Administration of certain fluoroquinolone antibacterials has been associated with prolongation of the QT interval on the electrocardiogram and, on rare occasions, ventricular arrhythmia. Blockade of the human cardiac K+ channel HERG often underlies such clinical findings. Therefore, we examined a series of seven fluoroquinolones for their ability to interact with this channel. Using patch-clamp electrophysiology, we found that all of the drugs tested inhibited HERG channel currents, but with widely differing potencies. Sparfloxacin was the most potent compound, displaying an IC50 value of 18 microM, whereas ofloxacin was the least potent compound, with an IC50 value of 1420 microM. Other IC50 values were as follows: grepafloxacin, 50 microM; moxifloxacin, 129 microM; gatifloxacin, 130 microM; levofloxacin, 915 microM; and ciprofloxacin, 966 microM. Block of HERG by sparfloxacin displayed a positive voltage dependence. In contrast to HERG, the KvLQT1/minK K+ channel was not a target for block by the fluoroquinolones. These results provide a mechanism for the QT prolongation observed clinically with administration of sparfloxacin and certain other fluoroquinolones because free plasma levels of these drugs after therapeutic doses approximate those concentrations that inhibit HERG channel current. In the cases of levofloxacin, ciprofloxacin, and ofloxacin, inhibition of HERG occurs at concentrations much greater than those observed clinically. The data indicate that clinically relevant HERG channel inhibition is not a class effect of the fluoroquinolone antibacterials but is highly dependent upon specific substitutions within this series of compounds. HERG channel affinity should be an important criterion for the development of newer fluoroquinolones.

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This comparison of the binding characteristics and pharmacology of 1,4-dihydropyridines indicates that the high-affinity binding sites studied in cardiac and smooth muscle cells represent a major site of action of these drugs, and that this site is the Ca2+ channel or a closely related protein. Electrophysiological studies suggest that the effects of both Ca2+ channel inhibitors and activators are voltage dependent. The apparent lack of agreement between the equilibrium dissociation constants for [3H]1 ,4-dihydropyridines and their potency in cardiac muscle may be due to conformational modifications that occur in the 1,4-dihydropyridine binding site as a result of voltage or other changes during membrane isolation. The selective effect of 1 ,4-dihydropyridines for vascular smooth muscle relative to cardiac muscle may be explained, in part, by differences in membrane potentials and Ca2+ channel regulatory mechanisms and, in part, by differences in receptor structure. 1,4-Dihydropyridine antagonists and activators appear to bind to a common site that is not the same as the binding site for nondihydropyridine Ca2+ channel antagonists.

The activities of menthol and peppermint oil were determined in guineapig ileal smooth muscle, in rat and guinea-pig atrial and papillary muscle, in rat brain synaptosomes and in chick retinal neurones by pharmacological 45Ca2+ uptake and radioligand binding assays. Menthol is a major constituent of peppermint oil and is approximately twice as potent as peppermint oil as an lnhibitor of K+ depolarization-induced and electrically stimulated responses in ileum and electrically stimulated atrial and papillary muscles. IC,, values in the ileal preparation ranged from 7.7 to 28.1 pg ml-' and in the cardiac preparations from 10.1 to 68.5 pg ml-I.Similar potencies were demonstrated against K+ depolarizationinduced "Ca2+ uptake in synaptosomes and against K+ depolarization and Bay K 8644-induced uptake in chick retinal neurons. IC,, values for menthol inhibition of K+ and Bay K 8644 responses in the retinal neurons were 1.1 x M (26.6 pg ml-I), respectively, and for peppermint oil were 20.3 and 41.7 pg ml-'respectively. Both menthol and peppermint oil inhibited specific M (17.2 pg ml-') and 1.75 x