the vasorelaxanteffect of the phosphodiesterase type 5 inhibitor sildenafil in penile resistance arteries.Materials and Methods: Second or third order branches of the horse deep intracavernous penile artery were mounted inmicrovascular myographs. The vasodilator effects of sildenafil and the NO donor SNAP (S-nitrosoacetyl-D,L-penicillamine)were evaluated in the absence and presence of the endothelium and inhibitors of the NO/cGMP (cyclic guanosine 3=,5=-monophosphate) pathway.Results: In phenylephrine precontracted, endothelium intact arteries sildenafil elicited potent relaxations that were mark-edly decreased by the blockade of soluble guanylate cyclase with ODQ (1H-[1,2,4]oxadiazolo[4-3a]quinoxalin-1-one). Endo-thelium removal and the inhibition of NO synthase with NG-nitro-L-arginine (L-NOARG) caused pronounced inhibition ofsildenafil elicited relaxations but not of SNAP induced responses. Combined treatment with the cyclooxygenase blockerindomethacin and L-NOARG caused significantly greater inhibition of sildenafil relaxations than that produced by L-NOARGalone. Inhibitors of the cGMP (PKG) and the cAMP (cyclic adenosine 3=,5=-monophosphate) dependent protein kinasesRp-8-Br-PET-cGMPS (�-phenyl-1, N2-etheno-8-bromoguanosine-3=,5=-cyclic monophosphorothioate, Rp-isomer) and Rp-8-CPT-cAMPS (Rp-8-CPT-cAMPS (8-(4-chlorophenylthio) adenosine-3=,5=-cyclic monophosphorothioate, Rp-isomer), respec-tively, inhibited the sildenafil concentration-relaxation curves. The relaxant responses of SNAP were markedly decreased byPKG inhibitor and to a lesser extent by cAMP dependent protein kinase inhibitor.Conclusions: The current results demonstrate a potent relaxant effect of sildenafil in penile resistance arteries due in partto cGMP accumulation and to the enhanced effects of basal released, endothelial derived NO acting through PKG activation.Cross-activation of the cAMP signaling pathway by sildenafil is also suggested.

Key Words: sildenafil, relaxation, endothelium, nitric oxide, penis

Penile erection occurs when NO released from nervesand endothelium upon sexual stimulation decreasespenile vascular resistance by relaxing tonically con-

stricted helicine arteries and corporeal smooth muscle.1,2

The essential role of NO in the physiology of erectionwas initially suggested in the early study of Klinge andSjostrand.3 NO activates smooth muscle soluble guanylatecyclase and increases intracellular cGMP, thus, stimulatingPKG. In turn, PKG can lower the intracellular Ca2� concen-tration or change Ca2� sensitivity of the contractile machin-ery to produce relaxation.1,2

Cyclic nucleotide PDEs degrade cGMP and cAMP and,therefore, PDE activity represents an important pathway inthe regulation of smooth muscle tone.1,2,4 By selectivelyinhibiting PDE5, which is the cGMP specific, cGMP bindingPDE, sildenafil citrate (Viagra®) compensates for decreasedNO release, cGMP production and impaired penile perfusion

Submitted for publication April 11, 2005.Supported by Grant SAF2002/02923 from DGI of the Spanish

Ministry of Science and Technology.* Correspondence and requests for reprints: Departamento de Fi-

siología, Facultad de Farmacia, Universidad Complutense de Ma-
drid, 28040-Madrid, Spain (telephone: 34 91 3947193; FAX: 34 913942267; e-mail: [email protected]).
0022-5347/06/1753-1164/0THE JOURNAL OF UROLOGY®

Copyright © 2006 by AMERICAN UROLOGICAL ASSOCIATION

1164

in male erectile dysfunction.1,4 This agent has proved to bean effective, safe and well tolerated drug for the oral treat-ment of organic impotence.1,4 In human and rabbit isolatedcorpus cavernosum sildenafil has a poor relaxant effect inthe absence of NO release but mainly it enhances the relax-ation and cGMP accumulation elicited by exogenous andneural released NO.5,6 Moreover, sildenafil potentiates theincreases in intracavernous pressure induced by pelvicnerve stimulation in anesthetized dogs.7 This initially led tothe suggestion that the action of sildenafil depends on pre-existing activation of the NO-cGMP system and it mightexplain why sexual arousal is necessary for the effectivenessof sildenafil in men.4,5,7 However, in large and resistancepenile arteries sildenafil acts as a powerful vasorelaxant andthe mechanisms underlying its erectogenic action seem to bedifferent.8,9 Thus, in bovine penile resistance arteries silde-nafil induces potent endothelium dependent relaxation anda modest enhancement in NO mediated neurogenic re-sponses.9

We further investigated the mechanisms underlying thedirect relaxant effect of sildenafil in penile resistance arter-ies in relation to the involvement of the vascular endothe-lium and L-arginine/NO pathway. Relaxations elicited by
sildenafil were compared to those of the NO donor SNAP.
Vol. 175, 1164-1170, March 2006Printed in U.S.A.

DOI:10.1016/S0022-5347(05)00320-4

SILDENAFIL RELAXATION IN PENILE RESISTANCE ARTERIES 1165

MATERIALS AND METHODS

Dissection and mounting. Penises from young healthyhorses were obtained once weekly at the local slaughter-house immediately after death and placed in cold PSS, com-posed of 119 mmol l-1 NaCl, 4.7 mmol l-1 KCl, 1.18 mmol l-1

KH2PO4, 1.17 mmol l-1 MgSO4, 1.5 mmol l-1 CaCl2, 0.027mmol l-1 ethylenediaminetetraacetic acid and 11 mmol l-1

glucose. Throughout the subsequent dissection the peniswas bathed with cold PSS (4C) gassed with 5% CO2/95% O2

to maintain pH at 7.4. The horse was used as experimentalanimal due to the close pharmacological reactivity betweenequine and human penile resistance arteries.2 Penile resis-tance arteries, which are second or third order branches ofthe deep penile artery and which have in the horse a nor-malized lumen diameter of 200 to 600 �m, were dissected bycarefully removing the adhering trabecular tissue.10 Arte-rial segments about 2 mm long were mounted as ring prep-arations on 2, 40 �m wires in double vascular myographs forisometric tension recording.

Arteries were allowed to equilibrate in PSS at 37C forabout 30 minutes The relationship between resting walltension and internal circumference was determined in eachartery. From this the internal circumference L100, corre-sponding to a transmural pressure of 100 mm Hg for arelaxed vessel in situ, was calculated. The arteries were setto internal circumference L1, as determined by the equation,L1 � 0.9 � L100, since force development is close to maximalat this internal circumference.10

Experimental procedure. Contractile ability of the ves-sels was tested at the beginning of each experiment bystimulating them twice with KPSS, which is equivalent toPSS but has NaCl exchanged with KCl on an equimolarbasis, giving a final concentration of 123.7 mM K�. Concen-tration-response curves for sildenafil and the NO donorSNAP were constructed for arteries precontracted withphenylephrine (0.3 to 1 �M) by about 40% to 60% of theKPSS contractile response. The role of endothelial cells insildenafil induced relaxation was tested in arteries in whichthe endothelium was mechanically removed, as previouslydescribed.10 To investigate a possible role of cGMP and NOin the relaxant responses of sildenafil the arteries wereincubated for 30 minutes with 5 �M of the guanylate cyclaseinhibitor ODQ or 100 �M of the NOS inhibitor L-NOARG.Earlier these concentrations were reported to maximallyinhibit NO and cGMP mediated relaxations, respectively.9,10

The effects of the PKG and PKA inhibitors were also as-sessed on relaxations elicited by sildenafil and SNAP byincubating the arteries for 45 minutes with Rp-8-Br-PET-cGMPS (3 �M) or Rp-8-CPT-cAMPS (50 �M), respectively.11

Statistical analysis. Mechanical responses of the arterieswere measured as force and are expressed as active walltension (�T), which is the increase in force (�F) divided bytwice the segment length. Relaxant responses are shown asa percent of the precontraction induced by phenylephrine.Sensitivity to the agonists is expressed in terms of pEC50 ��log(EC50). Results are expressed as the mean � SEM.Statistical differences between means were analyzed by Stu-dent’s unpaired 2-tailed t test. Means of multiple groups
were compared by 1-way ANOVA, followed by the Bonfer-
roni method as an a posterio test. Probability levels less than5% were considered significant.

RESULTS

Vasorelaxant effect of sildenafil in penile resistancearteries. A total of 45 endothelium intact penile resistancearteries with an l1 of 396 � 23 �m were precontracted withphenylephrine by 53% � 3% of the KPSS induced response(7.1 � 0.5 Nm-1). Sildenafil, the selective inhibitor of cGMPspecific PDE5, elicited potent and full concentration depen-

FIG. 1. Average concentration-response curves for relaxant effectsof sildenafil (A) and NO donor SNAP (B) in horse penile resistancearteries precontracted with phenylephrine (Phe) in absence andpresence of 5 �M of guanylate cyclase blocker ODQ. Results areexpressed as mean percent � SEM of contraction induced by phen-ylephrine in 4 or 5 arteries.

dent relaxations in 45 phenylephrine precontracted arteries

ts we

SILDENAFIL RELAXATION IN PENILE RESISTANCE ARTERIES1166

with a pEC50 of 7.72 � 0.04 and maximal relaxations thatwere 92.4% � 1.3% of the phenylephrine induced contrac-tion (fig. 1, A). Sildenafil induced relaxation was slow andlong lasting, especially at the highest drug concentrations.

Effect of endothelial cell removal and blockers ofguanylate cyclase, NOS and cyclooxygenase on silde-nafil and SNAP elicited relaxations. A 5 �M dose ofODQ, the selective blocker of guanylate cyclase, causedmarked inhibition of the relaxant responses elicited by sil-denafil and the NO donor SNAP (table 1 and fig. 1). How-ever, mechanical removal of the endothelium and NOSblockade differentially inhibited relaxations induced by thePDE5 inhibitor and the NO donor (figs. 2 and 3). Thus,sildenafil elicited relaxant responses were greatly decreasedby endothelial cell removal, whereas those induced by SNAPwere significantly enhanced (table 1 and fig. 2). Treatmentwith L-NOARG (100 �M) increased basal tone and causedprofound inhibition of sildenafil responses but did not alterrelaxant responses elicited by the NO donor (table 1 andfig. 3).

The cyclooxygenase inhibitor indomethacin (3 �M) did notalter sildenafil concentration-relaxation curves. However,combined treatment with indomethacin and L-NOARG (100�M) caused significantly greater inhibition of the relax-ations induced by the higher concentrations of sildenafilthan that produced by L-NOARG alone (table 1 and fig. 4).

Effect of inhibitors of PKG and PKA on sildenafil andSNAP elicited relaxations. Since cross-talk betweencGMP and cAMP has been demonstrated in penile erectiletissues, the effects of selective inhibitors of cGMP and cAMPdependent protein kinases on the relaxant responses of sil-denafil and SNAP were evaluated (table 2 and fig. 5). ThePKG blocker Rp-8-Br-PET-cGMPS (3 �M) caused 2.5 and7.8-fold decreases in the relaxant potency of sildenafil andSNAP, respectively, and decreased maximal responses withthis decrease more pronounced for SNAP (table 2, and fig. 5,

TABLE 1. Effect of endothelial cell removal and blockers of gusildenafil and SNAP in h

Mean � SEMpEC50 (�logEC50)

p value vs Control(unpaired t test)

Sildenafil:Control 7.58 � 0.13EndotheliumControl 7.62 � 0.095 �M ODQ 6.61 � 0.31 �0.05Control 7.75 � 0.09100 �M L-NOARG 7.06 � 0.14 �0.01Control 7.68 � 0.073 �M indomethacin 7.53 � 0.06Control 7.59 � 0.08100 �M L-NOARG �3 �M indomethacin

SNAP:Control 6.44 � 0.09Endothelium 6.95 � 0.06 �0.01Control 5.92 � 0.105 �M ODQ

Control 5.93 � 0.23100 �M L-NOARG 6.10 � 0.16

* Effective lumen diameter of penile resistance arteries at which experimen

A and C). Interestingly treatment with 50 �M of the PKA

inhibitor Rp-8-CPT-cAMPS also caused significant inhibi-tion of sildenafil induced responses with the potency of thisagent decreased 5.6-fold (table 2 and fig. 5, B). This proce-dure produced 2.6-fold rightward displacement of SNAP con-centration-response curves (table 2, fig. 5, D). Combinedtreatment with blockers of PKG and PKA caused additionalinhibition of the sildenafil relaxant responses compared tothat induced by either blocker alone (table 2).

DISCUSSION

The current results demonstrate a potent relaxant effect ofsildenafil in penile resistance arteries due in part to cGMPaccumulation and to the increased effects of basal releasedendothelial NO acting through PKG activation. Thus, byenhancing the effects of endothelial derived NO sildenafilactivates the NO-cGMP-PKG cascade, which has a main rolein the relaxation of penile resistance arteries.2,9,11 However,the involvement of a PKA mediated mechanism in the sil-denafil relaxant responses was also noted, thus, reinforcingthe concept of cross-talk between the cGMP and cAMP sig-naling pathways in penile erectile tissue relaxation.11,12

Sildenafil elicited potent relaxations of penile resistancearteries with the pEC50 for this relaxant effect in the rangeof the low therapeutic doses for erectile dysfunction.4 This isin agreement with results reported in bovine penile resis-tance arteries9 and the human penile dorsal artery8 but incontrast to findings in the corpus cavernosum from humansand other species, in which sildenafil produces little or norelaxation at high concentrations.5,6 In the current study themarked inhibition caused by the blockade of guanylate cy-clase in sildenafil induced responses indicates that thisagent relaxes smooth muscle by increasing the basal contentand effects of cGMP. NO is basally released from the endo-thelium in penile resistance arteries.10 Therefore, the pro-nounced inhibitory effect of endothelium removal andblockade of NO synthesis on relaxations elicited by sildenafil

te cyclase, NOS and cyclooxygenase on relaxation elicited byenile resistance arteries

an % Emax �SEM p value

No.Arteries

Mean l1 � SEM*(�m)

85.2 � 5.2 �0.001 vs control(unpaired t test)

5 287 � 4527.9 � 5.6 5 315 � 5689.4 � 3.3 �0.001 vs control

(unpaired t test)5 575 � 43

40.7 � 3.0 5 564 � 3996.2 � 1.3 �0.001 vs control


51.8 � 6.4 7 436 � 9896.2 � 1.7 5 314 � 1491.3 � 2.1 5 374 � 3693.1 � 3.6 6 365 � 3331.2 � 4.1 �0.001 vs control

(unpaired ttest), �0.05 vsL-NOARG

6 372 � 39

95.2 � 2.6 4 414 � 11593.7 � 4.5 4 335 � 6795.4 � 2.7 4 520 � 6423.8 � 2.9 �0.001 vs control


96.5 � 2.2 5 416 � 9887.5 � 5.3 5 538 � 62

re performed, determined as l1 � L1 ��1.

anylaorse p

Me

but not on those evoked by the NO donor SNAP suggests


that by inhibiting cGMP breakdown sildenafil augments therelaxing effects of endothelium derived NO. The endothe-lium and NO dependency of relaxations induced by PDE5inhibitors have been reported to be variable depending onthe vascular bed and the animal species. Thus, relaxantresponses induced by zaprinast, E4021 and sildenafil aredecreased by endothelial cell removal and NOS inhibition inthe aorta13 and in the large coronary arteries.14 However,sildenafil induced relaxations are independent of the endo-thelium in pulmonary arteries,15 suggesting that PDE5 ac-tivity is mainly present in smooth muscle cells. The currentresults showing that sildenafil mainly enhances the effects

FIG. 2. Average effect of mechanical endothelial cell removal onrelaxation to sildenafil (A) and SNAP (B) in penile resistance arter-ies, expressed as percent of contraction induced by phenylephrine(Phe). Points represent mean � SEM of 4 to 7 arteries.

of endothelium derived NO in penile resistance arteries

extend the previous concept that the erectogenic action ofthis drug is due to the facilitation of erectile tissue relax-ations mediated by neuronal derived NO.5,6,7,9 This wassupported by the observation that sildenafil potentiated re-laxations induced by the electrical filed stimulation of hu-man corpus cavernosum strips in vitro5 and erectile functionfollowing pelvic nerve stimulation in anesthetized dogs.7

Our data suggest that potentiation of the relaxant effects ofendothelial derived, basal released NO may also contributeto the vasodilator and erectogenic action of sildenafil. Aphysiological interaction between endothelial and neuralNO probably exists. Thus, it is likely that the neural NOmediated vasodilatation and increased blood flow into thecorpus cavernosum produced by parasympathetic activationduring the initiation of erection would lead to a shear-stressmediated stimulation of the endothelial lining in penile ar-teries, which in turn would release NO from the endothe-lium and produce further vasodilatation.2

Endothelial dysfunction is a key factor in the pathogen-esis of diabetic vascular disease, and constitutive NOS ac-tivity and endothelial NO release are impaired indiabetes.2,16 The current results demonstrating a role forendothelial NO in the vasodilator action of sildenafil wouldbe consistent with a recent report showing that this agentcannot normalize the impaired, endothelium dependent re-laxations of human penile resistance arteries in patientswith diabetes.17 Therefore, this could explain the lesser clin-ical efficacy of sildenafil in patients with diabetes who haveerectile dysfunction, in whom NO availability is de-creased.16,17

In bovine penile resistance arteries sildenafil relaxantresponses have been shown to be endothelium and prosta-noid dependent.9 Although in the current study NOS block-ade greatly decreased sildenafil responses, combinedtreatment with NOS and cyclooxygenase inhibitors pro-duced an additional inhibition of these relaxations, thus,suggesting that sildenafil may also enhance the basal re-lease and/or effects of prostanoids. Basal release of relaxantprostanoids has been shown in horse10 and human penileresistance arteries18 based on the pronounced increase inbasal tension and the inhibition of arachidonic acid elicitedrelaxation, respectively, produced by cyclooxygenase block-ade. Since vasodilator effects of prostanoids in penile tissuesare mediated by IP and EP receptors coupled to Gs proteinsand the adenylate cyclase pathway,10,18 the inhibitory effectof indomethacin on sildenafil induced relaxations of penileresistance arteries would suggest an indirect effect of thePDE5 inhibitor increasing cAMP levels stimulated by ba-sally released relaxant prostanoids.

The NO-cGMP cascade has an essential role in the relax-ation of penile arterial and corporeal smooth muscle leadingto erection.1,2 However, increasing evidence suggests cross-talk between cGMP and cAMP signaling pathways in theerectile tissues of the penis.11,12 In the current study the factthat the PKG and PKA blockers inhibited sildenafil inducedrelaxations indicates that sildenafil is increasing cGMP andcAMP intracellular levels or increased cGMP levels maycross-activate PKA. A stimulation of cAMP accumulation bysildenafil would be in agreement with that reported earlierin human corpus cavernosum and cardiac tissues, in whichat a concentration of 1 �M (which fully relaxes penile resis-tance arteries) sildenafil was shown to increase cAMP levels
by 4 and 2-fold, respectively, with the effect attributable to

se penile resistance arteries, expressed as percent of contraction inducedteries.


the inhibition of PDE3, which is the cGMP inhibited, cAMPspecific PDE.12 By inhibiting PDE5 and enhancing cGMPaccumulation sildenafil may exert an inhibitory effect onPDE3, thereby, augmenting intracellular cAMP levels. Thiswould be supported by the observation that, in addition toPDE5, PDE3 is also expressed in human corpus caverno-sum,5,6 and the selective inhibitor of this enzyme, milrinone,elicits full relaxations of trabecular tissue and penile resis-tance arteries.1,11 The fact that relaxations induced by thehighest doses of sildenafil were only partially decreased byguanylate cyclase blockade in penile resistance arteries fur-ther suggests that cAMP levels might also be enhanced bythis drug, as was shown earlier in the human corpus caver-nosum.12

The pronounced inhibition caused by PKA blockade insildenafil induced relaxations of penile resistance arteriesmay also suggest that increased cGMP levels stimulatedby the PDE5 inhibitor cross-activate PKA. This possibilityis further supported by the inhibitory effect of the PKAblocker on relaxations induced by the NO donor SNAP,although these relaxant responses were markedly antag-onized by the inhibition of soluble guanylate cyclase, thus,indicating that NO and sildenafil may relax penile arter-ies by cGMP dependent activation of PKA. The currentresults extend earlier data in PKG I deficient mice, inwhich endothelium dependent and NO relaxations areimpaired but high concentrations of NO can still relaxpreparations of corpus cavernosum,19 and large and resis-tance arteries by cross-activation of the cAMP signaling

FIG. 3. Effect of blockade of NOS with 100 �M L-NOARG on relaxaIsometric force recording of penile resistance artery with 310 �m effeby increasing concentrations of sildenafil in absence (top) and presPDE4 inhibitor rolipram fully relaxed artery. Vertical bar indicateconcentration-response curves for sildenafil (B) and SNAP (C) in horby phenylephrine (Phe). Points represent mean � SEM of 4 to 7 ar

tion to sildenafil (A and B) and SNAP (C) in penile resistance arteries.ctive lumen diameter contracted with 0.3 �M phenylephrine and relaxedence (bottom) of L-NOARG. Note that under NOS blockade conditionss force. Horizontal bar indicates time. Average effect of L-NOARG on

pathway.20

FIG. 4. Effect of 3 �M of cyclooxygenase blocker indomethacin aloneor in combination with 100 �M L-NOARG on average concentration-relaxation response curves to sildenafil in penile resistance arteries.
Results are expressed as mean percent of contraction � SEM elic-ited by phenylephrine (Phe) in 6 arteries.

ts we


CONCLUSIONS

Briefly, the current results demonstrate potent, endothe-lium dependent, NO mediated relaxations induced by silde-nafil in penile resistance arteries in vitro, which may explainin part the erectogenic effects of this drug in vivo. By inhib-iting cGMP breakdown sildenafil enhances the effects ofendothelial derived NO, which in turn activates PKG. How-ever, an important part of the sildenafil induced relaxationdepends on PKA activation, which reinforces the concept ofcross-linking of the cGMP and cAMP signaling pathways inpenile erectile tissues. Our results showing that the vasodi-lator effect of sildenafil on penile resistance arteries largelydepends on the production of endothelial NO may in partexplain the lesser clinical responsiveness to this drug in

FIG. 5. Effects of 3 �M of PKG inhibitor Rp-8-Br-PET-cGMPS (Aand C) and 50 �M of PKA inhibitor Rp-8-CPT-cAMPS (B and D) onrelaxation to sildenafil (A and B) and SNAP (C and D) in penile

TABLE 2. Effect of inhibitors of PKG (Rp-8-Br-PETS-cGMPS) aSNAP in horse pen

Mean � SEMpEC50

(�logEC50)

p Value vsControl

(unpaired t test)M

Sildenafil:Control 7.54 � 0.063 �M Rp-8-Br-PETS-cGMPS 7.14 � 0.06 �0.001

Control 7.82 � 0.0850 �M Rp-8-CPT-cAMPS 7.11 � 0.13 �0.01

Control 7.76 � 0.093 �M Rp-8-Br-PET-cGMPS� 50 �M Rp-8-CPT-cAMPS

6.92 � 0.08 �0.01

SNAP:Control 6.35 � 0.093 �M Rp-8-Br-PET-cGMPS 5.46 � 0.05 �0.001Control 5.95 � 0.0950 �M Rp-8-CPT-cAMPS 5.54 � 0.18 �0.05

* Effective lumen diameter of penile resistance arteries at which experimen

resistance arteries, expressed as mean percent � SEM of contrac-tion elicited by phenylephrine (Phe) in 4 to 6 arteries.

patients with erectile dysfunction affecting the vascular en-dothelium.

ACKNOWLEDGMENTS

Hugh Tyrell-Gray assisted with the manuscript. ManuelPerales and Francisco Puente provided technical assistance.Horse penises were provided by Segovia Slaughterhouse.

Abbreviations and Acronyms

cAMP � cyclic adenosine 3=,5=-monophosphate

cGMP � cyclic guanosine 3=,5=-monophosphate

EC50 � agonist concentrationproviding half maximalrelaxation

Emax � maximum relaxationexpressed as percent ofcontraction induced byphenylephrine

KPSS � high potassiumphysiological saline solution

L � artery internalcircumference

l1 � normalized internal lumendiameter

L-NOARG � NG-nitro-L-arginineNO � nitric oxide

NOS � NO synthaseODQ � 1H-[1,2,4]oxadiazolo[4-

3a]quinoxalin-1-onePDE � phosphodiesterase

PDE3 � PDE type 3PDE5 � PDE type 5PKA � cAMP dependent protein

kinasePKG � cGMP dependent protein

kinasePSS � physiological saline

solution

KA (Rp-8-CPT-cAMPS) on relaxation elicited by sildenafil andesistance arteries

% EmaxSEM p Value

No.Arteries

Mean l1 � SEM*(�m)

� 5.7 �0.05 vs control (unpairedt test) and vs PKG-I �PKA-I (1-way ANOVA �Bonferroni)

5 396 � 79� 8.1 5 366 � 86

� 1.3 �0.001 vs control(unpaired t test) � �0.01vs PKG-I � PKA-I (1-wayANOVA � Bonferroni)

5 301 � 12� 3.8 5 318 � 44

� 1.5 �0.01 vs control (unpairedt test)

5 305 � 14� 4.6 5 354 � 45

� 2.2 �0.001 vs control(unpaired t test)

4 463 � 115� 4.9 4 409 � 125� 4.3 6 409 � 38� 7.1 6 381 � 45

re performed determined as l1 � L1 ��1.

nd Pile r

ean�

91.271.9

94.073.6

94.555.6

91.455.593.582.4


Abbreviations and Acronyms continued

Rp-8-Br-PET-cGMPS � �-phenyl-1, N2-etheno-8-bromoguanosine-3=,5=-cyclicmonophosphorothioate,Rp-isomer

Rp-8-CPT-cAMPS � 8-(4-chlorophenylthio)adenosine-3=,5=-cyclicmonophosphorothioate,Rp-isomer

SNAP � S-nitrosoacetyl-D,L-penicillamine

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role of nitric oxide in the relaxation elicited by sildenafil in penile resistance arteries

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