Secretion of hCG from human placenta is regulated by agonists such as LHRH via an increase in [Ca²⁺]_i (Mathialagan & Rao, 1989). It has previously been proposed that LHRH raises [Ca²⁺]_i in human placenta by stimulation of L-type voltage-dependent Ca²⁺ channels (Sharma & Rao, 1997); however, there is little direct evidence for these channels in this tissue (Cronier et al. 1999). Rather, the role of voltage-independent Ca²⁺ channels for raising [Ca²⁺]_i in human placenta is becoming increasingly apparent (Clarson et al. 2002; Moreau et al. 2002). Therefore, in this study we have examined the effect of various blockers of Ca²⁺-permeable channels on LHRH-stimulated hCG secretion from human term placental villous fragments.

Fragments were dissected from term placentas, washed in control Tyrode buffer (mM: 135 NaCl, 5 KCl, 1 MgCl₂, 1.8 CaCl₂, 5.6 glucose, 10 Hepes; pH 7.4 with NaOH) and then incubated for 1 h (six fragments per vial) with 10 µM LHRH in control Tyrode buffer in the presence and absence of three channel blockers. The blockers examined were: 50 µM SKF96365 (blocks store-operated Ca²⁺ channels), 150 µM GdCl₃ (a blocker of Ca²⁺ entry pathways including non-selective cation channels) and 1 µM nifedipine (blocks L-type Ca²⁺ channels). Following incubation, hCG secretion was assessed by assay and placental fragment blotted wet weight was determined. Data were calculated as mIU ml^-1 (mg wet weight)^-1.

LHRH (10 µM) significantly increased hCG secretion. This was significantly reduced in the presence of 150 µM GdCl₃. SKF96365 (50 µM) reduced LHRH-stimulated secretion, which was significant at the 10 % level. Nifedipine (1 µM) had no effect on LHRH-stimulated hCG secretion (see Fig. 1). These data suggest that the role of L-type Ca²⁺ channels in LHRH-stimulated hCG secretion from human term placenta is inconclusive. It does, however, appear that voltage-independent Ca²⁺-permeable channels, such as non-selective cation and store-operated Ca²⁺ channels, are of greater importance for entry of Ca²⁺_O following LHRH stimulation and thus in hCG secretion from human term placenta.

This work was supported by the MRC.