Intracellular recordings for current and voltage clamping were obtained from 130 neuroendocrine cells of the pars intermedia (PI) in intact pituitaries maintained in vitro. Spontaneous and evoked action potentials were blocked by TTX or by intracellular injection of a local anesthetic, QX-222. After potassium (K+) currents were blocked by tetraethylammonium (TEA), 4-aminopyridine, and intracellular cesium (Cs+), 2 distinct calcium (Ca2+) spikes were observed which were differentiated by characteristic thresholds, durations, and amplitudes. Both Ca2+ spikes were blocked by cobalt (Co2+) but were unaffected by TTX or QX-222. The low-threshold spike (LTS) had a smaller amplitude and inactivated when membrane potential was depolarized past -40 mV or when evoked at a fast rate (greater than 0.5 Hz). The high-threshold spike (HTS) typically had a larger amplitude and longer duration, was not inactivated at potentials which inactivated the LTS, and could be evoked at rates of up to 10 Hz. Single-electrode voltage-clamp analysis revealed that 3 distinct components of the Ca2+ current were present in most cells. From a negative holding potential (-90 mV), 2 separate peak inward currents were observed; a low-threshold transient current, similar to a T-type Ca2+ current, activated at -40 mV, whereas a large-amplitude inactivating current activated above -20 mV. This large inactivating Ca2+ current was significantly inactivated at a holding potential of -40 mV or by brief prepulses to positive potentials, and was similar to an N-type Ca2+ current. A sustained Ca2+ current (L-type) was observed which was not altered by different holding potentials.(ABSTRACT TRUNCATED AT 250 WORDS)