Two types of calcium channels and hormone release in human pituitary tumor cells

Voltage-gated Ca2+ channel currents were examined in human growth hormone-producing cells using the whole cell variation of patch electrode voltage clamp. In 20 mM Ba2+, Na+-free medium, the inward current was composed of a fast-inactivating component (transient type) and a steady component (long-lasting type). These two types had different properties. The transient type inactivated with the depolarizing prepulse but the long-lasting type did not. The transient type was activated at more negative potential levels than the long-lasting type. Deactivation kinetics of the transient type were slower than that of the long-lasting type. Nitrendipine inhibited and Bay K 8644 enhanced Ca2+ channel currents, and the effects of these agents were more prominent on the long-lasting type than on the transient type. Hormone release was inhibited by nitrendipine and enhanced by Bay K 8644, indicating involvement of Ca2+ influx through voltage-gated channels in the release process.