Activity of high-threshold voltage activated neuronal Ca2+ channels, including dihydropyridine-sensitive (L-type) channels, rapidly disappears during cell dialysis in whole-cell recording conditions or after excision of a patch. To date, this phenomenom has been mainly related to phosphatase or protease activity. On the other hand, it has been suggested that Ca2+ channels may be regulated by G-proteins. Therefore, disruption of this regulatory pathway may also be involved directly or indirectly in the rundown process. Here, we show that treatment of cultured cerebellar granule cells with pertussis toxin (PTX) increases to 70% the probability for excising patches that display L-type Ca2+ channels activity in the inside-out recording configuration. Quantitative study indicates that, except a half decrease in the open probability, most features of the channel activity are retained after patch excision with minor modifications. The characteristics of the channel activity did not change with time during at least the first 9 min of the inside-out configuration. In addition, comparison of unitary currents recorded in the cell-attached, configuration on treated and nontreated cells demonstrates that the PTX treatment slows the activation kinetics of the current and increases the duration of channel openings evoked at -20 mV but not at 0 mV depolarizing potential. These data suggest that L-type Ca2+ channel activity are under a tonic regulation of a PTX-sensitive mechanism, which is implied in the run-down process.