Adenosine actions are preserved in corpus cavernosum from obese and type II diabetic db/db mouse

Abstract

Introduction: Erectile dysfunction (ED) in diabetes is associated with autonomic neuropathy and endothelial dysfunction. Whereas the nonadrenergic-noncholinergic (NANC)/neurogenic nitric oxide pathway has received great attention in diabetes-associated ED, few studies have addressed sympathetic overactivity.

Aim: To test the hypothesis that adenosine-induced inhibition of adrenergic-mediated contractile responses in mouse corpus cavernosum is impaired in the presence of diabetes.

Methods: The db/db (obesity and type II diabetes caused by a leptin receptor mutation) mouse strain was used as a model of obesity and type II diabetes, and standard procedures were performed to evaluate functional cavernosal responses.

Main outcome measures: Increased cavernosal responses to sympathetic stimulation in db/db mice are not associated with impaired prejunctional actions of adenosine.

Results: Electrical field stimulation (EFS)-, but not phenylephrine (PE)-, induced contractions are enhanced in cavernosal strips from db/db mice in comparison with those from lean littermates. Direct effects of adenosine, 2-chloro-adenosine, A(1) receptor agonist C-8031 (N6 cyclopentyladenosine), and sodium nitroprusside are similar between the strips from lean and db/db mice, whereas relaxant responses to acetylcholine and NANC stimulation are significantly impaired in the cavernosal strips from db/db mice. 5'-Iodotubercidin (adenosine kinase inhibitor) and dipyridamole (inhibitor of adenosine transport), as well as the A(1) agonist C-8031, significantly and similarly inhibit contractions induced by stimulation of adrenergic nerves in the cavernosal strips from lean and db/db mice.

Conclusions: Results from this study suggest that corpora cavernosa from obese and diabetic db/db mice display altered neural-mediated responses that would favor penile detumescence, i.e., increased contractile response to adrenergic nerve stimulation and decreased relaxant responses upon activation of NANC nerves. However, increased cavernosal responses to adrenergic nerve stimulation are not due to impaired negative modulation of sympathetic neurotransmission by adenosine in this diabetic model.

Figure 1

Effects of 5′-iodotubercidin (adenosine kinase inhibitor), dipyridamole (inhibitor of adenosine transport), and C-8031 (A₁ adenosine receptor subtype agonist) in the frequency-response curves elicited by electrical field stimulation (EFS) (1–32 Hz) in cavernosal strips from lean (○) and db/db (●) mice. Cavernosum strips were preincubated with Nω-nitro-L-arginine methyl ester, 10⁻⁴ M, and atropine, 10⁻⁶ M. After the completion of a control curve to EFS, the tissues were incubated in the presence of (A) vehicle (N = 10 and 8, respectively), (B) 5′-iodotubercidin (10⁻⁵ M, N = 5 and 6, respectively), (C) dipyridamole (10⁻⁶ M, N = 7 in each group), or (D) C-8031 (10⁻⁷ M, N = 8 and 6, respectively), and a second curve to EFS was performed. Experimental values of contraction are in millinewton, and data represent the mean ± SEM of N experiments. * = P < 0.05 compared with the values of cavernosal strips from lean mice; db/db = obesity and type II diabetes caused by a leptin receptor mutation.

Figure 2

Contractile responses to phenylephrine, alpha1-adrenergic receptor agonist, in cavernosal strips from lean (○) and db/db (●) mice. Phenylephrine concentration-response curves were performed in the absence (A) or presence (B) of Nω-nitro-L-arginine methyl ester (L-NAME), 10⁻⁴ M(N = 5 in all groups). Experimental values of contraction of cavernosal strips are in millinewton, and data represent the mean ± SEM of N experiments. db/db = obesity and type II diabetes caused by a leptin receptor mutation.

Figure 3

Effects of adenosine (A), 2-chloro-adenosine (B), and A₁ agonist C-8031 (C) in phenylephrine (PE)-contracted cavernosal strips from lean (○) and db/db (●) mice. Experimental values of the relaxations induced by adenosine (N = 4 and 6, respectively), 2-chloro-adenosine (N = 5 in each group), and C-8031 (N = 5 in each group) were calculated relative to the maximal changes from the contraction produced by PE in each tissue, which was taken as 100%. Data represent the mean ± SEM of N experiments. db/db = obesity and type II diabetes caused by a leptin receptor mutation.

Figure 4

Effects of nonadrenergic-noncholinergic nerves stimulation (A), acetylcholine (B), and sodium nitroprusside (C) in phenylephrine (PE)-contracted cavernosal strips from lean (○) and db/db (●) mice. Experimental values of the relaxations induced by electrical field stimulation stimulation (1–32 Hz) (N = 5 in each group), acetylcholine (N = 5in each group), and sodium nitroprusside (N = 5 in each group) were calculated relative to the maximal changes from the contraction produced by PE in each tissue, which was taken as 100%. Data represent the mean ± SEM of N experiments. * = P < 0.05 compared with the values of cavernosal strips from lean mice; db/db = obesity and type II diabetes caused by a leptin receptor mutation.