Plateau potentials recorded from lactating rat enriched lactotroph cells are triggered by thyrotropin releasing hormone and shortened by dopamine.

Enrichment of dispersed pituitary cells from normal lactating rats on a continuous BSA gradient permitted the isolation of two prolactin cell populations--light and heavy. Hormone release studies indicated that spontaneous prolactin (PRL) release of the heavy fraction cells was particularly sensitive to TRH stimulation (mean of 450%) and that this effect of TRH was totally inhibited by Ca2+ channel blockers (Co2+, Cd2+, Ni2+ and Mn2+). For this reason, the electrophysiological response to TRH was investigated on heavy fraction cells. Experiments performed on 264 cells after 4-12 days in culture showed that these cells could be divided into two groups. The first group, called high resting potential (HRP) cells, constituted 73% of the total and was characterized by a mean resting potential of -60 mV and a mean input membrane resistance of 700 M omega, and these cells displayed plateau potentials, which were triggered by application of brief (2 s), large (1 nA) depolarizing or hyperpolarizing current steps. The plateaux were characterized by a sustained depolarization at a potential near -20 mV and they were concomitant with an increase of the membrane conductance. The repolarization consisted of a slight, gradual hyperpolarization followed by a rapid return to the resting potential. The second group of cells (the remaining 27%; called low resting potential or LRP cells) was characterized by a mean resting potential of -45 mV and a mean input membrane resistance of 250 M omega. These cells were excitable, 30% of them displaying spontaneous activity but never showing plateaux. Electrophysiological experiments showed that the majority of the HRP cells (99%) responded to TRH but were insensitive to dopamine (DA) ejections (up to 10(-6) M) at resting potential. Ejection of TRH onto HRP cells induced a slow depolarization (10-15 mV) concomitant with a decrease of the membrane resistance. This led the cell membrane potential to a critical value (approximately -50 mV), at which the plateau response was triggered. The plateau lasted for about 10 s from potentials greater than -100 mV, and could reach 10 min from holding potentials close to the resting potential. The amplitude of the plateau varied according to the holding potential and the reversal potential was found to be -20 mV. Local application of tetraethylammonium chloride (TEA, 30 mM) only slightly affected the amplitude of the plateaux but they were shortened or totally blocked by Co2+, Cd2+ and Ni2+.(ABSTRACT TRUNCATED AT 400 WORDS)