The role of ATP sensitive potassium channels in myocardial protection

Dynamic activation of K(ATP) channels in rhythmically active neurons

1. The respiratory centre within the brainstem is one of the most active neuronal networks that generates ongoing rhythmic activity. Stabilization of such vital activity requires efficient processes for activity-correlated adjustment of neuronal excitability. Recent investigations have shown that a regulatory factor coupling electrical activity with cell metabolism comprises ATP-dependent K(+) channels (K(ATP) channels), which continuously adjust the excitability of respiratory neurons during normoxia and increasingly during hypoxia. 2. We used the single-cell antisense RNA amplification-polymerase chain reaction (PCR) technique to demonstrate that respiratory neurons co-express the sulphonylurea receptor SUR1 with the Kir6.2 potassium channel protein. 3. Single channel measurements on rhythmically active inspiratory neurons of the brainstem slice preparation of newborn mice revealed that K(ATP) channels are periodically activated in synchrony with each respiratory cycle. 4. The Na(+)-K(+)-ATPase was inhibited with ouabain to demonstrate that oscillations of the channel open probability disappear, although respiratory activity persists for a longer time. Such findings indicate that K(ATP) channel open probability reflects activity-dependent fluctuations in the ATP concentration within submembrane domains. 5. We also examined the effects of extracellular [K(+)] and hypoxia. All changes in the respiratory rhythm (i.e. changes in cycle length and burst durations) affected the periodic fluctuations of K(ATP) channel activity. 6. The data indicate that K(ATP) channels continuously modulate central respiratory neurons and contribute to periodic adjustment of neuronal excitability. Such dynamic adjustment of channel activity operates over a high range of metabolic demands, starting below physiological conditions and extending into pathological situations of energy depletion.

Cardiac troponin I is modified in the myocardium of bypass patients

BACKGROUND

Selective proteolysis of cardiac troponin I (cTnI) is a proposed mechanism of contractile dysfunction in stunned myocardium, and the presence of cTnI degradation products in serum may reflect the functional state of the remaining viable myocardium. However, recent swine and canine studies have not demonstrated stunning-dependent cTnI degradation.

METHODS AND RESULTS

To address the universality of cTnI modification, myocardial biopsy samples were obtained from coronary artery bypass patients (n=37) before and 10 minutes after removal of cross-clamp. Analysis of biopsy samples for cTnI by Western blotting revealed a spectrum of modified cTnI products in myocardium both before and after cross-clamp, including degradation products (7 products resulting from differential N- and C-terminal processing) and covalent complexes (3 products). In particular, a 22-kDa cTnI degradation product with C-terminal proteolysis was identified, which may represent an initial ischemia-dependent cTnI modification, similar to cTnI(1-193) observed in stunned rat myocardium. Although no systematic change in amount of modified cTnI was observed, subgroups of patients displayed an increase (n=10, 85+/-5% of cTnI remaining intact before cross-clamp versus 75+/-5% after) or a decrease (n=12, 67+/-5% before versus 78+/-5% after). Electron microscopy demonstrated normal ultrastructure in biopsy samples, which suggests no necrosis was present. In addition, cTnI modification products were observed in serum through a modified SDS-PAGE methodology.

CONCLUSIONS

cTnI modification, in particular proteolysis, occurs in myocardium of bypass patients and may play a key role in stunning in some bypass patients.

Analysis of single K(ATP) channels in mammalian dentate gyrus granule cells

ATP-sensitive potassium (K(ATP)) channels are heteromultimer complexes of subunits from members of the inwardly rectifying K(+) channel and the ATP-binding cassette protein superfamilies. K(ATP) channels couple metabolic state to membrane excitability, are distributed widely, and participate in a variety of physiological functions. Understood best in pancreatic beta cells, where their activation inhibits insulin release, K(ATP) channels have been implicated also in postischemia cardio- and neuroprotection. The dentate gyrus (DG) is a brain region with a high density of K(ATP) channels and is relatively resistant to ischemia/reperfusion-induced cell death. Therefore we were interested in describing the characteristics of single K(ATP) channels in DG granule cells. We recorded single K(ATP) channels in 59/105 cell-attached patches from DG granule cells in acutely prepared hippocampal slices. Single-channel openings had an E(K) close to 0 mV (symmetrical K(+)) and were organized in bursts with a duration of 19.3 +/- 1.6 (SE) ms and a frequency of 3.5 +/- 0.8 Hz, a unitary slope conductance of 27 pS, and a low, voltage-independent, probability of opening (P(open), 0.04 +/- 0.01). Open and closed dwell-time histograms were fitted best with one (tau(open) = 1.3 +/- 0.2 ms) and the sum of two (tau(closed,fast) = 2.6 +/- 0.9 ms, tau(closed,slow) = 302.7 +/- 67. 7 ms) exponentials, respectively, consistent with a kinetic model having at least a single open and two closed states. The P(open) was reduced ostensibly to zero by the sulfonylureas, glybenclamide (500 nM, 2/6; 10 microM,11/14 patches) and tolbutamide (20 microM, 4/6; 100 microM, 4/4 patches). The blocking dynamics for glybenclamide included transition to a subconductance state (43.3 +/- 2.6% of control I(open channel)). Unlike glybenclamide, the blockade produced by tolbutamide was reversible. In 5/5 patches, application of diazoxide (100 microM) increased significantly P(open) (0.12 +/- 0.02), which was attributable to a twofold increase in the frequency of bursts (8.3 +/- 2.0 Hz). Diazoxide was without effect on tau(open) and tau(closed,fast) but decreased significantly tau(closed,slow) (24.4 +/- 2.6 ms). We observed similar effects in 6/7 patches after exposure to hypoxia/hypoglycemia, which increased significantly P(open) (0.09 +/- 0.03) and the frequency of bursts (7.1 +/- 1.7 Hz) and decreased significantly tau(closed,slow) (29.5 +/- 1.8 ms). We have presented convergent evidence consistent with single K(ATP) channel activity in DG granule cells. The subunit composition of K(ATP) channels native to DG granule cells is not known; however, the characteristics of the channel activity we recorded are representative of Kir6.1/SUR1, SUR2B-based channels.

Potassium channel openers: are they effective as pretreatment or additives to cardioplegia?

BACKGROUND

This study was designed to test the hypothesis that the potassium channel opener pinacidil (Pin) as a pretreatment (PT) agent or additive to St. Thomas' solution (StT) could enhance myocardial protection.

METHODS

In a parabiotic rabbit Langendorff model, 36 hearts underwent global normothermic ischemia (1 hour) followed by reperfusion (30 minutes). Cardioplegia (50 mL, every 20 minutes) consisted of: StT; PinPT/StT, where Pin PT preceded StT arrest; Pin alone; Pin in StT (Pin/StT); and Pin in low potassium StT. Systolic function after reperfusion (percent recovery of developed pressure) and compliance (diastolic slope from pressure-volume relationship) were measured.

RESULTS

There was no significant difference between StT and PinPT/StT in percent recovery of developed pressure (51.54% +/- 3.5%, 42.17% +/- 4.0%, respectively) or compliance. Likewise, no significant differences occurred between Pin, StT, Pin/StT, and Pin in low potassium StT in percent recovery of developed pressure (58.99% +/- 4.8%, 51.54% +/- 3.5%, 53.09% +/- 3.2%, 66.43% +/- 7.3%, respectively) or compliance.

CONCLUSIONS

Pin is as effective a cardioplegic agent as StT; however, its use as a pretreatment or additive to traditional and Pin in low potassium StT provided no additional benefit in functional recovery.

Ischaemic preconditioning: mechanisms and potential clinical applications

PURPOSE

Brief ischaemic episodes, followed by periods of reperfusion, increase the resistance to further ischaemic damage. This response is called "ischaemic preconditioning." By reviewing the molecular basis and fundamental principals of ischaemic preconditioning, this paper will enable the anaesthetic and critical care practitioner to understand this developing therapeutic modality.

SOURCE

Articles were obtained from a Medline review (1960-1997; search terms: ischaemia, reperfusion injury, preconditioning, ischaemic preconditioning, cardiac protection). Other sources include review articles, textbooks, hand-searches (Index Medicus), and personal files. PRINCIPLE FINDING: Ischaemic preconditioning is a powerful protective mechanism against ischaemic injury that has been shown to occur in a variety of organ systems, including the heart, brain, spinal cord, retina, liver, lung and skeletal muscle. Ischaemic preconditioning has both immediate and delayed protective effects, the importance of which varies between species and organ systems. While the exact mechanisms of both protective components are yet to be clearly defined, ischaemic preconditioning is a multifactorial process requiring the interaction of numerous signals, second messengers and effector mechanisms. Stimuli other than ischaemia, such as hypoxic perfusion, tachycardia and pharmacological agents, including isoflurane, have preconditioning-like effects. Currently ischaemic preconditioning is used during minimally invasive cardiac surgery without cardiopulmonary bypass to protect the myocardium against ischaemic injury during the anastomosis.

CONCLUSION

Ischaemic preconditioning is a powerful protective mechanism against ischaemic injury in many organ systems. Future clinical applications will depend on the clarification of the underlying biochemical mechanisms, the development of pharmacological methods to induce preconditioning, and controlled trials in humans showing improved outcomes.

Attenuation of ischemic liver injury by augmentation of endogenous adenosine

Hepatic grafts from non-heartbeating donors may alleviate the organ shortage, but they inherently suffer from warm ischemia. In the present study, we tested our hypothesis that augmentation of endogenous adenosine by inhibition of nucleoside transport with R75231 attenuates ischemic liver injury. Adult female beagle dogs underwent 2-hr hepatic vascular exclusion with venovenous bypass. R75231 was given to the animals by continuous intravenous infusion for 30 min before ischemia at a dose of 0.1 mg/kg (Group 2, n=6), 0.05 mg/kg (Group 3, n=6), or 0.025 mg/kg (Group 4, n=6). Nontreated animals were used as the control (Group 1, n= 10). Animal survival, hepatic tissue blood flow, liver function, and histopathology were analyzed. Two-week animal survival was 30% in Group 1, 83% in Group 2, 100% in Group 3, and 100% in Group 4. Postreperfusion hepatic tissue blood flow was markedly improved by the treatment. Treatment significantly attenuated liver enzyme release, lipid peroxidation, and changes in adenine nucleotides and purine catabolites. Structural abnormality of the liver after reperfusion was markedly improved by R75231 treatment, showing better architecture and less neutrophil infiltration. Preischemic administration of a nucleoside transport inhibitor ameliorated ischemic liver injury due to the positive effects of augmented endogenous adenosine, and is applicable clinically when the liver is procured from a controlled non-heartbeating donor.