Basic Res Cardiol 91:31 - 34 (1996) 9 Steinkopff Verlag 1996

Criteria for a mediator or effector of myocardial preconditioning: Do KATechannels meet the requirements?

G. J. Gross D. A. Mei J. J. Schultz T. M i z u m u r a

Key words Preconditioning criteria m e m o r y - KATp channels - species d e p e n d e n c y -

t h r e s h o l d

-

Introduction A l t h o u g h a n u m b e r of mediators (7) have b e e n shown to mimic the protective effect of ischemia to precondition the heart, i.e., adenosine, acetylcholine, bradykinin, p h o r b o l esters, and several end effectors or targets (7) of these substances have b e e n identified, i.e., specific receptors, (A1, adenosine; M2 muscarinic; B 2, bradykinin) ATP-sensitive potassium channels (KATP), protein kinase C (PKC), no strict criteria have b e e n d e v e l o p e d which a r e necessary to confirm that a substance or effector is an i m p o r t a n t c o m p o n e n t of the preconditioning response. This review is an a t t e m p t to develop such criteria and to summarize data which suggest that the K~rp channel fits m o s t of these criteria and meets the requirements necessary for it to qualify as a c o m m o n p a t h w a y in the cardioprotective effect p r o d u c e d by a variety of stimuli in different species or models. Infarct size reduction will be the end point used to define preconditioning since this param e t e r is the one which was used to originally d e m o n s t r a t e this fascinating p h e n o m e n o n in canine hearts by M u r r y et

al. (6).

Criteria for mediator or effector of preconditioning (see Table 1) The criteria which have b e e n p r o p o s e d to be i m p o r t a n t for determining if a m e d i a t o r or effector is an i m p o r t a n t c o m p o n e n t of the cardioprotective response of ischemic preconditioning (PC) are the following: 1) Ischemic P C should be blocked by specific pharmacological antagonists of the p r o p o s e d m e d i a t o r or end effector; 2) P h a r m a cological agonists in the same class as the m e d i a t o r or which stimulate the end effector should mimic the effect of ischemic P C to reduce infarct size; 3) O t h e r stimuli which p r o d u c e a PC-like effect should be blocked by specific antagonists of the end effector or final c o m m o n pathway; 4) The time-course of the ischemic P C response ( " m e m o r y " ) should be mimicked or blocked by agonists

Table 1 Criteria f~ a mediat~ ~ effect~ ~ ischemic prec~176 ing (PC) - Ischemic PC should be blocked by specific pharmacological antagonists of the proposed mediator or end effector -

Pharmacological agonists in the same class as the mediator or which stimulate the end effector should mimic the effect of ischemic PC to reduce infarct size

- Other stimuli which produce a PK-like effect should be blocked by specific antagonists of the end effector or final common pathway - The time-course of the ischemic PC response ("memory") should be mimicked or blocked by agonists or antagonists of the end effector

Dr. G. J. Gross (EN) 9D. A. Mei 9J. J. Schultz 9T. Mizumura Department of Pharmacology & Toxicology Medical College of Wisconsin 8701 Watertown Plank Road Milwaukee, Wisconsin 53226, USA

- Pharmacological agonists of receptors or the end effector should decrease the threshold of time necessary to precondition the heart by ischemia - The mediator or effector of PC should be common to all species including man

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Basic Research in Cardiology,Vol. 91, No. 1 (1996) 9 SteinkopffVerlag 1996

or antagonists of the end effector; 5) Pharmacological agonists of receptors or the end effector should decrease the threshold of time necessary to precondition the heart by ischemia; 6) The mediator or effector of PC should be common to all species including man. Because of space limitations, the following discussion will be limited to the evidence supporting a role for the KATp channel as a final common pathway in the PC response primarily using studies performed in dogs in our laboratory for illustrative purposes.

Ischemic PC - Pharmacological antagonists The strongest evidence for a role of KATp channels in ischemic PC is based on studies using pharmacological antagonists, particularly glibenclamide. Our original work in dogs (2) showed that administration of glibenclamide before or immediately after the ischemic PC stimulus totally blocked its protective effect which suggested that the K~;rFchannel may be involved in both triggering and sustaining the beneficial effect of PC in dogs. Subsequently, we also showed that sodium 5-hydroxydecanoic acid (5-HD), a KATI' antagonist structurally unrelated to glibenclamide and possessing ischemia selectivity also abolished ischemic PC in dogs (1). Glibenclamide and 5-HD have also been shown to block ischemic PC in a number of related studies performed in pigs, rabbits and rats although there is some controversy in the latter two species.

Pharmacological agonists mimic ischemic PC Several studies in dogs have shown that KAT• channel openers mimic the effect of ischemic PC to reduce infarct size (4, 5). Most recently, Mizumura et al. (5) compared the effects of bimakalim, a KATp opener, and 5 min of ischemic PC to reduce infarct size, affect adenosine release from the ischemic area and affect neutrophil infiltration into the ischemic-reperfused area. Interestingly, bimakalim and ischemic PC, reduced infarct size, adenosine release and neutrophil infiltration to a nearly equal extent which suggests, but does not unequivocally prove, that these two stimuli may be acting via a common mechanism to reduce ischemic injury in the canine heart. Similar comparative studies have not been performed in other species and are necessary to strengthen the idea that KATP activation and ischemic PC share similar mechanisms of cardioprotection in all species.

PC mimetics blocked by specific antagonists of the end effector Recent work byYao and Gross (10) and Mei and Gross (3) has shown that other mediators or stimuli can produce a PC-like effect to reduce infarct size in dogs via activation of the KATP channel. Intracoronary administration of adenosine or acetylcholine have been shown to reduce infarct size in canine hearts similar to that produced by ischemic PC, and these protective effects were blocked by glibenclamide or 5-HD (10). Similarly, Mei and Gross (3) recently showed that 5 rain of hypoxia was as effective as 5 min of ischemia to precondition the canine heart, and this effect of hypoxia was also abolished by pretreatment with glibenclamide. Hartman and colleagues at Upjohn have also shown that glibenclamide blocks bradykinininduced PC in rabbits (personal communication). Therefore, it appears that the KATp channel may serve as a common effector in dogs and rabbits to various pharmacological or pathophysiological stimuli which induce a PC-like effect on infarct size.

Time-course or memory of ischemic PC - Mimicked or blocked by agonists or antagonists of the end effector One of the most interesting aspects of ischemic PC is the observation that there is a limited time following the PC stimulus during which the heart remembers that it is preconditioned. This phenomenon has been termed acute memory and has been shown to last for approximately 2 h in dogs (6). In this regard, recent evidence from our laboratory indirectly supports a role for the KATp channel as a necessary component of the acute memory phase of ischemic PC (12). We observed that glibenclamide, administered 50 rain after the ischemic PC stimulus totally abolished the protective effect of ischemic PC which was still present i h following preconditioning. We also found that a 10-min intracoronary infusion of both adenosine and bimakalim together produced a cardioprotective effect similar to that of 10 min of ischemic PC to reduce infarct size 60 min after the PC stimulus, whereas, either adenosine or bimakalim administered alone had no sustained cardioprotective effect in this canine model (12). Taken together, these results suggest that myocardial KaTP channels may be an essential component of the acute memory phase of PC. Recent evidence from our laboratory (unpublished results) also suggests that monophosphoryl lipid A (MLA), an endotoxin analogue, can produce a delayed (24 h later) cardioprotective response to reduce infarct size in dogs and rabbits similar to that of acute ischemic

G. J. Gross et al. Criteria for mediator of preconditioning PC and that this delayed protective effect is mediated via the KATe channel. Further work is necessary to determine if other stimuli known to produce a delayed cardioprotective response, i.e., ischemia, hypoxia, heat shock, adenosine, also act via the KATPchannel.

Pharmacological agonists of receptors or end effectors lower the threshold for ischemic PC In addition to ischemic PC having the property of memory, there is also a finite period of time or "threshold" required for ischemic PC to produce its cardioprotective effect. Generally, 2 - 3 rain of ischemia are necessary for the cardioprotective effect of PC to be manifest in various species including the dog. In this regard, recent results from our laboratory have shown that 3 min of ischemia is not sufficient to precondition the canine heart (11), however, when 0.3 /,g/min of the KATP opener bimakalim was infused simultaneously during the 3-min occlusion period, infarct size was reduced to an extent similar to that of 5 or 10 min of ischemic PC. These results further support the hypothesis that the KATe channel is an essential component in triggering as well as maintaining the PC response.

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although most studies with glibenclamide performed in rats have been negative. Perhaps more importantly, although infarct size was not the end point, a recent study of Tomai et al. (9) showed that glibenclamide blocked a PC-like effect produced by consecutive periods of angioplasty in man. Furthermore, results of Speechly-Dick et al. (8) in human atrial trabeculae preconditioned by hypoxia also suggest a key role for the KATPchannel in mediating PC in man.

Conclusions A set of criteria have been presented which we feel need to be met before one can assign an important role for a particular endogenous mediator, ion channel or enzyme as an important component of ischemic PC and evidence has been presented to suggest that the KATechannel meets most of the criteria listed. Obviously, this list is not all conclusive and other important factors will evolve as we increase our understanding of this fascinating phenomenon of ischemic PC.

Mediators or effectors of PC common to all species including man There is evidence to support the idea that the NATP channel is involved in ischemic PC in all species tested

Acknowledgements The authors wish to thank Ms. Carol Knapp for the excellent assistance in typing this manuscript. The work described was supported by NIH Grant HL 08311.

References 1. Auchampach JA, Grover GJ, Gross GJ (1992) Blockade of ischemic preconditioning in dogs by the novel ATP-dependent potassium channel antagonist sodium 5-hydroxydecanoate. Cardiovasc Res 26:1054-1062 2. Gross GJ, Auchampach JA (1992) Blockade of ATP-sensitive potassium channels prevents myocardial preconditioning in dogs. Circ Res 70:223-233 3. Mei DA, Gross GJ (1995) Evidence for the involvement of the ATP-sensitive potassium channel in a novel model of hypoxic preconditioning in dogs. Cardiovasc Res 30:222-230

4. Mizumura T, Nithipatikom K, Gross GJ (1995) Effects of nicorandil and glyceryl trinitrate on infarct size, adenosine release and neutrophil infiltration in the dog. Cardiovasc Res 29:482489 5. Mizumura T, Nithipatikom K, Gross GJ (1995) Bimakalim, an ATP-sensitive potassium channel opener, mimics the effects of ischemic preconditioning to reduce infarct size, adenosine release and neutrophil function in dogs. Circulation 92:1236-1245 6. Murry CE, Jennings RB, Reimer KA (1986) Preconditioning with ischemia: a delay of lethal cell injury in ischemic myocardium. Circulation 74:1124-1136

7. Parratt JR (1995) Possibilities for the pharmacological exploitation of ischemic preconditioning. J Mol Cell Cardiol 27:991-1000 8. Speechly-Dick ME, Grover GJ, Yellon DM (1995) Does ischemic preconditioning in the human atrium involve protein kinase C and the ATP-dependent K channel? Studies of contractile function in an in vitro model. Circ Res 77: 10301035

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9. Tomai F, Crea F, Gaspardone A, Versaci F, DePaulis R, Penta de Peppo A, Chiariello L, Gioffre PA (1994) Ischemic preconditioning during coronary angioplasty is prevented by glibenclamide, a selective ATP-sensitive K + channel blocker. Circulation 90:700-705

Basic Research in Cardiology, Vol. 91, No. 1 (1996) 9 Steinkopff Verlag 1996 10. Yao Z, Gross GJ (1993) Acetylcholine mimics ischemic preconditioning in dogs: role of nitric oxide, muscarinic receptors and the KATP channel. Circ Res 73:1193-1201 11. Yao Z, Gross GJ (1994) Activation of ATP-senstive potassium channels lowers threshold for ischemic preconditioning in dogs. Am J Physiol 267: H1888H1894

12. Yao Z, Mizumura T, Mei DA, Gross GJ (1996) Myocardial KATP channels may play a role in the memory of ischemic preconditioning in anesthetized dogs: Evidence for a synergistic effect between adenosine and the KATPchannel. Am J Physiol (in Review)