Changes in Calcium Channel Activity in Membranes from cis-Diammine- dichloroplatinum(II)-resistant and -sensitive LI 210 Cells1

t'a" channels from lipid and proteolipid fractions of oisplat in-sensitive and cisplatin-resistant cells were reconstituted and characterized in bilayer lipid membranes formed at the tips of patch-clamp micropipets. The characteristics of the Ca2* channels were typical for the endoplasmic reticulum membrane channel activity. They had a relatively large unit conductance and were modified by typical activators (nucleotides) and inhibitors (ruthenium red, verapamil). Different doses of nifedipine did not inhibit Ca2* channel activity. A substantial difference between the single-channel properties of the two types of investigated membranes was observed. The mean open time and the open state probability of channels reconstituted in bilayer lipid membranes from the membrane components of cisplatin-resistant cells were larger than those in bilayer lipid mem branes made from components of cisplat in-sensitive cells. Ruthenium red (7 x lir7 M) inhibited the channel activity in both types of membranes to the same level. The observed effects could be related to an increased Ca2* release from the intracellular Ca2* stores (endoplasmic reticulum system) accompanied by an enhanced intracytoplasmic Ca2* concentra tion in cisplatin-resistant cells. These changes in the Ca2* concentration level may be responsible for the higher antitumor drug efflux rate and the development of the drug resistance. The suggestion is made that specific inhibitors of the Ca2* transport across the membranes of the subcellular Ca2*-storing organdies may be tested as agents for overcom ing the antitumor drug resistance. calcium is controlled by the transport of Ca2* across the ER3 membranes (11, 12). Recently, Ca2+ channels from skeletal muscle SR were reconstituted in BLMs (13). Using the BLM method it has been demonstrated that Ca2+ channels with similar characteristics are present in the ER of nerve and other cells (14-16). Large single-channel unit conductance, fast ki netics, and other characteristics of these channels suggest that they are associated with the release of Ca2+ from the intracel lular stores in different cells (13-16). The properties of these channels differ significantly from those of the plasma membrane Ca2* channels which are respon sible for the Ca2* entry into the cell and have a much lower unit conductance. They are sensitive to verapamil and dihydropyridine agents such as nifedipine and have specific voltage-de pendent characteristics (17, 18). Cisplatin is considered a potent antitumor agent (19, 20). It has been used successfully in combination with other antitumor agents, such as Adriamycin, vinblastine, etc. (21-23). Ca2* channel blockers are capable of potentiating the cytotoxic ef fects of these drugs on cell lines showing resistance (24). In this communication, we report results obtained from stud ies on Ca2* channel activity in BLMs prepared from membrane components of cisplatin-resistant and -sensitive cells.