Analyzing the Effect of Drugs on Nitric Oxide Signaling Pathway in the Penis Essay


Nitric oxide ( NO ) is used as an of import secondary courier molecule in signaling tracts. It’s an of import vasodilative. One illustration of a tract that utilizes it is the Penile hard-on tract. The Penile Erection tract has been of important importance to scientists ( and the human race in general ) for a really long clip. There have been extended surveies into its workings, the signaling molecules involved and procedures. In the phallus, NO one time produced in a primary response cell can spread rapidly to the neighbouring cells to originate farther chemical reactions that lead to relaxation of musculuss and increased flow of blood, taking to penile hard-on. The importance of NO tract makes it an of import mark for drugs in the upsets of this organ. A mathematical theoretical account for the tract does non be. In this paper we have presented one such theoretical account that may be used to analyze the workings of the phallus hard-on pathway along with tendencies of the metabolites involved. A specific attending has been paid to PDE5 molecule because it is the enzyme affected by most external factors that choose to change the system. One such drug is Viagra which is a PDE5 inhibitor and a common and effectual intervention for Erectile Dysfunction in work forces.

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Materials and Methods:

The wiring diagram along with Km or Ki values is given below. A brief key is given as follows.

ACh: Acetyl Choline

Initial public offering3: Inositol Triphosphate

eNOS/nNOS: endothelial and neural Nitric Oxide Synthase

PDE5: Phosphodiesterase type 5

The IC50 value given for tadalfil is used to cipher KIfor the inhibitor.

Figure 1: Wiring Diagram for the NO Pathway

The above diagram shows the advancement of the NO signaling molecule from two beginnings. The Cavernous Nerve and the Cholinergic Nerve. The cavernous nervus releases No straight as a portion of the signaling procedure and the cholinergic nervus releases Acetyl-Choline ( ACh ) . ACh causes a cascade which activates NO Synthase to change over Arginine and O2to NO. The NO derived from both these beginnings in bend activates Guanyl Cyclase which converts GTP to cGMP. At this point the cGMP can either trip cGMP specific protein kinase which activates the tract for segregation of Ca2+in the endoplasmic Reticulum, this causes musculus relaxation. Other than this, it can be converted to 5’GMP by PDE5 which is a GMP specific phosphodiesterase.

Most effecters of the phallus hard-on tract act on PDE5 and suppress it. The usual action of PDE5 causes debasement of cGMP to 5’GMP, diminishing the cGMP concentration, doing fewer cGMP dependent protein kinase molecules to acquire activated and therefore unable to do hard-on. If PDE is inhibited cGMP accumulates and therefore the continuance of hard-on additions.

The Model:

We have modelled the system as a set of mono-directional reactions. The reaction rates are taken in the signifier of Km values which accommodates both contrary and forward reactions. Since the Km values don’t give an thought of the Vmax ( k2*Enzyme ) values we have considered Vmax value to be changeless for all reactions.

We simulated the behaviour of the system in MATLABThuliumas a Michaelis-Menten Metabolic Network. This had to be done because we had to pattern ingestion and production of metabolites in order to detect tendencies. Kmvalues were taken from BRENDA and Sabio-RK.

Kinetic Constants for simulations are defined as follows:

Degradation Rate

Endothelial Nitrogen Oxide Synthase Km

Neural Nitrogen Oxide Synthase Km

K2 value for all enzymes ( Michaelis Menten )

GCycl k value with NO

( The threshold value of NO is taken to be 0.022 )

GCycl k value without NO

K value for PDE5, uninhibited by Viagra

KIvalue of suppression by Viagra ( competitory suppression )

Kmof PDE5 with suppression of Viagra

( BRENDA ) , ( Sabio-RK )

Above, the values for NOS are considered for both NO making waies that take portion in the signaling procedure. We are sing the XT between both these tracts and integrating them into the theoretical account separately.

Ratess of alteration of compounds ( MM Kinetics )

Rate of Conversion of Arginine to NO

Rate of production of NO via the Cavernous Nerve

Rate of NO debasement

Rate of Conversion of GTP to cGMP

Rate of transition of cGMP to 5’GMP

In the presence of Viagra the v5 rate decrease linearly with clip ( non shown in above equation ) . This is to integrate the consequence of remotion of Viagra from the organic structure, therefore a reduced consequence with clip.

The undermentioned mathematical equations were used to specify system kineticss:

Consequences and Discussion:

The above set of equations and invariables is used to imitate the behaviour of the system. Discoursing some of the stuff of indispensable importance to the intent of the undertaking.

The Viagra drug serves 2 particular intents. First, it aids in musculus relaxation so that antecedently constricted arterias can pump blood into the relaxed principal cavernosum. Second, it maintains this province inside the phallus for a long adequate clip for one to “successfully execute intercourse” . We used this simulation to prove for this consequence and the undermentioned figures.

Figure 2: cGMP Levels in presence of Viagra

Figure 3: cGMP degrees in the absence of Viagra

The above figures show that in the presence of Viagra, non merely is the cGMP degree increased to around a 100 times more for a short period of clip but this province is besides sustained for a longer period of clip. If we take the threshold value required for an hard-on to be 0.65e-3 the consequence lasts about 15 times longer than the clip of hard-on in an ‘uninduced’ province. This seems to be a sensible estimate sing the effects of Viagra last for at least 4 hours. ( Eardley et al. 2002 )

An of import facet of the tract was the presence and modeling of NO release and action. NO is the initial signaling molecule and there are two beginnings doing its inflow into the system and demand to be simulated individually. Once this is done their consequence needs to be considered cumulatively to mensurate its entire consequence on the system.

Figure 4: Entire NO produced

Figure 5: No from Cavernous Nerve

Figure 6: No from Cholinergic Nerve

The above figures show that the entire concentration of NO hits a extremum at 0.035 units which stabilize at 0.02 units. A similar consequence is observed when visualising the behaviour of NO escape from the cavernous nervus, although it has a lower extremum. The end product from the Cholinergic nervus stabilizes at a lower degree than that of the sum or that of the cavernous nervus. This is used to exemplify that cavernous nervus provides the initial spike to NO sum in the system and the cholinergic nervus is used to stabilise its consequence. This clearly shows the XT between the two beginnings of NO influx into the system.


In this manner we have assessed through mathematical patterning the consequence of the inhibitor molecule on the system. The tadalfil drug works really expeditiously to assist with erectile disfunction jobs. Despite the efficiency of the molecule as a competitory inhibitor of GTP on the adhering site of PDE5, the map of NO is highly of import for its proper operation. NO degrees must at least be at radical degree for a important sum of GTP to be in the system doing musculus relaxation and in bend phallus hard-on.


Ian Eardley et Al. “ Onset and continuance of action of Viagra for the intervention of erectile disfunction. ”British Pharmacological Society( 2002 ) : 61-65. Online.

BRENDA. n.d. Online. , 28, April 2014.

Sabio-RK. n.d. Online. , 28, April 2014.

“ Azotic Oxide and Viagra. ” Concepts in Biochemistry. Wiley, 2002. Web. 02 Apr. 2014.

Philippides et Al. “ Four-dimensional Neuronal Signaling by Nitric Oxide: A Computational Analysis. ” The Journal of Neuroscience 20.3 ( 2000 ) : 1199-1207. Print.

Burnett, et Al. “ The Nitric Oxide Signaling Pathway in the Penis. “ Current Pharmaceutical Design 11.31 ( 2005 ) : 3987-994. Print.

Wood et Al. “ Models of the Diffusional Spread of Nitric Oxide: Deductions for Neural Nitric Oxide Signalling and Its Pharmacological Properties. “ Neuropharmacology 33.11 ( 1994 ) : 1235-244. Print.


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