Characterization of the maitotoxin‐activated cationic current from human skin fibroblasts

The maitotoxin (MTX)‐induced cationic current ( I mtx ) from human skin fibroblasts was characterized using the patch‐clamp technique in whole‐cell configuration. Under resting conditions (absence of MTX), the main current observed is produced by an outwardly rectifying K + channel which is inhibited by 1 m m TEA. The current reversal potential was −86 mV ( n = 12). MTX (500 p m ) activated a current with a linear current–voltage relationship and a reversal potential of −10 mV ( n = 10). Replacing the extracellular Na + and K + with N ‐methyl‐ d ‐glucamine (NMDG) caused a shift of the reversal potential to a value below −100 mV, indicating that Na + and K + , but not NMDG, carry I mtx . Further ion selectivity experiments showed that Ca 2+ carries I mtx also. The resulting permeability sequence obtained with the Goldman–Hodgkin–Katz equation yielded Na + (1) ≈ K + (1) > Ca 2+ (0.87). The I mtx activation time course reflected the changes in intracellular Ca 2+ and Na + measured with the fluorescent indicators fura‐2 and SBFI, respectively, suggesting that the activation of I mtx brings about an increment in intracellular Ca 2+ and Na + . Reducing the extracellular Ca 2+ concentration below 1.8 m m prevented the activation of I mtx and the increment in intracellular Na + induced by MTX. Mn 2+ and Mg 2+ could not replace Ca 2+ , but Ba 2+ could replace Ca 2+ . MTX activation of current in 10 m m Ba 2+ was approximately 50 % of that induced in the presence of 1.8 m m Ca 2+ . When 5 m m of the Ca 2+ chelator BAPTA was included in the patch pipette, MTX either failed to activate the current or induced a small current (less than 15 % of the control), indicating that intracellular Ca 2+ is also required for the activation of I mtx . Intracellular Ba 2+ can replace Ca 2+ as an activator of I mtx . However, in the presence of 10 m m Ba 2+ the activation by MTX of the current was 50 % less than the activation with n m concentrations of free intracellular Ca 2+ .