Angiotensin II type 1 receptor blockade abolishes specific K(ATP)channel gene expression in rats with myocardial ischemia

Subcellular trafficking and endocytic recycling of KATP channels

Sarcolemmal/plasmalemmal ATP-sensitive K+ (KATP) channels have key roles in many cell types and tissues. Hundreds of studies have described how the KATP channel activity and ATP sensitivity can be regulated by changes in the cellular metabolic state, by receptor signaling pathways and by pharmacological interventions. These alterations in channel activity directly translate to alterations in cell or tissue function, that can range from modulating secretory responses, such as insulin release from pancreatic β-cells or neurotransmitters from neurons, to modulating contractile behavior of smooth muscle or cardiac cells to elicit alterations in blood flow or cardiac contractility. It is increasingly becoming apparent, however, that KATP channels are regulated beyond changes in their activity. Recent studies have highlighted that KATP channel surface expression is a tightly regulated process with similar implications in health and disease. The surface expression of KATP channels is finely balanced by several trafficking steps including synthesis, assembly, anterograde trafficking, membrane anchoring, endocytosis, endocytic recycling, and degradation. This review aims to summarize the physiological and pathophysiological implications of KATP channel trafficking and mechanisms that regulate KATP channel trafficking. A better understanding of this topic has potential to identify new approaches to develop therapeutically useful drugs to treat KATP channel-related diseases.

Localization of ATP-sensitive K+ channel subunits in rat liver

BACKGROUND

ATP-sensitive K⁺ (K_ATP) channels were originally found in cardiac myocytes by Noma in 1983. K_ATP channels were formed by potassium ion-passing pore-forming subunits (Kir6.1, Kir6.2) and regulatory subunits SUR1, SU2A and SUR2B. A number of cells and tissues have been revealed to contain these channels including hepatocytes, but detailed localization of these subunits in different types of liver cells was still uncertain.

AIM

To investigate the expression of K_ATP channel subunits in rat liver and their localization in different cells of the liver.

METHODS

Rabbit anti-rat SUR1 peptide antibody was raised and purified by antigen immunoaffinity column chromatography. Four of Sprague-Dawley rats were used for liver protein extraction for immunoblot analysis, seven of them were used for immunohistochemistry both for the ABC method and immunofluorescence staining. Four of Wistar rats were used for the isolation of hepatic stellate cells (HSCs) and Kupffer cells for both primary culture and immunocytochemistry.

RESULTS

Immunoblot analysis showed that the five kinds of K_ATP channel subunits, i.e. Kir6.1, Kir6.2, SUR1, SUR2A, and SUR2B, were detected in liver. Immunohistochemical staining showed that Kir6.1 and Kir6.2 were weakly to moderately expressed in parenchymal cells and sinusoidal lining cells, while SUR1, SUR2A, and SUR2B were mainly localized to sinusoidal lining cells, such as HSCs, Kupffer cells, and sinusoidal endothelial cells. Immunoreactivity for SUR2A and SUR2B was expressed in the hepatocyte membrane. Double immunofluorescence staining further showed that the pore-forming subunits Kir6.1 and/or Kir6.2 colocalized with GFAP in rat liver sections and primary cultured HSCs. These K_ATP channel subunits also colocalized with CD68 in liver sections and primary cultured Kupffer cells. The SUR subunits colocalized with GFAP in liver sections and colocalized with CD68 both in liver sections and primary cultured Kupffer cells. In addition, five K_ATP channel subunits colocalized with SE-1 in sinusoidal endothelial cells.

CONCLUSION

Observations from the present study indicated that K_ATP channel subunits expressed in rat liver and the diversity of K_ATP channel subunit composition might form different types of K_ATP channels. This is applicable to hepatocytes, HSCs, various types of Kupffer cells and sinusoidal endothelial cells.

KATP Channels in the Cardiovascular System

KATP channels are integral to the functions of many cells and tissues. The use of electrophysiological methods has allowed for a detailed characterization of KATP channels in terms of their biophysical properties, nucleotide sensitivities, and modification by pharmacological compounds. However, even though they were first described almost 25 years ago (Noma 1983, Trube and Hescheler 1984), the physiological and pathophysiological roles of these channels, and their regulation by complex biological systems, are only now emerging for many tissues. Even in tissues where their roles have been best defined, there are still many unanswered questions. This review aims to summarize the properties, molecular composition, and pharmacology of KATP channels in various cardiovascular components (atria, specialized conduction system, ventricles, smooth muscle, endothelium, and mitochondria). We will summarize the lessons learned from available genetic mouse models and address the known roles of KATP channels in cardiovascular pathologies and how genetic variation in KATP channel genes contribute to human disease.

Neural control of ventricular rate in ambulatory dogs with pacing-induced sustained atrial fibrillation

BACKGROUND

We hypothesize that inferior vena cava-inferior atrial ganglionated plexus nerve activity (IVC-IAGPNA) is responsible for ventricular rate (VR) control during atrial fibrillation (AF) in ambulatory dogs.

METHODS AND RESULTS

We recorded bilateral cervical vagal nerve activity (VNA) and IVC-IAGPNA during baseline sinus rhythm and during pacing-induced sustained AF in 6 ambulatory dogs. Integrated nerve activities and average VR were measured every 10 seconds over 24 hours. Left VNA was associated with VR reduction during AF in 5 dogs (from 211 bpm [95% CI, 186-233] to 178 bpm [95% CI, 145-210]; P<0.001) and right VNA in 1 dog (from 208 bpm [95% CI, 197-223] to 181 bpm [95% CI, 163-200]; P<0.01). There were good correlations between IVC-IAGPNA and left VNA in the former 5 dogs and between IVC-IAGPNA and right VNA in the last dog. IVC-IAGPNA was associated with VR reduction in all dogs studied. Right VNA was associated with baseline sinus rate reduction from 105 bpm (95% CI, 95-116) to 77 bpm (95% CI, 64-91; P<0.01) in 4 dogs, whereas left VNA was associated with sinus rate reduction from 111 bpm (95% CI, 90-1250) to 81 bpm (95% CI, 67-103; P<0.01) in 2 dogs.

CONCLUSIONS

IVC-IAGPNA is invariably associated with VR reduction during AF. In comparison, right or left VNA was associated with VR reduction only when it coactivates with the IVC-IAGPNA. The vagal nerve that controls VR during AF may be different from that which controls sinus rhythm.

Angiotensin II and tumour necrosis factor alpha as mediators of ATP-dependent potassium channel remodelling in post-infarction heart failure

AIMS

Angiotensin II (Ang II) and tumour necrosis factor alpha (TNFalpha) are involved in the progression from compensated hypertrophy to heart failure. Here, we test their role in the remodelling of ATP-dependent potassium channel (K(ATP)) in heart failure, conferring increased metabolic and diazoxide sensitivity.

METHODS AND RESULTS

We observed increased expression of both angiotensinogen and TNFalpha in the failing rat myocardium, with a regional gradient matching that of the K(ATP) subunit Kir6.1 expression. Both angiotensinogen and TNFalpha expression correlated positively with Kir6.1 and negatively with Kir6.2 expression across the post-infarction myocardium. To further identify a causal relationship, cardiomyocytes isolated from normal rat hearts were exposed in vitro to Ang II or TNFalpha. We observed increased Kir6.1 and SUR subunit and reduced Kir6.2 subunit mRNA expression in cardiomyocytes cultured with Ang II or TNFalpha, similar to what was observed in failing hearts. In patch-clamp experiments, cardiomyocytes cultured with Ang II or TNFalpha exhibited responsiveness to diazoxide, in terms of both K(ATP) current and action potential shortening. This was not observed in untreated cardiomyocytes and resembles the diazoxide sensitivity of failing cardiomyocytes that also overexpress Kir6.1. Ang II exerted its effect through induction of TNFalpha expression, because TNFalpha-neutralizing antibody abolished the effect of Ang II, and in failing hearts, regional expression of angiotensinogen matched TNFalpha expression. Finally, Ang II and TNFalpha regulated K(ATP) subunit expression, possibly through differential expression of Forkhead box transcription factors.

CONCLUSION

This study identifies Ang II and TNFalpha as mediators of the remodelling of K(ATP) channels in heart failure.

Expression and function of ATP-dependent potassium channels in late post-infarction remodeling

Myocardial remodeling late after infarction is associated with increased incidence of fatal arrhythmias. Heterogeneous prolongation of the action potential in the surviving myocardium is one of the predominant causes. Sarcolemmal ATP-dependent potassium (K(ATP)) channels are important metabolic sensors regulating electrical activity of cardiomyocytes and are capable of considerably shortening the action potential. We determined whether ATP-dependent potassium channels generate or, on the contrary prevent the heterogeneity in action potential prolongation. Cardiomyocytes were obtained from the infarct border zone, the septum and the right ventricle of rat hearts 20 weeks after coronary occlusion when rats developed signs of heart failure. Expression of the conductance subunit Kir6.1, but not Kir6.2, and of all SUR regulatory subunits was increased up to 3-fold in cardiomyocytes from the infarct border zone. Concomitantly, there was a prominent response of the K(ATP) current to diazoxide that was not detectable in control cardiomyocytes. The action potential was prolonged in cardiomyocytes from the infarct border zone (74 ms) relative to sham (41 ms). However, activation of the K(ATP) channels by diazoxide reduced action potential duration to 42 ms. In myocytes of the septum and right ventricle, expression of channel subunits, duration of action potential, and sensitivity to diazoxide were only slightly increased relative to shams. In conclusion, the myocardium remodeled after infarction displays alterations of K(ATP) expression and function with spatial heterogeneity matching that of the action potential prolongation. Drugs selectively activating diazoxide-sensitive sarcolemmal K(ATP) channels should be considered in the prevention of arrhythmias in post-infarction heart failure.

Towards the understanding of the local hematopoietic bone marrow renin-angiotensin system

The classical view of the renin-angiotensin system (RAS) as a circulating endocrine system has evolved to organ- and tissue-based systems that perform paracrine/autocrine functions. Angiotensin II (Ang II), the dominant effector peptide of the RAS, regulates cellular growth in a wide variety of tissues in (patho)biological states. In 1996, we hypothesized that there exists a locally active RAS in the bone marrow affecting the growth, production, proliferation and differentiation of hematopoietic cells. Evidences supporting this hypothesis are growing. Ang II, through interacting with Ang II type 1 (AT1) receptor stimulates erythroid differentiation. This stimulatory effect of Ang II on erythropoiesis was completely abolished by a specific AT1 receptor antagonist, losartan. AT1a receptors are present on human CD34(+) hematopoietic stem cells. Ang II increases hematopoietic progenitor cell proliferation and this effect was also blocked by losartan. Angiotensin-converting enzyme (ACE) is involved in enhancing the recruitment of primitive stem cells into S-phase in hematopoietic bone marrow by degrading tetrapeptide AcSDKP. ACE inhibitors modified the circulating hematopoietic progenitors in healthy subjects. RAS may also affect pathological/neoplastic hematopoiesis. Renin has been isolated from leukemic blast cells. Higher bone marrow ACE levels in acute leukemic patients suggested that ACE is produced at higher quantities in the leukemic bone marrow. In this review, the 'State of the Art' of the local bone marrow RAS is summarized. A local RAS in the bone marrow can mediate, in an autocrine/paracrine fashion, some of the principal steps of hematopoietic cell production. To show a causal link between the components of RAS and the other regulatory hematopoietic growth factors is not only an academic curiosity. Elucidation of such a local bone marrow system may offer novel therapeutic approaches in pathologic/neoplastic conditions.

Regulation of cardiac inwardly rectifying potassium channels by membrane lipid metabolism

Types and distributions of inwardly rectifying potassium (Kir) channels are one of the major determinants of the electrophysiological properties of cardiac myocytes. Kir2.1 (classical inward rectifier K(+) channel), Kir6.2/SUR2A (ATP-sensitive K(+) channel) and Kir3.1/3.4 (muscarinic K(+) channels) in cardiac myocytes are commonly upregulated by a membrane lipid, phosphatidylinositol 4,5-bisphosphates (PIP(2)). PIP(2) interaction sites appear to be conserved by positively charged amino acid residues and the putative alpha-helix in the C-terminals of Kir channels. PIP(2) level in the plasma membrane is regulated by the agonist stimulation. Kir channels in the cardiac myocytes seem to be actively regulated by means of the change in PIP(2) level rather than by downstream signal transduction pathways.

Reciprocal regulation of expression of pore-forming KATP channel genes by hypoxia

The ATP-sensitive potassium (KATP) channel is thought to play an important role in the protection of heart and brain against tissue hypoxia. The genetic regulation of the components of the channel by hypoxia has not been previously described. Here, we investigated the regulation of the two pore-forming channel proteins, Kir6.1 and Kir6.2, in response to hypoxia in vivo and in vitro. We find that these two structurally-related inwardly-rectifying potassium channel proteins are reciprocally regulated by hypoxia in vivo, with upregulation of Kir6.1 and down-regulation of Kir6.2, thereby resulting in a significant change in the composition of the channel complex in response to hypoxia. In vitro we describe neuronal and cardiac cell lines in which Kir6.1 is up-regulated by hypoxia, demonstrating that Kir6.1 is a hypoxia-inducible gene. We conclude that the heart and brain display genetic plasticity in response to hypoxic stress through specific genetic reprograming of cytoprotective channel genes.