Sensitization of coronary alpha-adrenoceptor vasoconstriction in the prediabetic metabolic syndrome.

OBJECTIVE: This study tested whether alpha -adrenoceptor-mediated coronary vasoconstriction is augmented in the metabolic syndrome and is accompanied by the alteration of specific alpha(1)- and alpha(2)-coronary adrenoceptors. METHODS: Studies were conducted in control and chronically high-fat-fed (6 weeks of 60% calories from fat) dogs with metabolic syndrome. Alterations in coronary alpha(1B)-, alpha(1D)-, and alpha(2A)-adrenoceptor mRNA and protein expression were examined by real-time PCR and Western analyses, respectively. Coronary blood flow and its response to intracoronary infusion of either the alpha1-adrenoceptor agonist methoxamine (0.1-3 mg) or the alpha(2)-adrenoceptor agonist BHT-933 (0.1-3 mg) were measured in anesthetized dogs. RESULTS: Basal plasma epinephrine and norepinephrine levels were higher in the high-fat-fed dogs compared to controls. Real-time PCR revealed no alterations of coronary artery or arteriole alpha1B-, alpha(1D)-, and alpha(2A)-adrenoceptor mRNA expression. However, Western blot analysis showed a significant decrease in alpha(2A)-adrenoceptor protein density with no change in alpha(1B)- or alpha(1D)-adrenoceptors. Methoxamine and BHT-933 produced dose-dependent decreases in coronary blood flow, but the decrease in coronary flow to methoxamine was significantly greater (approximately 20%) in dogs with the metabolic syndrome. No differences in the coronary flow response to BHT-933 were noted. CONCLUSIONS: These results indicate that the metabolic syndrome is associated with sensitization of alpha1- and alpha2-adrenoceptor signaling that could significantly limit control of coronary blood flow when the sympathetic nervous system is activated.

Dysfunction of cardiac ryanodine receptors in the metabolic syndrome.

This study examined the hypothesis that the prediabetic metabolic syndrome alters expression, phosphorylation state and binding affinity of cardiac RyR2. Real-time PCR and Western blot analysis were used to assess mRNA and protein expression in the left ventricle, right ventricle and right atrium from control (n=5) and chronically high-fat-fed (n=5) dogs with the metabolic syndrome. Functional integrity of RyR2 was assessed by RyR2-Ser2809 phosphorylation and the receptor's ability to bind [3H]ryanodine. We found that RyR2 phosphorylation at Ser2809 was significantly elevated in right and left ventricle from high-fat-fed dogs compared to normal control dogs. This hyperphosphorylation was associated with a decrease in RyR2 binding affinity in right and left ventricle (high-fat diet=80.2 and 90.5 fmol/mg protein vs. control=243.6 and 200.9 fmol/mg protein, respectively) and a decrease in cardiac index in exercising dogs. RyR2 phosphorylation at Ser2809 and RyR2 binding affinity were not altered in the right atria of high-fat-fed dogs. In addition, no significant differences in cardiac RyR2 mRNA or protein expression were noted between groups. These data suggest that alterations in RyR2 could be an important mechanism of early cardiac dysfunction in obesity and insulin resistance.

Coronary vasomotor reactivity to endothelin-1 in the prediabetic metabolic syndrome.

OBJECTIVE: The purpose of the present investigation was to test the hypothesis that coronary vasoconstrictor responses to endothelin-1 are augmented in the prediabetic metabolic syndrome. METHODS: ELISA was used to measure plasma endothelin-1 and intracoronary endothelin-1 dose-response experiments were conducted in vivo on normal control and high-fat-fed prediabetic dogs. Additionally, isolated left circumflex (LCX) coronary arteries and arterioles (< 160 microm) were used for in vitro functional studies and molecular analyses (quantitative real-time PCR and Western blotting). RESULTS: Plasma endothelin-1 concentrations were not different between control and prediabetic dogs. Coronary vasoconstriction to endothelin-1 was similar in control and prediabetic dogs, both in vivo and in isolated arterioles. Nonetheless, real-time PCR analysis revealed significant decreases in ET(A) receptor transcript levels in LCX coronary arteries and arterioles. Also, Western blotting revealed a significant decrease in ET(A) receptor protein in LCX coronary arteries. CONCLUSIONS: The findings of the present investigation indicate that although ET(A) receptor-signaling is sensitized by induction of the metabolic syndrome, endothelin-mediated coronary vasoconstriction does not significantly contribute to coronary dysfunction at this early stage of prediabetes.

Coronary arteriolar vasoconstriction to angiotensin II is augmented in prediabetic metabolic syndrome via activation of AT1 receptors.

The metabolic syndrome is associated with activation of the renin-angiotensin system. However, whether the coronary vascular response to ANG II is altered under this condition is unknown. Experiments were conducted in control and chronically high-fat-fed dogs with the prediabetic metabolic syndrome both in vitro (isolated coronary arterioles, 60-110 microm) and in vivo (anesthetized and conscious). We found that plasma renin activity and ANG II levels are elevated in high-fat-fed dogs and that this increase in ANG II is associated with a significant increase in ANG II-mediated coronary vasoconstriction in isolated coronary arterioles and in anesthetized open-chest dogs. The vasoconstriction to ANG II is abolished by ANG II type 1 (AT1) receptor blockade. In conscious chronically instrumented dogs, AT1 receptor blockade with telmisartan improved the balance between coronary blood flow and myocardial oxygen consumption in the high-fat-fed dogs but not in normal control dogs, i.e., the relationship between coronary venous Po2 and myocardial oxygen consumption was shifted upward, toward normal control values. Quantitative assessment of coronary arteriolar AT1 and ANG II type 2 (AT2) receptor mRNA levels by real-time PCR revealed no significant difference between normal control and high-fat-fed dogs; however, Western blot analysis showed a significant increase in AT1 receptor protein level with no change in AT2 receptor protein density. These findings indicate that AT1 receptor-mediated coronary constriction is augmented in the prediabetic metabolic syndrome and contributes to impaired control of coronary blood flow via increases in circulating ANG II and/or coronary arteriolar AT1 receptor density.