Pharmacology of high‐threshold calcium currents in GH 4 C 1 pituitary cells and their regulation by activation of human D 2 and D 4 dopamine receptors

1 The objective of this study was to characterize the pharmacology of calcium currents in GH 4 C 1 pituitary cells and determine whether activation of heterologously expressed human dopamine receptors can regulate their function. Human D 2 (short), D 3 and D 4,2 receptor cDNA's were separately transfected into GH 4 C 1 cells and whole cell calcium currents were recorded by use of nystatin‐perforated patch clamp techniques. 2 High‐threshold calcium currents were antagonized in a biphasic manner by the dihydropyridine, nisoldipine. The half‐maximally effective concentration for each site was 0.2 n m (pIC 50 = 9.78 ± 0.21, n = 4) and 339 n m (pIC 50 = 6.47 ± 0.12, n = 4). The component of current inhibited by 10 n m nisoldipine was also blocked by ω‐conotoxin GVIA (30 ± 9% at 30 n m , n = 6) or by ω‐agatoxin IVA (34 ± 7% at 100 n m , n = 4). 3 Activation of either D 2 or D 4 receptors by dopamine (10 μ m ) or quinpirole (0.1 to 10 μ m ) reduced the peak calcium current by ca. 20% in the majority of cells studied. No inhibition was observed in control or D 3 transfected GH 4 C 1 cell lines. 4 The mobilisation of intracellular calcium by thyrotropin releasing hormone in hD 4 ‐GH 4 C 1 cells was also studied using Fura‐2 AM microspectrofluorimetry. Thyrotropin releasing hormone caused a concentration‐dependent increase in calcium mobilisation with an EC 50 of 7 n m . D 4 receptor activation had no effect upon either basal or hormone‐induced [Ca 2+ ] i transients. 5 These results demonstrate that GH 4 C 1 pituitary cells have at least two types of dihydropyridine‐sensitive high‐threshold calcium currents and that like D 2 receptors, human D 4 receptors can also regulate calcium channel function.