Our results demonstrate that transmural heterogeneity of repolarization is amplified under acquired long QT conditions and that the increase in TDR underlies the development of TdP in halothane- but not pentobarbital-anesthetized dogs. The data support an important contribution of M cells to TDR and to the development of TdP in the canine heart in vivo. Our data also highlight the importance of acceleration-induced prolongation of MAPD (a phenomena observed principally in M cells) in the development of TdP.

Characteristics of the M Cell. The discovery and characterization of the M cell, a unique cell type residing in the deep layers of the ventricular myocardium, has opened a new door in our understanding of the electrophysiology and pharmacolugy of the heart in both health and disease. The hallmark of the M cell is the ability of its action potential to prolong much more thau that of other ventricular myocardial cells in response to a slowing of rate and/or in response to agents that act to prolong action potential duration. Our goal in this review is to provide a comprehensive characterization of the M cell, its contribution to transmural heterogeneity, and its role in the normal electrical function of the heart. In the inscription of the ECG (particularly the T wave), and in the development of QT dispersion, T wave alternans, long QT intervals, and cardiac arrhythmias, such as torsades de pointes. Our secondary goal is to address the controversy that has arisen relative to the functional importance of the M cell in the normal heart. The controversy derives largely from the failure of some investigators to demonstrate transmural heterogeneity of repolarization in the dog in vivo under control conditions and after administration of quinidine. The inability to demonstrate transmural heterogeneity under these conditions may he due to the use of bipolar recording techniques that, in our experience, seriously underestimate transmural dispersion of repolarization (TDK). The use of sodium pentobarhital and a-chloralose as anesthesia also is problematic, becau.se these agents reduce or eliminate TDR by affecting a variety of ion channel currents. Finally, attempts to amplify transmural dispersion of repolarization with an agent such as quinidine must take into account that relatively high concentrations can result in effects opposite to those desired due to drug inhibition of multiple ion channels. These observations may explain the inability of earlier studies to detect the M cell.

Aims Obesity can modify the pharmacokinetics of lipophilic drugs. As b-adrenoceptor blockers (BB) are often prescribed for obese patients suffering from hypertension or coronary heart disease, this study compares the pharmacokinetics of lipophilic b-adrenoceptor blockers in obese and control subjects. Methods Nine obese (157±24% of ideal body weight (IBW) mean±s.d.) and nine non-obese healthy volunteers (98±10% IBW), aged 32±9 years, were included in the study. Subjects were randomly given a single i.v. infusion of one of the following racemic b-adrenoceptor blockers, whose doses (expressed as base per kg of IBW) were: propranolol (0.108 mg), labetalol (0.99 mg) and nebivolol (0.073 mg ). The plasma concentrations of unchanged drugs were measured by h.p.l.c. The ionisation constants and lipophilicity parameters of b-adrenoceptor blockers were assessed. Results The pharmacokinetic data for the three drugs were qualitatively similar. There was a trend towards a greater total distribution volume (V ss ) in obese patients than in controls. However, V ss expressed per kg body weight was slightly smaller in obese patients. The relationship between V ss and lipophilicity of five b-adrenoceptor was studied by combining the current results with those previously obtained with a moderately lipophilic drug (bisoprolol ) and a hydrophilic one (sotalol). The V ss of the five drugs was positively and well-correlated (r 2 =0.90; P<0.01) with their distribution coefficient at pH 7.4 (log D 7.4 ), but not with their partition coefficients. The linear regression coefficients for lean and obese subjects were very similar. Conclusions Lipophilic b-adrenoceptor blockers seem to diffuse less into adipose than into lean tissues. All electrical forms of the drugs (i.e. cations, neutral forms, or zwitterions) present at physiological pH contribute to their tissue distribution, in both obese and lean subjects.Their tissue distribution in obese patients could be restricted by the sum of hydrophobic forces and hydrogen bonds they elicit with macromolecules in lean tissues.

The coadministration of a class Ib agent can reduce the proarrhythmic potential of a class III drug in experimental animals predisposed to torsade de pointes arrhythmias and further suggests the clinical utility of such a strategy.

Chronic treatment of dogs with amiodarone induced a moderate prolongation of the QT interval and MAP duration without affecting transmural dispersion of repolarization and inhibited the effects of acute sotalol, including the prolongation of repolarization, the increase in transmural dispersion of repolarization and the induction of arrhythmias.