Properties of two types of calcium channels in clonal pituitary cells

The calcium currents of GH3 cells have been studied using the whole cell variant of the patch-clamp technique. Under conditions that eliminate sodium and potassium currents, we observed inward currents that activated within a few milliseconds, and deactivated with two time constants, approximately 150 microseconds and 3 ms at -80 mV, 18-20 degrees C. The components are called FD and SD (fast deactivating and slow deactivating). Both components are calcium currents, and are greatly reduced when magnesium is substituted for most of the calcium in the bath. In addition to (a) their different rates of deactivation, the two components differ in a number of other properties. (b) The SD component inactivates almost completely, with a time constant of 23 ms at 20 mV, 19 degrees C. The FD component, on the other hand, shows little or no sign of inactivation, and is almost the same in amplitude from 10 to 100 ms. The components thus seem quite independent of each other, and must arise from two independent sets of channels. (c) The FD channels activate more rapidly than SD at 20 mV, by a factor of approximately 2 as is shown in several ways. (d) In 10 Ca or 10 Ba, the activation curve for SD channels is approximately 20 mV more negative than for FD or Na channels. (e) FD channels conduct barium ions more effectively than calcium by a ratio of approximately 2. (f) FD channels "wash out" within minutes after the patch electrode breaks into a cell, whereas SD channel current remains relatively stable. It is argued that SD channels, because of their negative activation threshold, are involved in electrical events near threshold, and that FD channels are best suited for calcium injection once a spike has been initiated.