Maitotoxin-activated single calcium channels in guinea-pig cardiac cells

1. In order to clarify the mechanism of Ca-dependent excitatory action of maitotoxin (MTX), the most potent marine toxin known, patch-clamp techniques were used to analyse electrophysiological effects of MTX on guinea-pig isolated cardiac myocytes. 2. The whole-cell recordings showed that MTX (0.3 ng ml-1) produced a sustained inward current that was enhanced by adrenaline (2 microM) and abolished by Cd2+ (1 mM). 3. This current was predominantly carried by Ca2+ or Ba2+ and has an almost linear current-voltage relationship. 4. In cell-attached patches, MTX added to the pipette solution activated Ca channels with novel properties. The opening events of these channels occurred as long bursts, and the channel gating showed little voltage-dependence. 5. The unitary conductance was 12 pS in the presence of 50 mM Ba2+. Within a burst, the distribution of opening times was a single exponential with a mean open time of 10.4 ms. 6. The channel described here represents either a new class of voltage-independent Ca channel or an entirely modified form of voltage-gated Ca channel. This channel may account for the mechanism of enhanced Ca2+ influx through the cell membrane induced by MTX, and presumably regulates some ionic movements in myocardial cells.