Effects of streptozotocin-induced diabetes on the pharmacology of rat conduit and resistance intrapulmonary arteries

Cardiovascular Diabetology Original investigation Effects of streptozotocin-induced diabetes on the pharmacology of rat conduit and resistance intrapulmonary arteries Alison M Gurney*1 and Frank C Howarth2 0 Department of Physiology, Faculty of Medicine & Health Sciences, United Arab Emirates University , P.O. Box 17666, Al Ain, UAE 1 Faculty of Life Sciences, University of Manchester, Floor 2, Core Technology Facility , 46 Grafton Street, Manchester, M13 9NT , UK Background: Poor control of blood glucose in diabetes is known to promote vascular dysfunction and hypertension. Diabetes was recently shown to be linked to an increased prevalence of pulmonary hypertension. The aim of this study was to determine how the pharmacological reactivity of intrapulmonary arteries is altered in a rat model of diabetes. Methods: Diabetes was induced in rats by the -cell toxin, streptozotocin (STZ, 60 mg/kg), and isolated conduit and resistance intrapulmonary arteries studied 3-4 months later. Isometric tension responses to the vasoconstrictors phenylephrine, serotonin and PGF2, and the vasodilators carbachol and glyceryl trinitrate, were compared in STZ-treated rats and age-matched controls. Results: STZ-induced diabetes significantly blunted the maximum response of conduit, but not resistance pulmonary arteries to phenylephrine and serotonin, without a change in pEC50. Agonist responses were differentially reduced, with serotonin (46% smaller) affected more than phenylephrine (32% smaller) and responses to PGF2 unaltered. Vasoconstriction caused by K+induced depolarisation remained normal in diabetic rats. Endothelium-dependent dilation to carbachol and endothelium-independent dilation to glyceryl trinitrate were also unaffected. Conclusion: The small resistance pulmonary arteries are relatively resistant to STZ-induced diabetes. The impaired constrictor responsiveness of conduit vessels was agonist dependent, suggesting possible loss of receptor expression or function. The observed effects cannot account for pulmonary hypertension in diabetes, rather the impaired reactivity to vasoconstrictors would counteract the development of pulmonary hypertensive disease. - Background Hypertension is a recognised consequence of the poor control of blood glucose in diabetes. A recent study found that patients with diabetes mellitus also have an increased prevalence of pulmonary hypertension, independent of systemic hypertension, coronary artery disease, congestive heart failure or smoking [1]. In addition, pulmonary hypertension is more severe in patients with chronic obstructive pulmonary disease if they also have diabetes [2]. Furthermore, maternal diabetes is a risk factor for persistent pulmonary hypertension of the newborn [3]. Thus uncontrolled diabetes is a contributor to the development of pulmonary vascular disease. The underlying cause of systemic hypertension in diabetes is thought to be peripheral vasoconstriction [4] mediated, at least in part, by endothelial dysfunction [5,6] and enhanced smooth muscle contractility [7]. Little is known about the cause of pulmonary hypertension, but there is evidence from a rat model of diabetes, induced by streptozotocin (STZ) [8], that uncontrolled plasma glucose gives rise to structural [9] and biochemical [10] changes in pulmonary arteries. A study of salt-perfused rat lungs found that pulmonary vascular resistance was elevated in male, but not female rats subject to STZ-induced diabetes, along with reduced reactivity of the pulmonary vessels to vasoconstrictors [11]. A recent report suggests that, as in systemic vessels, STZ-induced diabetes in rats leads to loss of endothelium-dependent relaxation in the main intrapulmonary artery, possibly reflecting enhanced superoxide production from NADPH oxidase [12]. A single injection of STZ is widely used to generate a rat model of type I diabetes, which results from the selective toxicity of STZ towards the insulin-producing -cells in pancreatic islets [13]. A number of factors influence the vascular dysfunction that develops in this model, such as the age of the rats, the dose of STZ administered and the duration and severity of hyperglycaemia [8]. For example, the endothelium-dependent relaxation of rat aorta was enhanced 24 hrs after STZ injection, normal at 12 weeks and depressed at 8 weeks [6]. Impaired endothelial function was also more common in young adult humans with a longer duration of diabetes [14]. There may also be differential effects of diabetes on different blood vessels. For example, mesenteric arteries from STZ-treated rats displayed an enhanced vasoconstrictor response to noradrenaline and reduced sensitivity to endotheliumdependent vasodilators at a time (210 weeks after STZ injection) when aortae from the same animals responded normally [15]. Since the small resistance arteries in the lung are the main determinants of pulmonary vascular resistance and pulmonary arterial pressure, the aim of this study was to determine how chronic diabetes (34 months) affects these vessels. Reactivity to vasoconstrictors and vasodilators was investigated in intrapulmonary arteries (IPA) of 250 m in diameter and compared with larger conduit arteries from the same lungs, in STZ diabetic animals and age-matched controls. Methods Diabetes was induced in young male Wistar rats (200250 g) by a single intraperitoneal injection of 60 mg/kg STZ (Sigma) dissolved in citrate buffer, containing 100 mM citric acid and 100 mM sodium citrate at pH 4.5. This dose of STZ lies within the range used in most cardiovascular studies and produces moderate diabetes, in which blood glucose levels are 34 times normal, by causing substantial but incomplete depletion of pancreatic insulin [16]. Age-matched controls received an equivalent volume of vehicle. Control and STZ-treated animals were maintained on the same diet with freely available water. Animals were sacrificed at 34 months immediately after measuring body weight and blood glucose. A blood glucose level exceeding 200 mg/dL was considered diabetic. The lungs were quickly removed into physiological salt solution (PSS) of the following composition (mM): NaCl 112, KCl 5, KH2PO4 0.5, Na2HPO4 0.5, CaCl2 1.8, MgCl2 1, HEPES 10, glucose 11; pH7.3. The investigation was approved by the ethics committee of the Faculty of Medicine & Health Sciences, United Arab Emirates University and conforms to the Guide for the Care and Use of Laboratory Animals published by the National Institutes of Health (NIH Publication No. 85-23, revised 1996). Small vessel myography A section of conduit artery (~500 m diameter), from half way down the main artery running the length of the left lung lobe, was dissected out and mounted between two pins in a small vessel myograph chamber (Danish Myotechnology). Resistance-sized intrapulmonary arteries (< 250 m diameter) were dissected from the same lobe and mounted between two 40 m wires in separate myograph chambers. The vessels were ba (...truncated)