ABSTRACT
BACKGROUND: Extracellular and intracellular pH (pHo and pHi), which can be changed in various pathological conditions such as hypoxia, affects vascular contractility. To elucidate the mechanism to alter vascular contractility by pH, the effects of pH on reactivity to vasocontracting agents, intracellular Ca2+ influx, and Ca2+ sensitivity in vascular smooth muscle were examined. MATERIAL AND METHOD: Isometric contractions in rat superior mesenteric arteries (SMA) were observed. Intracellular Ca2+ concentration ([Ca2+]i) was recorded by microfluorometer using Fura-2/acetoxylmethyl ester in muscle cells. pHo was increased from 7.4 to 7.8 or decreased to 6.9 or 6.4. pHi was decreased by applying NH4+ or propionic acid or modulated by changing pHo after increasing membrane permeability using beta-escin. RESULT: Decreases in pHo from 7.4 to 6.9 or 6.4 shifted concentration-response curve by norepinephrine (NE) or serotonin (SE) to the right and significantly increased half maximal effective concentration (EC50) to NE or SE. Increase in pHo from 7.4 to 7.8 shifted concentration-response curve by norepinephrine (NE) or serotonin (SE) to the left and significantly reduced EC50 to NE or SE. NE increased [Ca2+]i in cultured smooth muscle cells from SMA and the increased [Ca2+]i was reduced by decreases in pHo. NE-induced contraction was inhibited by NH4+, whereas the resting tension was increased by NH4+ or propionic acid. When the cell membrane of SMA was permeabilized using beta-escin, SMA was contracted by increasing extracellular Ca2+ concentration from 0 to 10micrometer and the magnitude of contraction was decreased by a decrease in pHo and vice versa. CONCLUSION: From these results, it can be concluded that a decrease in pHo might inhibit vascular contraction by reducing the reactivity of vascular smooth muscle to vasoactive agents, Ca2+ influx and the sensitivity of vascular smooth muscle to Ca2+.
Subject(s)
Animals , Rats , Hypoxia , Calcium , Cell Membrane , Contracts , Diethylpropion , Escin , Hydrogen-Ion Concentration , Isometric Contraction , Membranes , Mesenteric Artery, Superior , Muscle Cells , Muscle, Smooth, Vascular , Myocytes, Smooth Muscle , Norepinephrine , Permeability , Propionates , SerotoninABSTRACT
BACKGROUND: The up-regulation of the nitric oxide (NO)-cGMP pathway might be involved in the change of vascular reactivity in rats 3 days after they suffer acute myocardial infarction. However, the underlying mechanism for this has not been clarified. MATERIAL AND METHOD: Acute myocardial infarction (AMI) was induced by occluding the left anterior descending coronary artery (LAD) for 30 min (Group AMI), whereas the sham-operated control rats were treated similarly without LAD occlusion (Group SHAM). The concentration-response relationships for phenylephrine (PE), KCl, acetylcholine (Ach) and sodium nitroprusside (SNP) were determined in the endothelium intact E(+) and endothelium denuded E(-) thoracic aortic rings from the rats 3 days after AMI or a SHAM operation. The concentration-response relationships of PE in the E(+) rings from the AMI rats were compared with those relationships in the rings pretreated with nitric oxide synthase (NOS) inhibitor N omega-nitro-L-arginine methyl ester (L-NAME) or the cyclooxygenase inhibitor indomethacin. The plasma nitrite/nitrate concentrations were checked via a Griess reaction. The cyclic GMP content in the thoracic aortic rings was measured by radioimmunoassay and the endothelial nitric oxide synthase (eNOS) mRNA expression was assessed by real time PCR. RESULT: The mean infarct size (%) in the rats with AMI was 21.3+/-0.62%. The heart rate and the systolic and diastolic blood pressure were not significantly changed in the AMI rats. The sensitivity of the contractile response to PE and KCl was significantly decreased in both the E(+) and E(-) aortic rings of the AMI group (p<0.05). L-NAME completely reversed these contractile responses whereas indomethacin did not (p<0.05). Moreover, the sensitivity of the relaxation response to Ach was also significantly decreased in the AMI group (p<0.05). The plasma nitrite and nitrate content (p<0.05), the basal cGMP content (p<0.05) and the eNOS mRNA expression (p=0.056) in the AMI rats were increased as compared with the SHAM group. CONCLUSION: Our findings indicate that the increased eNOS activity and the up-regulation of the NO-cGMP pathway can be attributed to the decreased contractile or relaxation response in the rat thoracic aorta 3 days after AMI.