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 nM (pIC(50) = 9.78 +/- 0.21, n = 4) and 339 nM (pIC(50) = 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 D-2 or D-4 receptors by dopamine (10 mu M) or quinpirole (0.1 to 10 mu 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 nM. D-4 receptor activation had no effect upon either basal or hormone-induced [Ca2+](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 Dq receptors can also regulate calcium channel function.