Pharmacology of high-threshold calcium currents in GH4C1 pituitary cells and their regulation by activation of human D2 and D4 dopamine receptors.

1. The objective of this study was to characterize the pharmacology of calcium currents in GH4C1 pituitary cells and determine whether activation of heterologously expressed human dopamine receptors can regulate their function. Human D2(short), D3 and D4.2 receptor cDNA's were separately transfected into GH4C1 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 nM (pIC50 = 9.78 +/- 0.21, n = 4) and 339 nM (pIC50 = 6.47 +/- 0.12, n = 4). The component of current inhibited by 10 nM nisoldipine was also blocked by omega-conotoxin GVIA (30 +/- 9% at 30 nM, n = 6) or by omega-agatoxin IVA (34 +/- 7% at 100 nM, n = 4). 3. Activation of either D2 or D4 receptors by dopamine (10 microM) or quinpirole (0.1 to 10 microM) reduced the peak calcium current by ca. 20% in the majority of cells studied. No inhibition was observed in control or D3 transfected GH4C1 cell lines. 4. The mobilisation of intracellular calcium by thyrotropin releasing hormone in hD4-GH4C1 cells was also studied using Fura-2 AM microspectrofluorimetry. Thyrotropin releasing hormone caused a concentration-dependent increase in calcium mobilisation with an EC50 of 7 nM. D4 receptor activation had no effect upon either basal or hormone-induced [Ca2+]i transients. 5. These results demonstrate that GH4C1 pituitary cells have at least two types of dihydropyridine sensitive high-threshold calcium currents and that like D2 receptors, human D4 receptors can also regulate calcium channel function.