Involvement Of Calcitonin Gene-related Peptide In The Cardiovascular Effect Of Nitroglycerin

BACKGROUNDNitroglycerin, a potent vasodilator, is widely used in treatment of angina pectoris by reducing proload and afterload and by dilating the main coronary arteries. The effect of nitroglycerin is ascribed to stimulation of nitric oxide (NO) and elevation of cyclic GMP levels. However, long-term administration of nitroglycerin can cause tolerance development leading to a marked attenuation in the magnitude of most of their pharmacological effects. The mechanism responsible for tolerance to nitroglycerin is not fully understood. There is substantial evidence to suggest that nitrate tolerance may be associated with a reduction of the content and activation of NO released from parent compounds.Previous investigations have shown that the regulatory effect of NO on vascular tone includes direct actions on vascular smooth muscle as well as modulation of sympathetic nerve transmission. There is evidence to show that NO is also capable of modulating peptidergic neurotransmission. CGRP, a predominant neurotransmitter incapsaicin-sensitive sensory nerves, is present in cardiovascular tissues. Studies have suggested that the release of CGRP is regulated by multiple factors including NO. Recently, it has been shown that vasodilator responses to nitroglycerin are related to stimulation of CGRP release in feline cerebral arterioles or rat aortas. We have also shown that cardioprotection induced by nitroglycerin is related to stimulation of CGRP release.The purposes of present studies were (1) to confirm the role of CGRP in the vasodilator response and depressor effect of nitroglycerin, (2) to examine the effect of CGRP on the development of tolerance to nitroglycerin, and (3) to explore whether the NO-cGMP pathway participates in regulation of the release of CGRP induced by nitroglycerin.METHODSThe experiment was divided into three parts:(1) Vasodilator responses to nitroglycerin Rats were anesthetized with sodium pentobarbital and the thoracic aorta was rapidly isolated and cut into rings of 4 mm length. The rings were suspended horizontally between two stainless-steel wires and mounted in organ chamber filled with warmed and oxygenated Krebs' solution. One of the ring ends was connected to a force transducer. The aortic ring was stretched with 2gresting force and precontracted with KC1 and phenylephrine. After the constriction stabilized, an accumulative concentration-response curve to nitroglycerin or CGRP was observed. For the studies on involvement of endogenous CGRP in vasodilator responses to nitroglycerin, the preparations were exposed to CGRP-(8-37) or capsaicin for 10 or 20 min, respectively, and then the response to nitroglycerin was tested.(2) The depressor effect of nitroglycerin After SD rats were anesthetized, a catheter was inserted into the left femoral artery to record blood pressure. Additional catheter was introduced into the right femoral artery for the withdrawal of reference arterial blood sample. Drugs were administered through a cannula inserted into the right femoral vein. In the case of vasodilator responses, sufficient time was allowed between each dose for blood pressure to return to a stable level, usually 5-6 min. For the studies on involvement of endogenous CGRP in the depressor effect of nitroglycerin and involvement of NO-cGMP pathway in the regulation of CGRP release, nitroglycerin was administered after pretreatment with capsaicin (50mg/kg, s.c, 4 days before the experiment) or methylene blue (3 mg/kg, i.v.) for 20 min. In order to rule out the effect of blood pressure on CGRP release, verapamil, a calcium antagonist, was used.(3) Nitroglycerin-induced tolerance Tolerance was induced by pretreatment with nitroglycerin (4.4 + 10-6 M) for 10 min in vitro and by treatment with nitroglycerin (10 mg/kg, s.c.) three times a day for 8 days.For the studies on the mechanism of the development of tolerance to nitroglycerin, drug was removed for 4 or 8 days or N-acetylcysteine or captopril was given in the tolerant rat.In the experiment in...