Tolerance to Nitroglycerin Induced by Isosorbide-5-Mononitrate Infusion in Vivo.

Abstract

Nitrates are still important drugs for clinical therapy of the patient with ischemic heart disease and for relief of anginal attack. Treatments with nitrates, however, develop vascular tolerance associated with attenuation of hemodynamic and antianginal effects.2,3) When a therapeutic plasma level of nitrates is maintained over 24 h, it may cause a tolerance to nitrates.2,3) Isosorbide-5-mononitrate (ISMN) is one of the active metabolites of isosorbide dinitrate and has been clinically used. 4,5) Because this agent is hard to metabolize further by the hepatic metabolic system, its therapeutic plasma level can be sustained for a longer period. 4,5) ISMN is clinically used for treatment of patients with hypertension or angina pectoris.6-8) Sublingual dosing of nitroglycerin (glyceryl trinitrate; GTN) is used for relief of the anginal pain of patients treated chronically with ISMN. Many clinical reports show lack of development of tolerance to ISMN. 7,9) However, we failed to find a report on whether the tolerance to GTN occurs when there has been chronical treatment with ISMN. In the present study, we compared tolerance to GTN in the rat infused continuously with ISMN to the rat infused continuously with GTN. MATERIALS AND METHODSThe investigation conformed to the Guiding Principles for the Care and Use of Experimental Animals in Hokkaido College of Pharmacy (published in 2001).Materials ISMN and GTN were supplied by Toa Eiyo Ltd., Fukushima, Japan. Osmotic pumps (Alzet Animal Preparation Male Sprague Dawley rats weighing 250-300 g were anesthetized with sodium pentobarbital (60 mg/kg, i.p.). Under artificial respiration, the left common carotid artery and the right external jugular vein were dissected free, and cannulated. A cannula introduced into the carotid artery was filled with saline solution containing heparin (100 units/ml), and connected to a pressure transducer. Heart rate was monitored from the ECG limb lead II. Mean arterial blood pressure and heart rate were recorded on a polygraph via a blood pressure amplifier (San-Ei 2238) and a bioelectric amplifier (San-Ei 1253A), respectively. ISMN and GTN dissolved in 99% ethanol were diluted with 5% glucose to prepare the solution at the concentration of 0.25-250 mg/kg and 0.25 mg/kg-2.5 mg/kg, respectively. Therefore, the final concentrations of ethanol and glucose were 5% and 5%, respectively. ISMN (0.25-250 mg/kg) or GTN (0.25 mg/kg-2.5 mg/kg) was injected through the right external jugular vein. Some rats were chronically infused with ISMN or GTN using an osmotic pump (ALZET ® Model 2001 and 2ML1) embedded subcutaneously in their back. The osmotic pump was filled with either 99% ethanol as vehicle, 250 mg/ml ISMN, 13 mg/ml GTN, or 20 mg/ml GTN, dissolved in 99% ethanol, and the solution was pumped out at 10 ml/h. ISMN was infused at the dose of 2.5 mg/h/rat for 3 or 7 d, whereas GTN was infused at 1.3 mg/h/rat for 7 d or 0.2 mg/h/rat for 3 d.Statistics The results are expressed as meansϮS.E.M. The significance of differences between groups was evaluated b...

Effect of nitric oxide on spinal evoked potentials and survival rate in rats with decompression sickness

“…Whereas the venous vessels are maximally dilated with relatively small doses of nitrates, the arterioles or resistance vessels dilate with high amounts of nitrates (1, 2), with GTN influencing the systemic vascular resistance far more potently than ISMN (26). So far, the optimal treatment drug (short-vs. long-acting NO donor), the optimal dosage, the optimal time of delivery before the decompression induced insult, and the exact physiological mechanism responsible for the therapeutic effect during DCS are unknown and remain to be established.…”

“…Therefore, to promote the beneficial effect, NO donors should be dispensed just before decompression. In a previous report by Mollerlokken et al (28) of DCS in a swine saturation model, it was found that GTN significantly decreased the intravascular bubble formation when given at a dosage twice that recommended in humans during a dive to 500 kPa (40 msw), subsequent to a linear decompression phase of 2 h. Accordingly, in groups 4 -6, the NO donors were administered 3-8 min before initiation of decompression and at a high dose that would inflict a hemodynamic impact during the decompression procedure (26,35). However, compared with the control group, NO donors in groups 4 -6 showed no protective effect on survival rate or spinal cord conduction when administered immediately before decompression.…”

“…However, if that is the case, it seems evident that removal of nuclei requires a therapeutic window exceeding the present interval from NO donor administration to the decompression-induced insult in groups 4 -6. Since the hemodynamic effect of NO donors wears off within minutes (26,35) when administered at a clinical dose and since the regeneration time for a depleted nuclei population is 10 -100 h (40), any protective effect of NO donors when administered before a dive advocates for a decrease in nuclei density as the predominant factor rather than conditioning by flow limitations. In a previous report by Dujic et al (8), divers received 0.4 mg GTN by oral spray 30 min before both a 30-min open water dive to 30 msw and a 80-min hyperbaric air dive to 18 msw, followed by a decompression phase of, respectively, 6 and 9 min.…”

“…The injection rate was 1 ml/min for groups 2, 4, and 6, 0.2 ml/min for group 5 (total infusion time of 5 min), and a bolus injection for group 3. The doses of the NO donors (except group 3) were chosen according to the hemodynamic effect reported in previous studies (26,35). The 58 rats from the 6 experimental groups were divided into the following: 1) group 1 (N ϭ 12), control group, saline intravenous injection initiated 10 min before compression; 2) group 2 (N ϭ 12), 300 mg/kg iv ISMN injection initiated 5-10 min before compression; 3) group 3 (N ϭ 8), 10 mg/kg ip GTN injection 30 min before compression; 4) group 4 (N ϭ 10), 300 mg/kg iv ISMN injection initiated 6 min before decompression; 5) group 5 (N ϭ 8), 1 mg/kg iv GTN injection initiated 8 min before decompression; and 6) group 6 (N ϭ 8), 75 g/kg iv GTN injection initiated 4 min before decompression.…”

“…Furthermore, it may be argued that, since NO donors cause immediate hemodynamic changes (26,35), administration of a NO donor before decompression should, everything else being equal, also result in an increased survival rate and a protective effect on spinal cord conduction, due to an enhanced tissue blood flow rate increasing N 2 elimination.…”