Tolerance to isosorbide dinitrate in isolated strips of rabbit corpus cavernosum

Lack of nitrate tolerance in isosorbide dinitrate- and sodium nitroprusside-induced relaxation of rabbit internal anal sphincter

AIM: To investigate the tolerance development against the relaxant effect of nitric oxide donating drug isosorbide dinitrate (ISDN) and sodium nitroprusside (SNP) in internal anal sphincter (IAS) smooth muscle.

METHODS: Relaxation responses of ISDN, and electrical field stimulation (EFS) were obtained before and after tolerance induction by ISDN incubation.

RESULTS: ISDN (10^-7-10^-4 mol/L) and SNP (10^-8-10^-4 mol/L) caused a concentration-dependent relaxation on the basal tonus of the isolated rabbit IAS strips. After a period of 2 h incubation of the 6 x 10^-4 mol/L ISDN the relaxation effects of ISDN and SNP did not change compared to control strips. EFS evoked frequency-dependent relaxation in internal anal sphincter smooth muscle and E_max obtained from control strips were not changed in ISDN tolerance-inducing condition. In this study nitrate tolerance was not observed in rabbit IAS smooth muscle.

CONCLUSION: This result shows that nitric oxide donating drugs relaxes the internal anal sphincter of the rabbits without the development of tolerance.

Impairment of endothelium- and nerve-mediated relaxation responses in the cavernosal smooth muscle of experimentally diabetic rabbits: role of weight loss and duration of diabetes

The effects of short- and long-term experimental diabetes on corporal nerve, endothelium and smooth-muscle responses were investigated, and the reasons for possible alterations in corporal smooth muscle responses such as hyperglycaemia, duration of experimental diabetes and/or altered tissue weight were evaluated. Rabbits were injected with alloxan (125 mg/kg) to induce diabetes. Age-matched non-diabetic and diabetic (3 and 9 weeks) and weight-matched non-diabetic groups (9 weeks) were used as control. In all groups, relaxation (carbachol, electrical field stimulation and sodiumnitroprusside) responses were examined. The relaxation responses were expressed as percentage of the precontraction to phenylephrine and as g response/g tissue weight. The effects of elevated glucose were also examined by incubating cavernosal strips in Krebs-Henseleit solution containing 44.4 mM glucose for 6 h. Cavernosal tissues of non-diabetic and 9-week diabetic rabbits were evaluated histologically. Sodiumnitroprusside (10(-7)-10(-4) M) responses were similar in all groups. Relaxation responses to electrical field stimulation (10 s train; amplitude 50 V; frequency 0.5-32 Hz; width 0.8 ms) were only attenuated in the 9-week diabetic group compared to the non-diabetic group. Carbachol (10(-8)-3 x 10(-5) M) responses were attenuated in both diabetic groups. When the relaxation responses expressed as g response/g tissue weight were evaluated, results were similar compared to those expressed as percentage of phenylephrine (10(-5) M). Neither carbachol nor electrical field stimulation mediated responses were impaired with glucose incubation. No morphological degenerations were observed in the endothelium. Diabetes may interfere with the synthesis and/or release of nitric oxide from both nerves and endothelium in corpus cavernosum, and alterations in endothelium-derived responses occur earlier than neurological disturbances. The sensitivity of cavernosal smooth muscle to nitric oxide did not alter in diabetes. Attenuation of responses was not due to decreased tissue weight caused by diabetes.

Penile erectile responses to cavernous nerve stimulation in rats are not affected by nitrate tolerance

The aim of this study was to elucidate whether penile erection induced by electrical stimulation of the cavernous nerve was affected in male rats with nitrate tolerance. Nitrate tolerance to nitroglycerin was induced by oral administration of isosorbide dinitrate (ISDN) at 1000 mg/kg to rats, once or twice a day for 5 or 6 days. The rats were anesthetized with sodium pentobarbital 18-24 h after the last dosing with ISDN or its vehicle. Penile erection induced by electrical stimulation was monitored by measuring the penile diameter sonomicrometrically. After measurement of the penile erectile response, nitroglycerin (3-300 microg/kg) was intravenously (i.v.) injected into eight rats treated with the vehicle or ISDN to examine its hypotensive effect. In the vehicle-treated rats, the maximal developed penile diameter (D-max) and the duration of penile erection (T50%, period of time from the maximum erection to its 50% decline) produced by electrical stimulation were 509+/-47 microm and 14.2+/-1.7 s, respectively. On the other hand, neither D-max nor T50% in ISDN-treated rats (509+/-36 microm and 13.1+/-1.3 s, respectively) was different from those in the vehicle-treated rats. However, the hypotensive effects of i.v. injected nitroglycerin were significantly attenuated in the ISDN-treated group as compared with the vehicle-treated group. It is concluded that nitrate tolerance fails to influence penile erection induced by cavernous nerve electrical stimulation in rats.

Does diabetes mellitus affect the progress of tolerance to isosorbide dinitrate (ISDN) in corporal tissue?

For erection to take place, the penile arteries and sinusoids have to dilate, thereby increasing the blood flow into the penis. There is increasing evidence that release of l-arginine derived nitric oxide (NO) from nonadrenergic-noncholinergic (NANC) nerves and from the sinusoidal endothelium is a major event in penile smooth muscle relaxation and promotes the endogenous formation of cyclic guanosine monophosphate (cGMP). Nitrovasodilators can be attributed to the activation of soluble guanylate cyclase, resulting in an increase in intracellular level of cyclic guanosine monophosphate, but prolonged exposure to high levels of nitroglycerine and other organic nitroesters induces tolerance against the cardiovascular effect. In this study, the aim was to determine the effect of diabetes on the corporal smooth muscle relaxant effect of ISDN and the effect of diabetes on the process of tolerance to the drug. For this purpose, alloxan-induced diabetic rabbits were used to form diabetes group. The responses of the corpus cavernous strips obtained from control and alloxan-induced diabetic rabbit were studied in organ chamber. In conclusion, prolonged in vitro exposure of corpus cavernosum strips obtained from control and diabetic groups to high concentrations of ISDN caused significant desensitization to the relaxant effect the drug. So, prolonged exposure of corporal tissue to the agents like nitroglycerine, used for treatment of impotence, may render ineffective the therapy in diabetic erectile impotence. However, intolerance to nitric oxide provides a rationale for the concept of using nitro oxide agents (like SNP) in the treatment of diabetic erectile dysfunction.