PHOSPHOLIPASE-C-BETA AND MEMBRANE ACTION OF CALCITRIOL AND ESTRADIOL

We have shown that estrogens and calcitriol, the hormonally active for m of vitamin D, increase the concentration of intracellular calcium ([ Ca2+]i) within 5 s by mobilizing calcium from the endoplasmic reticulu m and the formation of inositol 1,4,5-trisphosphate and diacylglycerol . Because the activation of effecters as phospholipase C (PLC) coupled to G-proteins is the early event in the signal transduction pathway l eading 60 the inositol 1,4,5-trisphosphate formation and to [Ca2+](i) increase, we described different PLC isoforms (beta 1, beta 2, gamma 1 , and gamma 2, but not beta 4) in female rat osteoblasts using Western immunoblotting. The data showed that phospholipase C beta was involve d in the mobilization of Ca2+ from the endoplasmic reticulum of Fura-2 -loaded confluent osteoblasts by calcitriol and 17 beta estradiol, and PLC gamma was ineffective. The data also showed that only a PLC beta 1 linked to a Pertussis toxin-insensitive G protein and a PLC beta 2 c oupled to a Pertussis toxin-sensitive G-protein are involved in the ef fects of calcitriol and 17 beta estradiol on the mobilization of Ca2from intracellular Ca2+ stores. In conclusion, these results may be an important step toward understanding membrane effects of these steroid s and may be an additional argument in favor of membrane receptors to steroid hormones.