Multiplicity of effectors of the cardioprotective agent, diazoxide

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Abstract

Diazoxide has been identified over the past 50 years to have a number of physiological effects, including lowering the blood pressure and rectifying hypoglycemia. Today it is used clinically to treat these conditions. More recently, another important mode of action emerged: diazoxide has powerful protective properties against cardiac ischemia. The heart has intrinsic protective mechanisms against ischemia injury; one of which is ischemic preconditioning. Diazoxide mimics ischemic preconditioning. The purpose of this treatise is to review the literature in an attempt to identify the many effectors of diazoxide and discuss how they may contribute to diazoxide's cardioprotective properties. Particular emphasis is placed on the concentration ranges in which diazoxide affects its different targets and how this compares with the concentrations commonly used to study cardioprotection. It is concluded that diazoxide may have several potential effectors that may potentially contribute to cardioprotection, including KATP channels in the pancreas, smooth muscle, endothelium, neurons and the mitochondrial inner membrane. Diazoxide may also affect other ion channels and ATPases and may directly regulate mitochondrial energetics. It is possible that the success of diazoxide lies in this promiscuity and that the compound acts to rebalance multiple physiological processes during cardiac ischemia.

Introduction

Diazoxide (CAS Number: 364-98-7; 7-chloro-3-methyl-2H-1,2,4-benzothiadiazine 1,1-dioxide; Fig. 1) has a molecular weight of 230.7 and a molecular formula of C8H7ClN2O2S. It is a white powder insoluble in water, but soluble in organic solvents (e.g. 10 mg/ml in DMSO). The dogma has arisen in recent years (particularly in the cardioprotection literature) that diazoxide is an agent with a unique molecular target. This is not the case and the purpose of this literature review is to highlight the multiplicity of diazoxide effectors to assist in a better understanding of mechanisms involved in the established cardioprotective effects of this compound.

Section snippets

History

In the early 1960's, a study was designed to examine possible non-diuretic mechanisms by which benzothiadiazines lower blood pressure — diazoxide was found to directly cause vasodilation of blood vessels independent of diuretic actions (Rubin et al., 1962). Early reports, however, also demonstrated that some hypotensive drugs such as diazoxide led to elevated blood glucose levels (hyperglycemia) (Wolff, 1964, Okun et al., 1964). The following years saw a large rise in publications, mostly

Clinical use

In tablet form (e.g. Proglycem, FDA approved in 1976) diazoxide is prescribed orally (usually 2 to 3 times daily) for the management of symptomatic hypoglycemia. Side effects include shortness of breath, swelling in extremities, tachycardia, chest pain, blurred vision, bruising or bleeding, unusual weakness; and decreased frequency of urination. Intravenously (e.g. Hyperstat) diazoxide is indicated as a peripheral vasodilator for short-term use in the emergency reduction of blood pressure in

Diazoxide is cardioprotective against ischemic insults

During the treatment of patients with hypotension, early studies suggested an increase in myocardial injury with diazoxide (e.g. chest pain and ST elevation) (Kanada et al., 1976, O'Brien et al., 1975). These effects may have been related to the hypotensive action of the drug. Most controlled animal studies to date, however, as well as in vitro studies with human cardiac tissues, suggest that diazoxide has cardioprotective properties (Garlid et al., 1997, Nakai and Ichihara, 1994, Wang et al.,

Diazoxide activates pancreatic β-cell KATP channels

Diazoxide has long been recognized to be hyperglycemic by inhibiting insulin release from pancreatic β-cells (Wolff et al., 1963, Loubatieres et al., 1966, Okun et al., 1964). The mechanism was found to be an increased membrane K+ permeability (measured with Rb+ flux assays), leading to membrane hyperpolarization, inhibition of Ca2+ influx (Henquin & Meissner, 1982) and diminished insulin secretory release. Shortly after the discovery of cardiac KATP channels (Noma, 1983, Trube and Hescheler,

Diazoxide activates smooth muscle KATP channels

The hypotensive properties of diazoxide have been described over 50 years ago (Rubin et al., 1962). As is the case for other KATP channel openers (Gross et al., 1989, Quayle et al., 1997, Sakamoto et al., 1989), the vasodilatory effects of diazoxide in vascular smooth muscle are due to the opening of vascular KATP (or KNDP) channels (Standen et al., 1989), which leads to local relaxation in smooth muscle due to an increased membrane K+ permeability, subsequent inhibition of excitability, lowered

Does diazoxide activate ‘cardiac’ sarcolemmal KATP channels?

Before addressing this question, it would be instructive to examine the mechanism by which diazoxide opens KATP channels. KATP channels are heterooctamers of four Kir6.1 and/or Kir6.2 subunits in complex with SURx subunits (Nichols, 2006). Two genes (ABCC8 and ABCC9) respectively give rise to SUR1 and SUR2 subunits, each of which may be alternatively spliced. The most commonly described splice variants studied are SUR1, SUR2A and SUR2B (Nichols, 2006). When comparing the effects of diazoxide on

Diazoxide activates mitochondrial KATP channels

A K+ selective ion channel, blocked by ATP with an IC50 of 800 μM, was initially described in the inner membrane of fused giant mitoplasts prepared from rat liver mitochondria using patch clamp methods (Inoue et al., 1991). A similar mitochondrial KATP (mKATP) channel was recorded when purified membranes of rat liver or bovine heart were reconstituted in lipid bilayer membranes (Bednarczyk et al., 2005, Nakae et al., 2003, Paucek et al., 1992, Zhang et al., 2001). Patch clamping of the inner

Diazoxide improves mitochondrial function

As early as 1969, diazoxide has been recognized as an inhibitor of the mitochondrial complex II protein, succinate dehydrogenase (SDH) (Schafer et al., 1969). This inhibition, which is also observed in the heart (Hanley and Daut, 2005, Hanley et al., 2002), occurs at concentrations often used to study cardioprotection (K1/2 = 32 μM; Table 1) and similar to those affecting other diazoxide effectors (Table 2). As expected, SDH inhibition by diazoxide leads to increased flavoprotein fluorescence (

Are endothelial KATP channels involved?

Although diazoxide can cause vasodilation in the absence of an intact endothelium (Antoine et al., 1992), several reports indicate the endothelium to be another diazoxide target (Feleder & Adler-Graschinsky, 1997). In humans, plethysmography recordings of forearm blood flow demonstrated that endothelial function is impaired by ischemia (induced by blood pressure cuff inflation), which is prevented by IPC or diazoxide pre-administration (800 μg/min for 20 min) (Broadhead et al., 2004).

Diazoxide regulates neurotransmitter release

A novel role for KATP channels has been described in the sympathetic nervous system, where it was found that norepinephrine (NE) release is inhibited by active KATP channels (Oe et al., 1999). The data supporting this finding are that KATP channel openers (including 100 μM diazoxide) inhibit NE release as well as the increase in atrial rate induced by electrical stimulation of the sympathetic ganglion (Mohan and Paterson, 2000, Oe et al., 1999). In contrast, KATP channel blockers had the

Other diazoxide effectors

Other than those discussed above, diazoxide affects the functions of several other proteins and biological processes. For example, in the cardioprotective range, 100 μM diazoxide causes a significant increase in ATPase activity in purified cardiac membranes isolated from guinea pig hearts (Bienengraeber et al., 2000). Stimulation of mitochondrial ATPase activity has also been reported (Dzeja et al., 2003, Portenhauser et al., 1971), which is due to the stimulation of the F0F1 ATP synthase (

Conclusion

A drug does not have to be selective in order to be effective. It has been argued that an ideal drug may be one whose efficacy is not solely based on a single target, but rather on rebalancing several biological effectors/processes that contribute to the etiology, pathogeneses, and progression of diseases (i.e. a promiscuous drug) (Mencher & Wang, 2005). Diazoxide appears to be such as a drug: while it is clear that diazoxide has potent cardioprotective properties, a review of the literature

Conflict of interest statement

The author declares that there are no conflicts of interest.

Funding

This work was supported by the National Institutes of Health grant HL093563.

Acknowledgments

The author is grateful to Dr Kathleen C Kinnally, NYU College of Dentistry, for reading the manuscript and for making useful suggestions.

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