Stereoisomers of calcium antagonists distinguish a myocardial and vascular mode of protection against cardiac ischemic injury

Electrophysiological effect of l-cis-diltiazem, the stereoisomer of d-cis-diltiazem, on isolated guinea-pig left ventricular myocytes

l-cis-Diltiazem, the stereoisomer of the L-type Ca(2+) channel blocker d-cis-diltiazem, protects cardiac myocytes from ischemia and reperfusion injury in the perfused heart and from veratridine-induced Ca(2+) overload. We determined the effect of l-cis-diltiazem on the voltage-dependent Na(+) current (I(Na)) and lysophosphatidylcholine-induced currents in isolated guinea-pig left ventricular myocytes by a whole-cell patch-clamp technique. l-cis-Diltiazem inhibited I(Na) in a dose-dependent manner without altering the current-voltage relationship for I(Na) (K(d) values : 729 and 9 microM at holding potentials of -140 and -80 mV, respectively). A use-dependent block of I(Na), the leftward shift of the steady-state inactivation curve and the delay of recovery from inactivation suggest that l-cis-diltiazem has a higher affinity for the inactivated state of Na(+) channels. In addition to I(Na), the lysophosphatidylcholine-induced currents were inhibited by l-cis-diltiazem in a similar concentration range. It is suggested that inhibition of both Na(+) channels and lysophosphatidylcholine-activated non-selective cation channels contributes to the cardioprotective effect of l-cis-diltiazem.

Protective effect of l-cis-diltiazem on hypercontracture of rat myocytes induced by veratridine

The protective effect of l-cis-diltiazem, the stereoisomer of d-cis-diltiazem, was studied against the veratridine-induced hypercontracture of rat myocytes. Veratridine increased both [Na+]i and [Ca2+]i, but did not cause hypercontracture in the absence of extracellular Ca2+. Both l-cis-diltiazem (0.1-10 microM) and d-cis-diltiazem (10-30 microM) inhibited the hypercontracture and the increase in [Ca2+]i in a concentration-dependent manner. However, l-cis-diltiazem did not exert a negative inotropic effect in K+ (20 mM)-depolarized rat papillary muscles even at a dose of 10 microM. As seen in the case of tetrodotoxin, l-cis-diltiazem and d-cis-diltiazem also suppressed the increase in [Na+]i. The results show that l-cis-diltiazem prevents the veratridine-induced hypercontracture of myocytes by suppression of the [Ca2+]i increase. The attenuation of the [Ca2+]i increase by l-cis-diltiazem was not dependent on inhibition of Ca2+ channels, but was partly due to inhibition of excessive Na+ entry via veratridine-modified Na+ channels.

Reversal of acute theophylline toxicity by calcium channel blockers in dogs and rats

Theophylline, widely used in the treatment of pulmonary diseases, has a narrow therapeutic index; the recommended plasma levels being 10-20 micrograms/ml in humans. The misuse or abuse of theophylline can cause life-threatening central nervous system and cardiovascular effects. Increased intracellular Ca2+ levels are thought to play an important role in theophylline toxicity and death. The objective of this study was to determine whether Ca2+ channel blockers, e.g. verapamil, nifedipine, or diltiazem, prevent sudden death caused by theophylline treatment in rats and dogs. Groups of Sprague-Dawley rats were treated with theophylline alone (150 mg/kg i.p.) or with theophylline pretreatment followed by administration of verapamil (0.25 to 0.5 mg/kg i.p.), nifedipine (0.25 to 1.0 mg/kg i.p.), or diltiazem (0.5 to 1.0 mg/kg i.p.), 2.5 to 15 min later. The rats were observed for toxic signs and survival over a period of 15 days. All three calcium channel blockers significantly reduced the theophylline-induced sudden death in rats. In a separate study, neither verapamil (0.5 mg/kg i.p.) nor nifedipine (1.0 mg/kg i.p.) prevented the theophylline-induced myocardial necrosis in the rat. In beagle dogs, verapamil (0.5 mg/kg i.v.) prevented theophylline (15 mg/kg/min i.v. for 10 min)-induced hypotension, arrhythmias, and sudden death. Our results support previously reported findings that calcium plays a major role in theophylline-induced toxicity and death.

Beneficial effects of felodipine on myocardial and coronary function during low-flow ischemia and reperfusion

An acute coronary occlusion causes severe low-flow ischemia in the occluded region. Calcium antagonists have the potential to reduce the rate of ischemic injury by decreasing myocardial oxygen demand, as well as by other mechanisms, especially when given prior to the onset of ischemia. However, their clinical use may be limited by their negative inotropic effects. The purpose of this study was to assess the effects of felodipine as a potentially protective agent against myocardial ischemia and reperfusion injury, independent of any negative inotropic actions, when given after the onset of low-flow ischemia. Isolated isovolumic (balloon-in-LV), blood-perfused rabbit hearts, paced at a constant heart rate, were subjected to 90 minutes of low-flow ischemia at a coronary perfusion pressure of 10 mmHg, which reduced coronary blood flow to 22-24% of baseline. After 15 minutes of low-flow ischemia, hearts received 2 x 10(-6) M felodipine (n = 7) or no drug (controls, n = 8). Felodipine was given until 15 minutes of reperfusion. During low-flow ischemia both groups of hearts had identical coronary blood flow, heart rate, left ventricular (LV) developed pressure, lactate production, and O2 consumption. However, felodipine markedly protected against ischemic diastolic dysfunction. At the end of low-flow ischemia, LV end-diastolic pressure (LVEDP) had increased from 10 +/- 1 to 28 +/- 5 mmHg in the felodipine group, while in the controls LVEDP increased to 48 +/- 8 mmHg (p < 0.05). During 30 minutes of reperfusion, felodipine had a beneficial effect upon coronary blood flow (initial postischemic hyperemia 245 +/- 38% of baseline in the felodipine group vs. 124 +/- 18% in the controls; p < 0.01) Felodipine markedly improved the recovery of contractile function [LV developed pressure recovered from a baseline of 104 +/- 4 to 75 +/- 6 mmHg (72%) in the felodipine group vs. 34 +/- 10 mmHg (32%) in the control group; p < 0.01], as well as diastolic function (LVEDP = 25 +/- 4 mmHg in the felodipine group vs. 61 +/- 10 mmHg in the controls; p < 0.05), and ATP levels (8.5 +/- 1.4 mumoles/g d.w. in the felodipine group vs. 3.9 +/- 1.4 mumoles/g d.w. in the control group, p < 0.05). Felodipine, given after the onset of low-flow ischemia, protects the myocardium during both ischemia and reperfusion by mechanisms other than reducing myocardial oxygen demand.

On the other hand: the stereoselectivity of drug action at ion channels

Ion channels are pharmacological receptors with specific drug binding sites. These binding sites define specific structure-function relationships for the actions of drug classes. Interpretation of these structure-function relationships may be complex because of state-dependent drug-channel interactions. These state-dependent interactions determine affinity and access of drug to binding sites and may result in both quantitative and qualitative changes in structure-function relationships including stereoselectivity. A channel-active drug may exhibit antagonist or activator properties according to membrane potential and the stereoselectivity of interaction may also change with channel state.

Enantioselective gallopamil protein binding

The protein binding of the enantiomers of gallopamil has been investigated in solutions of human serum albumin, alpha 1-acid glycoprotein and serum. Over the range of concentrations attained after oral gallopamil administration, the binding of both enantiomers to albumin, alpha 1-acid glycoprotein, and serum proteins was independent of gallopamil concentration. The binding to both human serum albumin (40 g/liter) [range of fraction bound (fb) R: 0.624 to 0.699; S: 0.502 to 0.605] and alpha 1-acid glycoprotein (0.5 g/liter) (range of fb R: 0.530 to 0.718; S: 0.502 to 0.620) was stereoselective, favoring the (R)-enantiomer (predialysis gallopamil concentrations 2.5 to 10,000 ng/ml). When the enantiomers (predialysis gallopamil concentration 10 ng/ml) were studied separately in drug-free serum samples from six healthy volunteers the fraction of (S)-gallopamil bound (fb: 0.943 +/- 0.016) was lower (P < 0.05) than that of (R)-gallopamil (fb: 0.960 +/- 0.010). The serum protein binding of both (R)- and (S)-gallopamil was unaffected by their optical antipodes (fb R: 0.963 +/- 0.011; S: 0.948 +/- 0.015) indicating that at therapeutic concentrations a protein binding enantiomer-enantiomer interaction does not occur. The protein binding of (R)- and (S)-gallopamil ex vivo 2 h after single dose oral administration of 50 mg pseudoracemic gallopamil (fb R: 0.960 +/- 0.010: predialysis [R] 6.9 to 35.3 ng/ml; S: 0.943 +/- 0.016: predialysis [S] 9.5 to 30.7 ng/ml) was comparable to that observed in vitro in drug-free serum. Gallopamil metabolites formed during first-pass following oral administration, therefore, do not influence the protein binding of (R)- or (S)-gallopamil.

Chiral aspects of drug action at ion channels: a commentary on the stereoselectivity of drug actions at voltage-gated ion channels with particular reference to verapamil actions at the Ca2+ channel

Ion channels may be considered as pharmacological receptors possessing specific drug binding sites with defined structure-activity relationships. Accordingly drug binding to ion channels is stereoselective. Interpretation of this stereoselectivity may be complex because of the existence of differences in affinity and access to different channel states. Such state-dependent interactions may give rise to quantitative and qualitative differences in stereoselectivity. The implications of such differences are reviewed for drug action at Na+, K+ and Ca2+ channels. Detailed attention is paid to the actions of verapamil enantiomers in the cardiovascular system where activities differ in vascular and cardiac tissues because of state-dependent interactions and stereoselective first-oass metabolism.