Constitutively active calcium channels in plasma membrane of T-lymphocytes

Abstract

Compensated influx and efflux of calcium ions maintain the constancy of Ca2+ concentration in cytoplasm of quiescent cells under variable external conditions. In cell plasma membrane there exist several types of Ca2+ channels with different properties, regulation mechanisms, and pharmacology. Using fluorescent Ca2+-sensitive probes, we have shown here that in T-lymphocytes under resting conditions, Ca2+ influx occurs through special constitutively active Ca2+ channels, permeable to Ni2+ and Mn2+. These channels differ from the receptor-activated SOC channels, from Ca2+ channels activated by arachidonic acid, and from calmidazolium-activated channels. Ca2+ influx rate in quiescent cells increases with a rise in temperature (Q10 =1.9). The strong dependence of the constitutively active channel activity on temperature coincided with the plasma membrane Ca2+-ATPase dependence, indicating that intracellular enzymes regulate the channel activity. To identify the constitutively active channel, we analyzed the effects of L-type Ca2+ channels, SOC channels, Ca2+-independent phospholipase A2, and calmodulin inhibitors. Of all inhibitors listed only dihydropyridine blocker of L-type voltage-dependent Ca2+ channels, isradipin, at a concentration of 1.5 μM completely suppressed calcium influx. However, the channels did not exhibit sensitivity to changes in membrane potential. Our observations testify to the existence of a new nonselective Ca2+ channel in T-lymphocyte plasma membrane and characterize the new channels pharmacologically. The results obtained are important for understanding the regulation mechanisms of Ca2+ channels in plasma membrane of non-excitable cells.