MOST LACTOTROPHS FROM LACTATING RATS ARE ABLE TO RESPOND TO BOTH THYROTROPIN-RELEASING-HORMONE AND DOPAMINE

Intracellular free calcium concentration ([Ca2+](i)) was measured with video imaging in lactotrophs from lactating rats. The median resting [Ca2+](i) was 24 nM (85 cells). The great majority of cells responded to thyrotropin-releasing hormone (TRH) with an increase in [Ca2+](i) ( median peak [Ca2+](i) after TRH = 298 nM; n = 73). In 77% of these cel ls this [Ca2+](i) increase was biphasic, with [Ca2+](i) remaining high after the initial peak (median [Ca2+](i) 90 s after TRH application = 104 nM; n = 56); the second phase depended on calcium influx. Most ce lls also responded to dopamine (DA), after TRH had been applied. DA re duced or abolished TRH-induced calcium influx and also reduced resting [Ca2+](i) if this was above its initial value. A few lactotrophs resp onded to TRH only after DA application and withdrawal. We conclude tha t the population of lactotrophs in lactating rats is heterogeneous, bu t is not composed of two distinct sub-groups defined by their responsi veness to TRH or DA. Copyright (C) 1996 Elsevier Science Ireland Ltd.