Supersensitivity of the adenylyl cyclase system in acute myocardial ischemia: evaluation of three independent mechanisms

Phospholamban phosphorylation in ischemia-reperfused heart. Effect of pacing during ischemia and response to a beta-adrenergic challenge

The status of phospholamban (PLB) phosphorylation in the ischemia-reperfused hearts remains controversial. Although a decrease in the phosphorylation of both PLB residues (Ser16, PKA site, and Thr17, CaMKII site) was previously reported, experiments from our laboratory failed to detect this decrease. In an attempt to elucidate the cause for this discrepancy, experiments were performed in Langendorff-perfused rat hearts with two main goals: (1) To determine whether keeping pacing during ischemia, a protocol followed in other ischemia-reperfusion models, decreases the phosphorylation of PLB residues, below pre-ischemic values; (2) To investigate whether a maximal beta-adrenergic challenge allows to detect a decrease in the ability of PLB to be phosphorylated in ischemia-reperfused hearts. Hearts were submitted to a global ischemia/reperfusion protocol (20/30 min) with (P) or without (NP) pacing during ischemia, and phosphorylation of PLB residues was assessed by immunodetection. The recovery of contractility upon reperfusion was lower in P vs. NP hearts. Ser16 of PLB, was phosphorylated at the end of ischemia in NP hearts. This increase appeared earlier in P hearts and was significantly diminished by catecholamine depletion and beta-blockade. Thr17 site was phosphorylated at the beginning of ischemia and the onset of reperfusion. The ischemia-induced phosphorylation of Thr17 was higher and more sustained in P vs. NP hearts, and inhibited by the calcium channel blocker, nifedipine, whereas the reperfusion-induced increase in Thr17 phosphorylation was similar in P and NP hearts and was significantly diminished by the Na+/Ca2+ exchanger inhibitor KB-R7943. Phosphorylation of PLB residues did not decrease below basal levels at any time during ischemia and reperfusion. However, the phosphorylation, inotropic and lusitropic response to beta-adrenergic stimulation was significantly decreased both in P and NP hearts.

Differences in response to activation of adenylyl cyclase by various stimulants in human myocardium

The response of adenylyl cyclase complex in human atrial tissue removed at corrective surgery of normoxemic and hypoxemic congenital heart defects in children to various stimulants was evaluated and related to the oxygenation state of the myocardium. When comparing response to stimulation in normoxemic and hypoxemic atria a higher basal as well as stimulated adenylyl cyclase activity was found in hypoxemic atria; an insignificant stimulatory effect of isoprenaline in normoxemic hearts became significant in the atria of hypoxemic patients. Hypoxemic samples also showed two times higher activity when the total catalytic activity was evaluated by the stimulation with forskolin. Higher stimulatory effect of Gpp/NH/p was also observed in hypoxemic than in normoxemic state. Increased adenylyl cyclase activity might represent one of adaptive mechanisms to hypoxemia in patients with congenital heart defects.

Differential presynaptic modulation of noradrenaline release in human atrial tissue in normoxia and anoxia

1. Presynaptic modulation of noradrenaline release in human atrial tissue specimens was investigated under normoxic and anoxic conditions. 2. Noradrenaline release was induced by electrical stimulation and release during experimental intervention (S2) was compared with release during a preceding control stimulation (S1). The results were expressed as the geometric means and 95% confidence intervals of the S2/S1 ratio. 3. The alpha 2-adrenoceptor agonist, UK 14304 (0.1 mumol-1) significantly inhibited noradrenaline release, resulting in a S2/S1 ratio of 0.49 (0.40-0.59), and the a 2-adrenoceptor antagonist, yohimbine (1 mumol l-1) increased noradrenaline release (S2/S1 1.83 [1.43-2.35]) during normoxia. Both compounds were ineffective during anoxia. 4. Adenosine (30 mumol-1) inhibited noradrenaline release with a S2/S1 ratio of 0.54 (0.42-0.66). The adenosine antagonist, 8-phenyltheophylline, alone had no effect during normoxia. During anoxia, neither adenosine nor 8-phenyltheophylline altered noradrenaline release. 5. The beta 2-adrenoceptor agonist, terbutaline (1 mumol l-1) increased (1.53 [1.14-2.01]) and the beta-adrenoceptor antagonist, pindolol (1 mumol l-1) suppressed noradrenaline release (0.62 [0.49-0.79]) under normoxic conditions. During anoxia, pindolol significantly inhibited noradrenaline release with a S2/S1 ratio of 0.66 (0.51-0.85), whereas terbutaline did not influence noradrenaline release. 6. Angiotensin II (0.1 mumol l-1 enhanced noradrenaline release resulting in a S2/S1 ratio of 1.44 (1.34-1.54), while the angiotensin II antagonist, losartan (1 mumol l-1) had no effect on noradrenaline release during normoxia. Conversely, angiotensin II did not increase noradrenaline release and losartan significantly inhibited noradrenaline release to a S2/S1 ratio of 0.60 (0.46-0.77) during anoxia. 7. In conclusion, human cardiac tissue possesses presynaptic inhibitory alpha 2-adrenoceptors and adenosine receptors, as well as facilitatory beta 2-adrenoceptors and angiotensin II receptors regulating noradrenaline release under normoxic conditions. During anoxia the responses to alpha 2-adrenoceptors and adenosine receptor stimulation are lost, whereas facilitatory responses to beta 2-adrenoceptors and adenosine II receptor stimulation are maintained and these receptors appear to be maximally stimulated. This differential presynaptic modulation in anoxia may contribute to enhanced sympathetic activity in ischaemia.

The guanine nucleotide-binding regulatory proteins (G proteins) in myocardium with ischemia

Guanine nucleotide-binding regulatory proteins (G proteins) play a major role in the regulation of a number of physiological processes, such as stimulation or Inhibition of adenylate cyclase activity or gaiting of ionic channels. Myocardial ischemia could induce the changes in receptor-G protein signal transduction system in the heart. Therefore, this article will focus on the role and alterations of G proteins (especially, Gs and Gi) in myocardial ischemia. The Gi protein rapidly loses functional activity during very early myocardial ischemia. In contrast to Gi protein, the function of Gs protein during this phase has not been evaluated. Moreover, the changes in Gs protein after 30 min of ischemia are contradictory. However, the sensitization of the adenylate cyclase activity in the very early phase of acute ischemia is gradually replaced by a decrease in adenylate cyclase activity with prolonged ischemia. The decrease in the function and amount of Gs protein may be one of the factors that induce these changes. The function of Gs protein was also decreased in the canine hearts with ischemia and reperfusion. In contrast to ischemia and reperfusion, there are no significant alterations in G proteins and modulation of adenylate cyclase in the stunned myocardium. It has become increasingly evident that Gi protein may play an important role in the cardioprotective effects of preconditioning. When beta-adrenoreceptor densities are reduced in chronic myocardial ischemia, decreased in the amount and function of Gi protein and increased amount of Gs protein may play the role in preservation of the adenylate cyclase activity. These alterations in G proteins may play the important role in the myocardial function during myocardial ischemia.

Regional myocardial downregulation of the inhibitory guanosine triphosphate-binding protein (Gi alpha 2) and beta-adrenergic receptors in a porcine model of chronic episodic myocardial ischemia

Regional myocardial ischemia is associated with increased levels of adenosine and norepinephrine, factors that may alter activation of the beta-adrenergic receptor (beta AR)-G protein-adenylyl cyclase pathway in the heart. We have used the ameroid constrictor model to determine whether alterations in myocardial signal transduction through the beta AR-G protein-adenylyl cyclase pathway occur in the setting of chronic episodes of reversible ischemia. Pigs were instrumented with ameroid occluders placed around the left circumflex coronary artery. 5 wk later, after ameroid closure, flow and function were normal in the ischemic bed, but flow (P = 0.001) and function (P < 0.03) were abnormal when metabolic demands were increased. The ischemic bed showed a reduction in myocardial beta AR number (P < 0.005). Despite regional downregulation of myocardial beta AR number, adenylyl cyclase activity was similar in the ischemic and control beds. Quantitative immunoblotting showed that the cardiac inhibitory GTP-binding protein, Gi alpha 2, was decreased in the ischemic bed (P = 0.02). In contrast, the cardiac stimulatory GTP-binding protein, Gs alpha, was increased in endocardial sections from the ischemic bed (P = < 0.05). Decreased Gi alpha 2 content was associated with decreased inhibition of adenylyl cyclase. Reduced Gi alpha 2 content, in conjunction with increased Gs alpha content in the endocardium, may provide a means by which adrenergic activation is maintained in the setting of chronic episodic myocardial ischemia.